Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        2 IVLTQSPASLAVSLGQRATISCRASESVdnyGISFLNWFQQKPGQPPKLLIYAASNQGSGVPARFSGSGSGTYFSLNIHP 81
Cdd:cd04980   1 IVMTQSPASLSVSPGERVTISCKASQSI---SSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTISS 77

                        90       100
                ....*....|....*....|....*....
6D9G_D       82 MEEDDTAVYFCQQTKGVSWTFGGGTKVEI 110
Cdd:cd04980  78 VEPEDAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        2 IVLTQSPASLAVSLGQRATISCRASESVdnyGISFLNWFQQKPGQPPKLLIYAASNQGSGVPARFSGSGSGTYFSLNIHP 81
Cdd:cd04980   1 IVMTQSPASLSVSPGERVTISCKASQSI---SSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTISS 77

                        90       100
                ....*....|....*....|....*....
6D9G_D       82 MEEDDTAVYFCQQTKGVSWTFGGGTKVEI 110
Cdd:cd04980  78 VEPEDAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin C1-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        119 VSVFPPSSEQLtSGGASVVCFLNNFYPRDINVKWKIDGSERQNGVlNSWTDQDSKDSTYSMSSTLTLTKDEYERHNSYTC 198
Cdd:pfam07654   1 VYVFPPSPEEL-GKPNTLTCLVTGFYPPDITVTWLKNGQEVTEGV-KTTPPSPNSDWTYQLSSYLTVTPSDWESGDEYTC 78

                  ....*..
6D9G_D        199 EATHKTS 205
Cdd:pfam07654  79 RVEHEGL 85

Immunoglobulin V-set domain; This domain is found in antibodies as well as neural protein P0 and CTL4 amongst others.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D          6 QSPASLAVSLGQRATISCRASESVDNyGISFLNWFQQKPGQPPKLLIYAASNQG--SGVPARFSGSGSGTY--FSLNIHP 81
Cdd:pfam07686   1 QTPREVTVALGGSVTLPCTYSSSMSE-ASTSVYWYRQPPGKGPTFLIAYYSNGSeeGVKKGRFSGRGDPSNgdGSLTIQN 79

                          90       100
                  ....*....|....*....|....*....
6D9G_D         82 MEEDDTAVYFCQQTKGVSWTFGGGTKVEI 110
Cdd:pfam07686  80 LTLSDSGTYTCAVIPSGEGVFGKGTRLTV 108

Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        2 IVLTQSPASLAVSLGQRATISCRASESVdnyGISFLNWFQQKPGQPPKLLIYAASNQGSGVPARFSGSGSGTYFSLNIHP 81
Cdd:cd04980   1 IVMTQSPASLSVSPGERVTISCKASQSI---SSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTISS 77

                        90       100
                ....*....|....*....|....*....
6D9G_D       82 MEEDDTAVYFCQQTKGVSWTFGGGTKVEI 110
Cdd:cd04980  78 VEPEDAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin (Ig) lambda light chain variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, lambda type, variable (V) domain. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        3 VLTQsPASLAVSLGQRATISCRASESvdNYGISFLNWFQQKPGQPPKLLIYAASNQGSGVPARFSGSGSGTYFSLNIHPM 82
Cdd:cd04984   1 VLTQ-PSSLSVSPGETVTITCTGSSG--NISGNYVNWYQQKPGSAPRYLIYEDKHRPSGIPDRFSGSKSGNTASLTISGA 77

                        90       100
                ....*....|....*....|....*.
6D9G_D       83 EEDDTAVYFCQQTKGVSWTFGGGTKV 108
Cdd:cd04984  78 QTEDEADYYCQVWDSNSYVFGGGTKL 103

