PGAP1 (Bst1 in yeast) functions as a GPI inositol-deacylase; this deacylation is important for the efficient transport of GPI-anchored proteins from the endoplasmic reticulum to the Golgi body []. Mutations of the PGAP1 gene cause mental retardation, autosomal recessive 42 (MRT42) [].
The sequences found in this family are similar to PGAP1 (). This is an endoplasmic reticulum membrane protein with a catalytic serine-containing motif that is conserved in a number of lipases. PGAP1 functions as a GPI inositol-deacylase; this deacylation is important not only for the efficient transport of GPI-anchored proteins from the endoplasmic reticulum to the Golgi body but also for efficient degradation of misfolded GPI-anchored proteins in the ER [, ].
PGAP2, also known as FRAG1, is involved in the lipid remodeling steps of glycosylphosphatidylinositol (GPI)-anchor maturation []. Immediately after attachment of GPI to newly synthesized proteins, inositol-linked palmitate is removed by deacylase PGAP1 in the ER. The inositol-deacylation is essential for subsequent steps in fatty acid remodeling in the Golgi. PGAP3 removes an unsaturated fatty chain at sn-2 and PGAP2 reacylation of sn-2 with stearic acid follows [].
Metallophosphoesterase 1 (MPPE1), also known as PGAP5, acts as a phosphodiesterase removing an ethanolamine phosphate (EtN-P) from mannose 2 of the glycosylphosphatidylinositol (GPI) anchor, thus permitting efficient endoplasmic reticulum (ER) to Golgi transport of GPI proteins[]. Before GPI-anchored proteins exist the ER, two remodeling reaction, inositol deacylation by PGAP1 and removal of the second EtNP by PGAP5, occur in mammalian cells. The MPPE1 gene has been identified as a candidate susceptibility gene for Bipolar disorder [].MPPE1 belongs to the metallophosphatase (MPP) superfamily. MPPs are functionally diverse, but all share a conserved domain consisting of a double β-sheet sandwich with a di-metal active site made up of residues located at the C-terminal side of the sheets. This domain is thought to allow for productive metal coordination.