In the seventh reaction of GPI synthesis, a second mannose residue is added to the glycolipid on the lumenal face of the endoplasmic reticulum membrane. PIG-V was identified as the catylyst, or a component of the catalyst, of this reaction, on the basis of its ability to correct the metabolic defects of yeast and mammalian mutant cells arrested at this stage of the GPI synthetic process (Fabre et al. 2005; Kang et al. 2005).
glycolipid mannosyltransferase activity of PIGV [endoplasmic reticulum membrane]