Immunoglobulin variable domain (IgV); The members here are composed of the immunoglobulin variable domain (IgV). The IgV family contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology, and are components of immunoglobulin (Ig) and T cell receptors. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding. A predominant feature of most Ig domains is the disulfide bridge connecting 2 beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E and, D strands in one sheet and A', G, F, C, C', and C" strands in the other.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        4 LTQSPASLAVSLGQRATISCRASESVDNYGISflnWFQQKPGQPPKLLIYAASNQGS---GVPARFSGSGSG-TYFSLNI 79
Cdd:cd00099   1 VTQSPRSLSVQEGESVTLSCEVSSSFSSTYIY---WYRQKPGQGPEFLIYLSSSKGKtkgGVPGRFSGSRDGtSSFSLTI 77

                        90       100       110
                ....*....|....*....|....*....|....
6D9G_D       80 HPMEEDDTAVYFCQQT---KGVSWTFGGGTKVEI 110
Cdd:cd00099  78 SNLQPEDSGTYYCAVSesgGTDKLTFGSGTRLTV 111

Immunoglobulin C-Type;

                           10        20        30        40        50        60        70
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
6D9G_D         134 ASVVCFLNNFYPRDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDEYERHNSYTCEATHKTSTSP 208
Cdd:smart00407   2 ATLVCLVSGFYPPDITVTWLRNGQEVTEGVSTTDPLKNS-DGTYFLSSYLTVPASTWESGDVYTCQVTHEGLKEP 75

CH3 domain (third constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, gamma, and delta chains, and CH4 domain (fourth constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, epsilon, and mu chains; member of the C1-set of I; The members here are composed of the third and fourth immunoglobulin constant domain (IgC) of alpha, delta, gamma and alpha, epsilon, and mu heavy chains, respectively. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQLTSGG-ASVVCFLNNFYPRDINVKWKIDGSERQNGVLNSWTDQDSKDSTYSMSSTLTLTKDEYERHNS 195
Cdd:cd05768   1 PSVYLLPPPEEELSLNEtVTLTCLVKGFYPEDIFVSWLQNGEPLPSADYKTTAPVPESDGSFFVYSKLNVSTADWNSGDV 80

                        90       100
                ....*....|....*....|.
6D9G_D      196 YTCEATHKTSTSPIV-KSFNR 215
Cdd:cd05768  81 FSCVVGHEALPLQFTqKSIDK 101

Immunoglobulin V-set domain; This domain is found in antibodies as well as neural protein P0 and CTL4 amongst others.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D          6 QSPASLAVSLGQRATISCRASESVDNyGISFLNWFQQKPGQPPKLLIYAASNQG--SGVPARFSGSGSGTY--FSLNIHP 81
Cdd:pfam07686   1 QTPREVTVALGGSVTLPCTYSSSMSE-ASTSVYWYRQPPGKGPTFLIAYYSNGSeeGVKKGRFSGRGDPSNgdGSLTIQN 79

                          90       100
                  ....*....|....*....|....*....
6D9G_D         82 MEEDDTAVYFCQQTKGVSWTFGGGTKVEI 110
Cdd:pfam07686  80 LTLSDSGTYTCAVIPSGEGVFGKGTRLTV 108

CH2 domain (second constant Ig domain of the heavy chain) in immunoglobulin mu chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the second immunoglobulin constant domain (IgC) of mu heavy chains. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQLTSGG-ASVVCFLNNFYPRDINVKWKIDGSERQ--NGVLNSWTDQDSKdSTYSMSSTLTLTKDEYERH 193
Cdd:cd16093   2 PTVSLHAPSREEFLGNRtATFVCLATGFSPKTISFKWLRNGKEVTssTGAVVEEPKEDGK-TLYSATSFLTITESEWKSQ 80

                        90
                ....*....|...
6D9G_D      194 NSYTCEATHKTST 206
Cdd:cd16093  81 TEFTCEFKHKGEI 93

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D           8 PASLAVSLGQRATISCRASESVDNYgisfLNWFQQKPgqppKLLIYaasnqgsgvPARFSGSGSGTYFSLNIHPMEEDDT 87
Cdd:smart00410   1 PPSVTVKEGESVTLSCEASGSPPPE----VTWYKQGG----KLLAE---------SGRFSVSRSGSTSTLTISNVTPEDS 63

