RMAL IS A GLYCOSPHINGOLIPID-ASSOCIATED PROTEIN OF MYELIN AND APICAL MEMBRANES OF EPITHELIAL-CELLS IN KIDNEY AND STOMACH

rMAL, the rat myelin and lymphocyte protein, is a small hydrophobic pr otein of 17 kDa with four putative transmembrane domains and is expres sed in oligodendrocytes and Schwann cells, the myelinating cells of th e nervous system. In addition, transcript expression has been found in kidney, spleen, and intestine. Confocal microscopy and immunoelectron microscopy with an affinity-purified antibody localized rMAL to compa ct myelin in a pattern similar to the structural myelin proteins: myel in basic protein and proteolipid protein. In kidney and stomach epithe lia, rMAL is located almost exclusively on the apical (luminal) membra nes of the cells lining distal tubuli in kidney and the glandular part of the stomach. Biochemical analysis of plasma membranes isolated fro m spinal cord and kidney demonstrated that rMAL is a proteolipid that is present in detergent insoluble complexes typical for proteins assoc iated with glycosphingolipids. Lipid and protein analysis showed a co- enrichment of glycosphingolipids and rMAL protein within these complex es, indicating a close association of rMAL to glycosphingolipids in my elin and in kidney in vivo. We conclude that specific rMAL-glycosphing olipid interactions may lead to the formation and maintenance of stabl e protein-lipid microdomains in myelin and apical epithelial membranes . They may contribute to specific properties of these highly specializ ed plasma membranes.