Nuclear envelope localization of PIG-B is essential for GPI-anchor synthesis in Drosophila

Abstract

Membrane lipid biosynthesis is a complex process that takes place in various intracellular compartments. Glycosylphosphatidylinositol (GPI), a lipid involved inmembrane anchoring of some proteins, is synthesized by the PIG enzymes. Most PIGs are localized to the endoplasmic reticulum (ER), but Drosophila PIG-B (DmPIG-B) is localized to the nuclear envelope (NE). To determine whether the NE localization of DmPIG-B is functionally important, we defined the determinants of localization and generated an ER-localized form, denoted DmPIGB[ ER]. The enzymatic activity of DmPIG-B[ER] was comparable to that of NE-localized DmPIG-B[NE]. Expression of DmPIG-B[ER] inefficiently rescued the lethality of the PIG-B mutant, whereas DmPIG-B[NE] rescued this lethality fully. DmPIG-B[ER] was preferentially degraded by lysosomes, suggesting that the NE localization is essential for function and stability of the protein. In addition,we found that the region of the ERproximal to theNE is the site of translation of GPI-anchored proteins and addition of GPI. Thus, the NE and proximal ERmay provide a platformfor efficientGPI anchoring.