                           90       100
                   ....*....|....*....|...
6D9G_D          88 AVYFCQQTKGVSWTFgGGTKVEI 110
Cdd:smart00410  64 GTYTCAATNSSGSAS-SGTTLTV 85

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DR; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DR. HLA-DR is an MHC class II cell surface receptor encoded by the human leukocyte antigen complex on chromosome 6 region 6p21.31. HLA-DR is also involved in several autoimmune conditions, disease susceptibility, and disease resistance including seronegative-rheumatoid arthritis, penicillamine-induced myasthenia, schizophrenia, Goodpasture syndrome, systemic lupus erythematosus, Alzheimers, tuberculoid leprosy, and Hashimoto's thyroiditis. HLA-DR molecules are upregulated in response to signaling. HLA-DR is an alphabeta heterodimer cell surface receptor, each subunit of which contains two extracellular domains, a membrane-spanning domain, and a cytoplasmic tail. Both alpha and beta chains are anchored in the membrane. The DR beta chain is encoded by 4 loci, however no more than 3 functional loci are present in a single individual, and no more than two on a single chromosome. Sometimes an individual may only possess 2 copies of the same locus, DRB1*. The HLA-DRB1 locus is ubiquitous and encodes a very large number of functionally variable gene products (HLA-DR1 to HLA-DR17). The HLA-DRB3 locus encodes the HLA-DR52 specificity, is moderately variable and is variably associated with certain HLA-DRB1 types. The HLA-DRB4 locus encodes the HLA-DR53 specificity, has some variation, and is associated with certain HLA-DRB1 types. The HLA-DRB5 locus encodes the HLA-DR51 specificity, which is typically invariable, and is linked to the HLA-DR2 types. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQLTSGGAsVVCFLNNFYPRDINVKWKIDGSERQNGVLNSWTDQDSkDSTYsmsSTLTLTKDEYERHNSY 196
Cdd:cd21000   4 PKVTVYPAKTQPLQHHNL-LVCSVNGFYPGSIEVRWFRNGQEEKAGVVSTGLIQNG-DWTF---QTLVMLETVPRSGEVY 78

                        90
                ....*....|...
6D9G_D      197 TCEATHKTSTSPI 209
Cdd:cd21000  79 TCQVEHPSVTSPL 91

Tapasin-R immunoglobulin-like domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin-like domain on Tapasin-R. Tapasin is a V-C1 (variable-constant) immunoglobulin superfamily molecule present in the endoplasmic reticulum (ER), where it links MHC class I molecules to the transporter associated with antigen processing (TAP). Tapasin-R is a tapasin-related protein that contains similar structural motifs to Tapasin, with some marked differences, especially in the V domain, transmembrane and cytoplasmic regions. The majority of Tapasin-R is located within the ER; however, there may be some expression of Tapasin-R at the cell surface. Tapasin-R lacks an obvious ER retention signal.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPpSSEQLTSGGASVVCFLNNFYPRDINVKWKID---GSERQNGVLNSW--TDQDSKDSTYSMSSTLTLTKDEYE 191
Cdd:cd05771   1 PRVRLSP-KNLVKPDLPQTLSCHIAGYYPLDVDVEWLREepgGSESQVSRDGVSlsSHRQSVDGTYSISSYLTLEPGTEN 79

                        90       100
                ....*....|....*....|.
6D9G_D      192 RHNSYTCEATHKTSTSPIVKS 212
Cdd:cd05771  80 RGATYTCRVTHVSLEEPLSVS 100

