GPI anchoring leads to sphingolipid-dependent retention of endocytosed proteins in the recycling endosomal compartment

Glycosylphosphatidylinositol (GPI) anchoring is important for the function of several proteins in the context of their membrane trafficking pathways. We have shown previously that endocytosed GPI-anchored proteins (GPI-APs) a re recycled to the plasma membrane three times more slowly than other membr ane components. Recently, we found that GPI-APs are delivered to endocytic organelles, devoid of markers of the clathrin-mediated pathway, prior to th eir delivery to a common recycling endosomal compartment (REC). Here we sho w that the rate-limiting step in the recycling of GPI-APs is their slow exi t from the REC; replacement of the GPI anchor with a transmembrane protein sequence abolishes retention in this compartment. Depletion of endogenous s phingolipid levels using sphingolipid synthesis inhibitors or in a sphingol ipid-synthesis mutant cell line specifically enhances the rate of endocytic recycling of GPI-APs to that of other membrane components. We have shown p reviously that endocytic retention of GPI-APs is also relieved by cholester ol depletion. These findings strongly suggest that functional retention of GPI-APs in the REC occurs via their association with sphingolipid and chole sterol-enriched sorting platforms or 'rafts'.