GPI-anchored proteins associate to form microdomains during their intracellular transport in Caco-2 cells

In this study, we have investigated the possibility that glycosyl-phosphatidylinositol (GPI)-anchored proteins form insoluble membrane complexes in Caco-2 cells and that transmembrane proteins are associated with these complexes. GPI-anchored proteins were mainly resistant to Triton X-100 (TX-100) extraction at 4 degrees C but fully soluble in n-octyl-glucoside. Resistance to Triton X-100 extraction was not observed in the endoplasmic reticulum but appeared during transport through the Golgi complex. It was not dependent upon N-glycosylation processing, or pH variation from 6.5 to 8.5, and was not affected by sterol-binding agents. Other apical or basolateral transmembrane proteins were well solubilized in TX-100, with the exception of sucrase-isomaltase, which was partly insoluble. We isolated a membrane fraction from Caco-2 cells that contained GPI-anchored proteins and sucrase-isomaltase but no antigen 525, a basolateral marker, or dipeptidylpeptidase IV, an apical one. These data suggest that GPI-anchored proteins cluster to form membrane microdomains together with an apical transmembrane protein, providing a possible apical sorting mechanism for intestinal cells in vitro that might be related to apical sorting in MDCK cells, and that other mechanisms might exist to sort proteins to the apical membrane.