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and I-A; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of human histocompatibility antigen (HLA) DQ and mouse I-A. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E have the same basic features insofar as peptide loading and presentation, they differ in that each interacts with distinctly different sets of peptides, and in the incidence of deletion of their genes. A structural understanding of the similarities and differences between I-A and I-E may help with understanding their roles in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. Human HLA-DR, -DQ, and -DP are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. HLA-DQ is involved in the autoimmune diseases celiac disease and diabetes mellitus type. DQ is one of several antigens involved in rejection of organ transplants. DQ2 is encoded by the HLA-DQB1*02 allele group. DQ6 is encoded by the HLA-DQB1*06 allele group. DQ2 beta-chains combine with alpha-chains, encoded by genetically linked HLA-DQA1 alleles, to form the cis-haplotype isoforms. These isoforms, nicknamed DQ2.2 and DQ2.5, are also encoded by the DQA1*0201 and DQA1*0501 genes, respectively. DQ6 beta-chains combine with alpha-chains, encoded by genetically linked HLA-DQA1 alleles, to form the cis-haplotype isoforms. For DQ6, however, cis-isoform pairing only occurs with DQ1 alpha-chains. There are many haplotypes of DQ6. Susceptibility to Leptospirosis infection was found associated with undifferentiated DQ6. DQ8 is determined by the antibody recognition of beta8 and this generally detects the gene product of DQB1*0302. DQ8 is commonly linked to autoimmune disease in the human population. DQ8 is the second most predominant isoform linked to celiac disease and the DQ most linked to Type 1 diabetes. DQ8 increases the risk for rheumatoid arthritis and is linked to the primary risk locus for RA, HLA-DR4. DR4 also plays an important role in Type 1 diabetes. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune response. They are expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice, and induced in nonprofessional APCs, such as keratinocyctes; they are expressed on the surface of activated human T cells and on T cells from other species. MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes; these peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC, and bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      115 AAPTVSVFPPSSEQLTSGGAsVVCFLNNFYPRDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKdeyERHN 194
Cdd:cd21001   2 VEPTVTISPSRTEALNHHNL-LVCSVTDFYPGQIKVRWFRNDQEETAGVVSTPLIRNG-DWTFQILVMLEMTP---QRGD 76

                        90
                ....*....|....*
6D9G_D      195 SYTCEATHKTSTSPI 209
Cdd:cd21001  77 VYTCHVEHPSLQSPI 91

CH3 domain (third constant Ig domain of heavy chains) in immunoglobulin heavy alpha, epsilon, and mu chains, and CH2 domain (second constant Ig domain of the gheavy chain) in immunoglobulin heavy gamma chain; member of the C1-set of Ig superfamily (IgSF) ; The members here are composed of the third immunoglobulin constant domain (IgC) of the gamma heavy chains and the second immunoglobulin constant domain (IgC) of alpha, epsilon, and mu heavy chains. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQL-TSGGASVVCFLNNFYPRD-INVKWKidgseRQNG--VLNS-WTDQDSKDSTYSMSSTLTLTKDEYE 191
Cdd:cd07696   1 VSVFLIPPSPKDLfLTKSAKVTCLVVDLTSIEeVNVTWS-----REDGneVLAStTNPEKHYNATLSVVSTLTVCADDWD 75

                        90       100
                ....*....|....*....|.
6D9G_D      192 RHNSYTCEATHKTSTSPIVKS 212
Cdd:cd07696  76 NGKTFKCKVTHPDLPSPIVKS 96

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II beta chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain has two globular domains (N- and C-terminal) and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQLTSGgASVVCFLNNFYPRDINVKWKIDGSERQNGVlNSWTDQDSKDSTYSMSSTLTLTKdeyERHNSY 196
Cdd:cd05766   4 PSVKVSPTKTGPLEHP-NLLVCSVTGFYPAEIEVKWFRNGQEETAGV-VSTELIPNGDWTFQILVMLETTP---RRGDVY 78

