Phosphoethanolamine extracted from PE is attached at different sugars that make up the glycan core , and in the final step, preformed GPI is attached via a phosphoethanolamine linker (extracted from PE) to the C-terminus of the target protein by a multi-subunit GPI transamidase .
PIGT codes for a subunit of the transamidase enzyme complex that links the GPI anchor to the target protein.
The Riezman group reported yeast GAA1 (for GPI Anchor Attachment 1), the first component of GPI transamidase to be identified.
The Conzelmann group identified yeast GPI8 as a catalytic component of GPI transamidase. (51) The Medof group identified a human homolog of Gpi8p, termed PIG-K.
Ohishi isolated the GPI transamidase complex and identified two more components.
Ohishi cloned the PIGT gene based on the N-terminal sequence of the other component, which he found in the isolated GPI transamidase. (53) PIG-T is a 578-amino-acid protein with one transmembrane domain near the C-terminus.
(55) PIG-U protein was identified as the fifth component of the isolated GPI transamidase. PIG-U is a 435-amino-acid protein with multiple transmembrane domains.
In the ER of the PIGV-defective cells, the C-terminal GPI attachment signal peptide of a nascent protein, such as alkaline phosphatase, is cleaved by GPI transamidase and the major part of the protein is either secreted without GPI-anchoring or degraded by ER-associated degradation.
The removal of the C-terminal GPI signal sequence and its replacement with GPI on the lumen of rER are catalyzed by the GPI transamidase (GPIT), which is a complex consisting of the membrane proteins PIG-K/Gpi8, GAA-1, PIG-S/Gpi-17, PIGT/Gpi16, and PIG-U/Cdc91 [125,126].
Kang et al., "Human PIG-U and yeast Cdc91p are the fifth subunit of GPI transamidase that attaches GPI-anchors to proteins," Molecular Biology of the Cell, vol.
(15) have reported patients who were seropositive for Chagas disease only by inhibition transamidase
assay but negative with conventional serologic testing.
Thrombin also converts factor XIII into a transamidase
that forms covalent bonds within the loose clot and stabilizes it.