Immunoglobulin domain; The members here are composed of the immunoglobulin (Ig) domain found in the Ig superfamily. The Ig superfamily is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. Members of this group are components of immunoglobulin, neuroglia, cell surface glycoproteins, including T-cell receptors, CD2, CD4, CD8, and membrane glycoproteins, including butyrophilin and chondroitin sulfate proteoglycan core protein. A predominant feature of most Ig domains is a disulfide bridge connecting the two 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" in the other. The structures in C1-set are smaller than those in the V-set; they have one beta sheet that is formed by strands A, B, E, and D and the other by strands G, F, C, and C'. Moreover, a C1-set Ig domain contains a short C' strand (three residues) and lacks A' and C" strand. Unlike other Ig domain sets, C2-set structures do not have a D strand. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

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                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1046856867 149 NITLTCSVEEEEKNVVYSWSPSGEE---------------SNVLQIFHSPMDQKLTYTCTASNPVSNNSDSVTV 207
Cdd:pfam00047  13 SATLTCSASTGSPGPDVTWSKEGGTlieslkvkhdngrttQSSLLISNVTKEDAGTYTCVVNNPGGSATLSTSL 86

Immunoglobulin domain; Members of the immunoglobulin superfamily are found in hundreds of proteins of different functions. Examples include antibodies, the giant muscle kinase titin and receptor tyrosine kinases. Immunoglobulin-like domains may be involved in protein-protein and protein-ligand interactions.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1046856867 149 NITLTCSVEEEEKNVVYSWSPSGEE---------------SNVLQIFHSPMDQKLTYTCTASNPVSNNSDSVTV 207
Cdd:pfam00047  13 SATLTCSASTGSPGPDVTWSKEGGTlieslkvkhdngrttQSSLLISNVTKEDAGTYTCVVNNPGGSATLSTSL 86

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.

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                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1046856867 144 LNNTCNITLTCSVEEEEKNVVYSWSPSGEE-----------------SNVLQIFHSPMDQKLTYTCTASNPVSNNSDSVT 206
Cdd:cd04986    16 LGSNASLTCTLSGLKDPEGATFTWEPSGGKeaiqgpperdscgcysvSSVLPGCAEPWNSGDTFSCTVTHPESKGTLTAT 95

                  .
gi 1046856867 207 V 207
Cdd:cd04986    96 I 96

N-terminal immunoglobulin (Ig)-like domain of T-cell surface antigen CD2, and similar domains; The members here are composed of the N-terminal immunoglobulin (Ig)-like domain (or domain 1) of T-cell surface antigen Clusters of Differentiation (CD) 2 and similar proteins. CD2 is a T-cell specific surface glycoprotein and is critically important for mediating adhesion between T cells and antigen-presenting cells or between cytolytic T cells and target cells. CD2 is located on chromosome 1 at 1p13 in humans and on chromosome 3 in mice. CD2 contains an extracellular domain with two or Ig-like domains, a single transmembrane segment, and a cytoplasmic region rich in proline and basic residues.

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                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1046856867  26 CSMVLNGILGESVTLPVDiKDGQKINNIAW--TSKSSVAFVT---PKVGGHeltvtqdiYKGRLEVTNQTYNLIIRNLRM 100
Cdd:cd05775     1 SSGEVYGALGGNVTLTIS-SLQDDIDEIKWkkTKDKIVEWENnigPTYFGS--------FKDRVLLDKESGSLTIKNLTK 71

                          90       100
                  ....*....|....*....|...
gi 1046856867 101 KDAGTYKADI---NEKNREKTIT 120
Cdd:cd05775    72 EDSGTYELEItstNGKVLSSKFT 94

First immunoglobulin variable (IgV) domain of poliovirus receptor (PVR, also known as CD155 and necl-5), and similar domains; The members here are composed of the first immunoglobulin (Ig) domain of poliovirus receptor (PVR, also known as CD155 and nectin-like protein 5 (necl-5)). Poliovirus (PV) binds to its cellular receptor (PVR/CD155) to initiate infection. CD155 is a membrane-anchored, single-span glycoprotein; its extracellular region has three Ig-like domains. There are four different isotypes of CD155 (referred to as alpha, beta, gamma, and delta), that result from alternate splicing of the CD155 mRNA, and have identical extracellular domains. CD155-beta and CD155-gamma are secreted; CD155-alpha and CD155-delta are membrane-bound and function as PV receptors. The virus recognition site is contained in the amino-terminal domain, D1. Having the virus attachment site on the receptor distal from the plasma membrane may be important for successful initiation of infection of cells by the virus. CD155 binds in the poliovirus "canyon" with a footprint similar to that of the intercellular adhesion molecule-1 receptor on human rhinoviruses. This group also includes the first Ig-like domain of nectin-1 (also known as poliovirus receptor related protein(PVRL)1; CD111), nectin-3 (also known as PVRL 3), nectin-4 (also known as PVRL4; LNIR receptor)and DNAX accessory molecule 1 (DNAM-1; CD226).

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                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1046856867  32 GILGESVTLPVDIKD--GQKINNIAW-----TSKSSVAFVTPKVGGHeltvTQDIYKGRLEVTNQTYN-----LIIRNLR 99
Cdd:cd05718    11 GFLGGSVTLPCSLTSpgTTKITQVTWmkigaGSSQNVAVFHPQYGPS----VPNPYAERVEFLAARLGlrnatLRIRNLR 86

                  ....*..
gi 1046856867 100 MKDAGTY 106
Cdd:cd05718    87 VEDEGNY 93

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

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                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1046856867  27 SMVLNGILGESVTLPVDIKD--GQKINNIAW----TSKSSVAFVTPKVGGHELTVTQDIYKGRLEVTNQTYNLIIRNLRM 100
Cdd:pfam07686   3 PREVTVALGGSVTLPCTYSSsmSEASTSVYWyrqpPGKGPTFLIAYYSNGSEEGVKKGRFSGRGDPSNGDGSLTIQNLTL 82

                          90       100
                  ....*....|....*....|....*
gi 1046856867 101 KDAGTYKADI--NEKNREKTITKIY 123
Cdd:pfam07686  83 SDSGTYTCAVipSGEGVFGKGTRLT 107

First immunoglobulin (Ig) domain of Cluster of Differentiation (CD) 4; member of the V-set of IgSF domains; The members here are composed of the first immunoglobulin (Ig) domain of Cluster of Differentiation (CD) 4. CD4 and CD8 are the two primary co-receptor proteins found on the surface of T cells, and the presence of either CD4 or CD8 determines the function of the T cell. CD4 is found on helper T cells, where it is required for the binding of MHC (major histocompatibility complex) class II molecules, while CD8 is found on cytotoxic T cells, where it is required for the binding of MHC class I molecules. CD4 contains four immunoglobulin domains, with the first three included in this hierarchy. The fourth domain has a general Ig architecture, but has slight topological changes in the arrangement of beta strands relative to the other structures in this family and is not specifically included in the hierarchy.

                          10        20
                  ....*....|....*....|....*..
gi 1046856867  90 TYNLIIRNLRMKDAGTYKADINEKNRE 116
Cdd:cd07690    66 SFPLIIKNLKIEDSDTYICEVEDKKEE 92