TABLE 12.1. [Components of the core mammalian...]. - Essentials of Glycobiology

Warning: The NCBI web site requires JavaScript to function. more...

TABLE 12.1.

Components of the core mammalian and Saccharomyces cerevisiae glycosylphosphatidylinositol (GPI)-biosynthetic machinery

Step 1. UDP-GlcNAc:PI α1-6 N-acetylglucosaminyltransferase (GlcNAc-T) transfers GlcNAc from UDP-GlcNAc to PI to produce GlcNAc-PI

Components

PIG-A^a^,^b (Gpi3), 484 (452) AA, 1 (1) TM, catalytic subunit

PIG-H^b (Gpi15), 188 (229) AA, 2 (2) TM, associated with PIG-A

PIG-C^b (Gpi2), 292 (280) AA, 8 (6) TM, associated with PIG-Q (with Gpi1)

PIG-Q^b (Gpi1), 581 (609) AA, 6 (6) TM, associated with PIG-C

PIG-P^b (Gpi19), 134 (140) AA, 2 (2) TM, associated with PIG-A, PIG-Q (with Gpi2)

DPM2 (Yil102c-A), 84 (75) AA, 2 (2) TM, subunit of Dol-P-Man synthase

PIG-Y^b (Eri1), 71 (68) AA, 2 (2) TM, associated with PIG-A

Note: PIG-A/PIG-H complex is associated with the PIG-C/PIG-Q complex. Active site faces the cytoplasm. In yeast, but not in mammals, Ras physically associates with GlcNAc-T. Ras2 inhibits GlcNAc-T in S. cerevisiae. Ras1 activates GlcNAc-T in C. albicans.

Step 2. GlcNAc-PI de-N-acetylase de-N-acetylates GlcNAc-PI to GlcN-PI

Components

PIG-L^b (Gpi12), 252 (304) AA, 1 (1) TM

Note: The active site of PIG-L (Gpi12) faces the cytoplasm. It is generally stimulated by divalent cations but species-specific differences in metal ion preferences seem to exist.

Step 3. Inositol acyltransferase (inositol acyl-T) acylates C2 hydroxyl of myo-inositol, using acyl-CoA as donor, to produce GlcN-aPI

Components

PIG-W^b (Gwt1), 504 (490) AA, 13 (12) TM

Note: The active site of PIG-W (Gwt1) faces the ER lumen, which requires the GPI intermediate to be flipped inwards from the cytoplasmic face. A flippase for this process has not yet been identified. In mammals, the first stage of lipid remodeling to replace the diacyl form with the diradyl form also occurs at this step. The mechanistic details of the process are unclear.

Step 4. Mannosyltransferase-1 (MT-1) transfers Man-1 from Dol-P-Man to produce Man-GlcN-aPI

Components

PIG-M^b (Gpi14), 423 (403) AA, 8 (8) TM, catalytic subunit

PIG-X (Pbn1), 252 (416) AA, 1 (1) TM, associated with PIG-M

Note: The active site of PIG-M (Gpi14) faces the ER lumen. A conserved DXD motif is critical for activity. PIG-M and PIG-X must be co-expressed to rescue a gpi14 mutant strain. Yeast Arv1 is probably involved in flipping of the GPI intermediate and/or its delivery to MT-1. Human ARV1^b (271 AA, 3 TM) complements yeast Arv1 (321 AA, 3 TM); its endogenous role in the GPI pathway is unknown.

Step 5. Mannosyltransferase-2 (MT-2) transfers Man-2 from Dol-P-Man to produce Man-Man-GlcN-aPI

Components

PIG-V^b (Gpi18 with Pga1), 493 (433, 198) AA, 8 (8+2) TM

Note: The active site of PIG-V (Gpi18) faces the ER lumen. PIG-V complements both Gpi18 and Pga1.

Step 6. Ethanolamine phosphate transferase-1 (EtNPT-1) transfers EtNP-1 from phosphatidylethanolamine (PE) to produce Man-(EtNP)Man-GlcN-aPI

Components

PIG-N^b (Mcd4), 931 (919) AA, 15 (14) TM

Note: The active site of PIG-N (Mcd4) faces the ER lumen. PIG-N, PIG-O and PIG-G possess a conserved phosphatase motif.

Step 7. Mannosyltransferase-3 (MT-3) transfers Man-3 from Dol-P-Man to produce Man-Man-(EtNP)Man-GlcN-aPI

Components

PIG-B^a^,^b (Gpi10), 554 (616) AA, 9 (9) TM

Note: The active site of PIG-B faces the ER lumen. Dol-P-Man is the donor of Man. MT-3 in some protozoan parasites may also add Man-4.

Step 8. Ethanolamine phosphate transferase -3 (EtNPT-3) transfers EtNP-3 from PE to produce (EtNP)Man-Man-(EtNP)Man-GlcN-aPI

Components

PIG-O^b (Gpi13), 880 (1017) AA, 14 (13) TM, catalytic subunit

PIG-F^b (Gpi11), 219 (219) AA, 6 (4) TM, associated with PIG-O

Note: The active site of PIG-O faces the ER lumen. Gpi11 is not a major participant at this step in yeast.