                        90
                ....*....|...
6D9G_D      197 TCEATHKTSTSPI 209
Cdd:cd05766  79 TCQVEHSSLQSPL 91

Signal-regulatory protein (SIRP) immunoglobulin-like domain 3; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig)-like domain in Signal-Regulatory Protein (SIRP), domain 3 (C1 repeat 2). The SIRPs belong to the "paired receptors" class of membrane proteins that comprise several genes coding for proteins with similar extracellular regions but very different transmembrane/cytoplasmic regions with different (activating or inhibitory) signaling potentials. They are commonly on NK cells, but are also on many myeloid cells. Their extracellular region contains three Immunoglobulin superfamily domains a single V-set and two C1-set IgSF domains. Their cytoplasmic tails that contain either ITIMs or transmembrane regions that have positively charged residues that allow an association with adaptor proteins, such as DAP12/KARAP, containing ITAMs. There are 3 distinct SIRP members: alpha, beta, and gamma. SIRP alpha (also known as CD172a or SRC homology 2 domain-containing protein tyrosine phosphatase substrate 1/Shps-1) is a membrane receptor that interacts with a ligand CD47 expressed on many cells and gives an inhibitory signal through immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic region that interact with phosphatases SHP-1 and SHP-2. SIRP beta has a short cytoplasmic region and associates with a transmembrane adapter protein DAP12 containing immunoreceptor tyrosine-based activation motifs to give an activating signal. SIRP gamma contains a very short cytoplasmic region lacking obvious signaling motifs but also binds CD47, but with much less affinity.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVfppsSEQLTSGG--ASVVCFLNNFYPRDINVKWKIDGSERQNGVLNSWTDqdSKDSTYSMSSTLTLTKDEYERHN 194
Cdd:cd16085   3 PTLEV----TQQPTMVWnqVNVTCQVEKFYPQRLQLTWLENGNVSRTETPSTLTV--NKDGTYNWTSWLLVNVSAHREDV 76

                ....*...
6D9G_D      195 SYTCEATH 202
Cdd:cd16085  77 VLTCQVEH 84

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of the major histocompatibility complex (MHC) class II alpha chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      116 APTVSVFPpsSEQLTSGGASV-VCFLNNFYPRDINVKWKIDGSERQNGVlnSWTD-QDSKDSTYSMSSTLTLTKdeyERH 193
Cdd:cd05767   2 PPEVTVFP--KSPVELGEPNTlICFVDNFFPPVINVTWLRNGQPVTDGV--SETVfLPREDHSFRKFSYLPFTP---SEG 74

                        90
                ....*....|....*...
6D9G_D      194 NSYTCEATHKTSTSPIVK 211
Cdd:cd05767  75 DIYDCRVEHWGLEEPLLK 92

CH1 domain (first constant Ig domain of the heavy chain) in immunoglobulin heavy alpha chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin constant-1 set domain of alpha chains. It belongs to a family composed of the first immunoglobulin constant-1 set domain of alpha, epsilon, gamma, and mu heavy chains. This domain is found on the Fab antigen-binding fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      120 SVFPPSSEQ-LTSGGASVVCFLNNFYPRDINVKWKIDGSErqnGVLNSWTDQDSKDSTYSMSSTLTLTKDEYERHNSYTC 198
Cdd:cd21818   3 TVFPLSLCPsLSSDPVVIGCLVQGFFPEPVNVTWNYSGKG---GTARNFPAMLASGGRYTQSSQLTLPADQCPEGEAYKC 79

