PIG-S and PIG-T, essential for GPI anchor attachment to proteins, form a complex with GAA1 and GPI8

Many eukaryotic cell surface proteins are anchored to the plasma membrane v ia glycosylphosphatidylinositol (GPI). The GPI transamidase mediates GPI an choring in the endoplasmic reticulum, by replacing a protein's C-terminal G PI attachment signal peptide with a preassembled GPI. During this transamid ation reaction, the GPI transamidase forms a carbonyl intermediate with a s ubstrate protein. It was known that the GPI transamidase is a complex conta ining GAA1 and GP18. Here, we report two new components of this enzyme: PIG -S and PIG-T. To determine roles for PIG-S and PIG-T, we disrupted these ge nes in mouse F9 cells by homologous recombination. PIG-S and PIG-T knockout cells were defective in transfer of GPI to proteins, particularly in forma tion of the carbonyl intermediates. We also demonstrate that PIG-S and PIG- T form a protein complex with GAA1 and GP18, and that PIG-T maintains the c omplex by stabilizing the expression of GAA1 and GP18. Saccharomyces cerevi siae Gpi16p (YHR188C) and Gpi17p (YDR434W) are orthologues of PIG-T and PIG -S, respectively.