CAVEOLIN AND MAL, 2 PROTEIN-COMPONENTS OF INTERNAL DETERGENT-INSOLUBLE MEMBRANES, ARE IN DISTINCT LIPID MICROENVIRONMENTS IN MDCK CELLS

The MAL proteolipid and caveolin have been identified as components of internal detergent-insoluble membrane microdomains enriched in glycol ipids and cholesterol. We have addressed the study of the glycolipid-e nriched membranes in cells expressing endogenously only either MAL (Ju rkat T cells) or caveolin (epithelial A498 cells) and in polarized MDC K cells which express both proteins simultaneously. Subcellular fracti onation by centrifugation to equilibrium in sucrose gradients of Trito n X-100 cell extracts from Jurkat and A498 cells revealed that MAL and caveolin are incorporated in detergent-insoluble buoyant membranes in dependently of the expression of each other and indicated the existenc e in these cells of insoluble membrane microdomains with either MAL or caveolin. Immunofluorescence analysis in MDCK cells indicated that bo th MAL and caveolin were located in the Golgi region, whereas caveolin was found in addition at the cell surface. Biochemical analysis in th ese cells revealed the existence of distinct membrane microenvironment s differentially susceptible to detergent solubilization containing ei ther internal MAL or internal plus surface caveolin. The observed hete rogeneity within the internal glycolipid-enriched membrane fraction su ggests the existence of distinct specialized lipid microenvironments i n MDCK cells. (C) 1997 Academic Press.