                ....*...
6D9G_D      199 EATHKTST 206
Cdd:cd21818  80 SVQHYSPS 87

Class I major histocompatibility complex (MHC) alpha chain, alpha3 immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class I alpha chain. Class I MHC proteins bind antigenic peptide fragments and present them to CD8+ T lymphocytes. Class I molecules consist of a transmembrane alpha chain and a small chain called the beta-2-microglobulin. The alpha chain contains three extracellular domains, two of which fold together to form the peptide-binding cleft (alpha1 and alpha2), and one which has an Ig fold (alpha3). Peptide binding to class I molecules occurs in the endoplasmic reticulum (ER) and involves both chaperones and dedicated factors to assist in peptide loading. Class I MHC molecules are expressed on most nucleated cells.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      116 APTVSVF-PPSSEqltsGGASVVCFLNNFYPRDINVKWKIDGSE----------RQNGvlnswtdqdskDSTYSMSSTLT 184
Cdd:cd07698   2 PPKVHVThHPRSD----GESTLRCWALGFYPAEITLTWQRDGEDqtqdmelvetRPNG-----------DGTFQKWAAVV 66

                        90       100
                ....*....|....*....|....*.
6D9G_D      185 LTKDEYERhnsYTCEATHKTSTSPIV 210
Cdd:cd07698  67 VPSGEEQR---YTCHVQHEGLPEPLT 89

Immunoglobulin (Ig) heavy chain (H), variable (V) domain; The members here are composed of the immunoglobulin (Ig) heavy chain (H), variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which can associate with any of the heavy chains. This family includes alpha, gamma, delta, epsilon, and mu heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        4 LTQSPASLaVSLGQRATISCRAS-ESVDNYGIsflNWFQQKPGQPPKLL----------IYAASNQGSgvpARFSGSGSG 72
Cdd:cd04981   2 LQESGPGL-VKPGQSLKLSCKASgFTFTSYGM---GWVRQAPGKGLEWIgliypgggdtYYADSFKGR---FTITRDTSK 74

                        90       100       110       120
                ....*....|....*....|....*....|....*....|.
6D9G_D       73 TYFSLNIHPMEEDDTAVYFCQQTKGVSWTF-----GGGTKV 108
Cdd:cd04981  75 STAYLQLNSLTSEDTAVYYCARGLGGYGYSyfdywGQGTTV 115

Immunoglobulin (Ig)-like domain of Programmed Cell Death 1 (PD1); The members here are composed of the immunoglobulin (Ig)-like domain of Programmed Cell Death 1 (PD1; also known as CD279/cluster of differentiation 279). PD1 is a cell surface receptor that is expressed on T cells and pro-B cells. The protein's structure includes an extracellular IgV domain followed by a transmembrane region and an intracellular tail. Activation of CD4+ T cells, CD8+ T cells, NKT cells, B cells, and monocytes induces PD-1 expression, immediately after which it binds two distinct ligands, PD-L1 (also known as B7-H1 or CD274/cluster of differentiation 274) and PD-L2, also known as B7-DC. PD-1 plays an important role in down regulating the immune system by preventing the activation of T-cells, reducing autoimmunity and promoting self-tolerance. The inhibitory effect of PD-1 is accomplished by promoting apoptosis in antigen specific T-cells in lymph nodes while simultaneously reducing apoptosis in regulatory T cells. A class of drugs that target PD-1, known as the PD-1 inhibitors, activate the immune system to attack tumors and treat cancer. Comparisons between the mouse PD-1 (mPD-1) and human PD-1 (hPD-1) reveals that unlike the mPD-1 which has a conventional IgSF V-set domain, hPD-1 lacks a C" strand, and instead the C' and D strands are connected by a long and flexible loop. In addition, the BC loop is not stabilized by disulfide bonding to the F strand of the ligand binding beta sheet. These differences result in different binding affinities of human and mouse PD-1 for their ligands.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D        4 LTQSPASLAVSLGQRATISCRASESVDNYgisFLNWFQQKPG-QPPKLLIYAASNQGSGVPARFSGSG--SGTYFSLNIH 80
Cdd:cd16088   2 PTFSPALLVVTEGANATFTCSFSNTSESF---VLNWYRLSPSnQTDKLAAFPEDRSQPGQDWRFRVTQlpNGRDFHMSVV 78

