GLYCOSPHINGOLIPID-ENRICHED, DETERGENT-INSOLUBLE COMPLEXES IN PROTEIN SORTING IN EPITHELIAL-CELLS

In simple epithelial cells, the delivery of apical and basolateral pro teins to the cell surface is mediated by sorting in the trans-Golgi ne twork and transport via separate vesicular carriers. In order to ident ify the molecular machinery involved in protein sorting, we have recen tly studied a detergent-insoluble complex in Madin-Darby canine kidney (MDCK) cells, following CHAPS extraction of exocytic carrier vesicles , specifically including the apical marker protein influenza hemagglut inin (HA). Previously, a Triton X-100 insoluble membrane residue that was enriched in glycosylphosphatidylinositol-anchored (GPI) proteins a nd glycolipids was characterized and implicated in transport to the ap ical cell surface [Brown, D., & Rose, J. (1991) Cell 68, 533-544]. In this report, the protein compositions of the CHAPS and Triton complexe s have been compared by two-dimensional gel analysis. Only a few major membrane proteins are found in the complexes. The protein composition s are qualitatively similar, but differ quantitatively in the individu al components. The CHAPS complex is depleted of GPI-linked proteins an d retains a minor fraction of lipids similar in composition to that of the Triton X-100 insoluble complex. We propose that in vivo the compl exes form part of a sorting platform that mediates protein segregation and delivery to the apical cell surface.