PIG-S is one of several key, core, components of the glycosylphosphatidylinositol (GPI) trans-amidase complex that mediates GPI anchoring in the endoplasmic reticulum. Anchoring occurs when a protein's C-terminal GPI attachment signal peptide is replaced with a pre-assembled GPI [1]. Mammalian GPITransamidase consists of at least five components: Gaa1, Gpi8, PIG-S, PIG-T, and PIG-U, all five of which are required for function. It is possible that Gaa1, Gpi8, PIG-S, and PIG-T form a tightly associated core that is only weakly associated with PIG-U. The exact function of PIG-S is unclear [2].

1: PIG-S and PIG-T, essential for GPI anchor attachment to proteins, form a complex with GAA1 and GPI8. Ohishi K, Inoue N, Kinoshita T; EMBO J. 2001;20:4088-4098. PMID:11483512

2: A conserved proline in the last transmembrane segment of Gaa1 is required for glycosylphosphatidylinositol (GPI) recognition by GPI transamidase. Vainauskas S, Menon AK; J Biol Chem. 2004;279:6540-6545. PMID:14660601