                        90
                ....*....|..
6D9G_D       81 PMEEDDTAVYFC 92
Cdd:cd16088  79 RARRNDSGTYLC 90

CH1 domain (first constant Ig domain of the heavy chain) in immunoglobulin heavy epsilon and gamma chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the first immunoglobulin constant-1 set domain of epsilon and gamma chains. It belongs to a family composed of the first immunoglobulin constant-1 set domain of alpha, delta, epsilon, gamma, and mu heavy chains. This domain is found on the Fab antigen-binding fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      120 SVFP--PSSEQLTSGGASVVCFLNNFYPRDINVKWKidgserqNGVLNSWTD-----QDSKDsTYSMSSTLTLTKDEYER 192
Cdd:cd21817   3 SVFPlaPCCKSTNGSSVTLGCLVTGYFPEPVTVTWN-------SGSLTSGVKtfpavLQSSG-LYTTSSQVTVPSSSWGS 74

                        90
                ....*....|....*....
6D9G_D      193 hNSYTCEATHKTSTSPIVK 211
Cdd:cd21817  75 -QTFTCNVEHKPSSTKVDK 92

Immunoglobulin (Ig)-like domain of platelet-derived growth factor (PDGF) receptors (R), alpha and beta; member of the I-set of IgSF domains; The members here are composed of the immunoglobulin (Ig)-like domain of platelet-derived growth factor (PDGF) receptors (R), alpha (also known as cluster of differentiation (CD) 140a), and beta (also known as CD140b). PDGF is a potent mitogen for connective tissue cells. PDGF-stimulated processes are mediated by three different PDGFs (PDGF-A,PDGF-B, and PDGF-C). PDGFRalpha binds to all three PDGFs, whereas the PDGFRbeta binds only to PDGF-B. PDGFRs alpha and beta have similar organization: an extracellular component with five Ig-like domains, a transmembrane segment, and a cytoplasmic portion having protein tyrosine kinase activity. In mice, PDGFRalpha and PDGFRbeta are essential for normal development.

                        10        20        30        40        50        60
                ....*....|....*....|....*....|....*....|....*....|....*....|....*..
6D9G_D      148 INVKWKIDGSERQNGVLNSwtdQDSKDSTYSMSSTLTLTKDEYERHNSYTCEATHKTSTSPIVKSFN 214
Cdd:cd05861  32 VDLEWTYPGKESGRGIEPV---EEFKVPPYHLVYTLTIPSATLEDSGTYECAVTEATNDHQDEKKIN 95

Class Ib major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen (HLA) F; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class Ib major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen alpha chain F (HLA-F). HLA-F, encoded by the HLA-F gene in humans, belongs to the non-classical HLA class I heavy chain paralogs. This class I molecule mainly exists as a heterodimer associated with the invariant light chain beta-2-microglobulin. HLA-F molecules can interact with both activating and inhibitory receptors on immune cells, such as NK cells, and can present a diverse panel of peptides. Class I MHC proteins bind antigenic peptide fragments and present them to CD8+ T lymphocytes. Class I molecules consist of a transmembrane alpha chain and a small chain called the beta-2-microglobulin. The alpha chain contains three extracellular domains, two of which fold together to form the peptide-binding cleft (alpha1 and alpha2), and one which has an Ig fold (alpha3). Peptide binding to class I molecules occurs in the endoplasmic reticulum (ER) and involves both chaperones and dedicated factors to assist in peptide loading. Class I MHC molecules are expressed on most nucleated cells.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      111 KRADAAPTVSVFPPSSEQltsgGASVVCFLNNFYPRDINVKWKIDGSERQNGVLNSWTdQDSKDSTYSMSSTLTLTKDEY 190
Cdd:cd21023   1 QRADPPKAHVAHHPISDH----EATLRCWALGFYPAEITLTWQRDGEEQTQDTELVET-RPAGDGTFQKWAAVVVPPGEE 75