Step 9. Ethanolamine phosphate transferase-2 (EtNPT-2) transfers EtNP-2 from PE to produce (EtNP)Man-(EtNP)Man-(EtNP)Man-GlcN-aPI

Components

PIG-G^b (Gpi7), 983 (830) AA, 13 (9) TM, catalytic subunit

PIG-F^b (Gpi11), 219 (219) AA, 6 (4) TM, associated with PIG-G

Note: The active site of PIG-G faces the ER lumen. In yeast, Gpi11 is not critical for Gpi7 activity, but prior addition of Man-4 by MT-4 (Smp3, 516 AA, 7 TM) is crucial. In humans, the action of a MT-4, PIG-Z (579 AA, 4 TM), is tissue-specific.

Step 10. GPI transamidase transfers the complete GPI precursor to proteins

Components

GPAA1^b (Gaa1), 621 (614) AA, 7 (6) TM

PIG-K^b (Gpi8), 367 (411) AA, 1 (1) TM, catalytic subunit

PIG-T^a^,^b (Gpi16), 557 (610) AA, 1 (1) TM, disulfide bonded to PIG-K

PIG-S^b (Gpi17), 555 (534) AA, 2 (2) TM

PIG-U^b (Gab1), 435 (394) AA, 8 (8) TM

Note: PIG-K/Gpi8, a cysteine protease-like endopeptidase, cleaves GPIsp. GPAA1/Gaa1, a metallo-protease/synthase, is predicted to catalyze amide bond formation between the GPI anchor and protein. Active sites of both subunits face the ER lumen. Gpi8 is not disulfide linked to Gpi16 in yeast.

Step 11. Inositol deacylase deacylates from inositol of GPI-APs

Components

PGAP1^b (Bst1), 922 (1029) AA, 6 (8) TM

Note: The active site of PGAP1 faces the ER lumen. A conserved Ser in the active site is essential for activity.

Step 12. Ethanolamine phosphoesterase-1 removes EtNP from Man-2 of GPI-APs

Components

PGAP5 (Ted1), 396 (473) AA, 2 (1) TM

Note: The active site of PGAP5 faces the ER lumen. It is a Mn⁺⁺-dependent phosphoesterase. In yeast, Cdc1 (491 AA, 3 TM), a paralog of Ted1, is the ethanolamine phosphoesterase-2 that removes EtNP from Man-1 and enables cell wall localization of the GPI-AP. Recent reports suggest that Cdc1 localizes to the Golgi (see Figure 12.2).

Step 13. ER to Golgi transport cargo receptor transports GPI-APs to the Golgi after packaging them into COPII-coated vesicles

Components

TMED9 (Erp1), 235 (219) AA, 1 (1) TM

TMED2 (Emp24), 201 (203) AA, 1 (1) TM

TMED5 (Erp2), 229 (215) AA, 1 (1) TM, GPI-binding subunit

TMED10 (Erv25), 219 (211) AA, 1 (1) TM

Step14. Fatty-acid remodeling step 1 involves removal of the sn-2 fatty acid to generate a lyso-PI in the Golgi

Components

PGAP3^b (Per1), 320 (357) AA, 7 (6) TM, GPI-specific phospholipase A2

Note: In yeast, this reaction is mediated by Per1 in the ER lumen.

Step15. Fatty-acid remodeling step 2 involves reacylation at sn-2 with a saturated fatty acid

Components

PGAP2^b (Gup1), 254 (560) AA, 5 (10) TM.

Note: PGAP2 is a Golgi protein. Gup1 is an ER membrane-bound acyltransferase and not a structural homolog of PGAP2. The diacylglycerol on many GPI-APs of S. cerevisiae, may ultimately be replaced with ceramide in the ER by Cwh43 (Fatty-acid remodeling step 3; 953 AA, multiple TM) before Step 13 (above).

: The step numbers refer to Figure 12.2. The size of the predicted protein in amino acids (AA) and the number of predicted transmembrane (TM) domains are given alongside the protein names. The names and numbers in brackets correspond to the yeast homologs. The numbers of transmembrane domains in proteins with more than five transmembrane domains are predicted from their amino acid sequences rather than being experimentally determined in most cases.
: ER, endoplasmic reticulum.
a: Mutations in this human gene may produce paroxysmal nocturnal hemoglobinuria (PNH) (Chapter 46).
b: Mutations in this human gene cause inherited GPI deficiencies (IGDs) (Chapter 45).

From: Chapter 12, Glycosylphosphatidylinositol Anchors

Essentials of Glycobiology [Internet]. 4th edition.

Varki A, Cummings RD, Esko JD, et al., editors.

Cold Spring Harbor (NY): Cold Spring Harbor Laboratory Press; 2022.

The content of this book is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 Unported license. To view the terms and conditions of this license, visit https://creativecommons.org/licenses/by-nc-nd/4.0/

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.