                        90       100
                ....*....|....*....|
6D9G_D      191 ERhnsYTCEATHKTSTSPIV 210
Cdd:cd21023  76 QR---YTCHVQHEGLPQPLI 92

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and related proteins; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) DQ. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. Two autoimmune diseases in which HLA-DQ is involved are celiac disease and diabetes mellitus type 1. DQ is one of several antigens involved in rejection of organ transplants. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQLTSGGaSVVCFLNNFYPRDINVKWKIDGSERQNGVlnSWTDQDSK-DSTYSMSSTLTLTKDEYErhnS 195
Cdd:cd21008   3 PEVTVFPKSPVTLGQPN-TLICLVDNIFPPVINITWLSNGHSVTEGV--SETSFLSKsDHSFLKISYLTFLPSADD---I 76

                        90
                ....*....|....*.
6D9G_D      196 YTCEATHKTSTSPIVK 211
Cdd:cd21008  77 YDCKVEHWGLDKPLLK 92

Class Ia major histocompatibility complex (MHC) immunoglobulin domain of MIC-A and MIC-B; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class Ia major histocompatibility complex (MHC) immunoglobulin domain of MIC-A and MIC-B. MIC-A and MIC-B are homologs that serve as stress-inducible antigens on epithelial and epithelially derived cells. Both serve as ligands for the widely expressed activating immunoreceptor NKG2D, a C-type lectin-like activating immunoreceptor. MIC-B is very similar in structure to MIC-A and likely interacts with NKG2D in an analogous manner. The interdomain flexibility observed in the MIC-A structures, a feature unique to MIC proteins among MHC class I proteins and homologs, is also displayed by MIC-B, with an interdomain relationship intermediate between the two examples of MIC-A structures. Mapping sequence variations onto the structures of MIC-A and MIC-B reveals patterns completely distinct from those displayed by classical MHC class I proteins, with a number of substitutions falling on positions likely to affect interactions with NKG2D, but with other positions lying distant from the NKG2D binding sites or buried within the core of the proteins. Members of the IgC family are components of immunoglobulin, T-cell receptors, CD1 cell surface glycoproteins, secretory glycoproteins A/C, and major histocompatibility complex (MHC) class I/II molecules. In immunoglobulins, each chain is composed of one variable domain (IgV) and one or more IgC domains. These names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. The IgV domain is responsible for antigen binding and the IgC domain is involved in oligomerization and molecular interactions.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVfppSSEQLTSGGASVVCFLNNFYPRDINVKWKIDGSERQNGVlNSWTD-QDSKDSTYSMSSTLTLTKDEYERhns 195
Cdd:cd21017   5 PMVNV---TRSEASEGNITVTCRASGFYPWNITLSWRQDGVSLSHDT-QQWGDvLPDGNGTYQTWVATRICQGEEQR--- 77

                        90
                ....*....|....*.
6D9G_D      196 YTCEATHKTS--TSPI 209
Cdd:cd21017  78 FTCYMEHSGNhsTHPV 93

CH2 domain (second constant Ig domain of the heavy chain) in immunoglobulin heavy alpha chain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the second immunoglobulin constant-1 set domain (IgC) of alpha heavy chains. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. This group belongs to the C1-set of IgSF domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules, and in various T-cell receptors.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
6D9G_D      117 PTVSVFPPSSEQLTSG-GASVVCFLNnfyprdinvkwkidGSERQNGVLNSWTDQDSKDST-----------YSMSSTLT 184
Cdd:cd04986   2 PRLSLQRPALEDLLLGsNASLTCTLS--------------GLKDPEGATFTWEPSGGKEAIqgpperdscgcYSVSSVLP 67

                        90       100
                ....*....|....*....|....*...
6D9G_D      185 LTKDEYERHNSYTCEATHKTSTSPIVKS 212
Cdd:cd04986  68 GCAEPWNSGDTFSCTVTHPESKGTLTAT 95