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 protein of 17 kDa with four putative transmembrane domains and is expressed in oligodendrocytes and Schwann cells, the myelinating cells of the 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 compact myelin in a pattern similar to the structural myelin proteins: myelin basic protein and proteolipid protein. In kidney and stomach epithelia, rMAL is located almost exclusively on the apical (luminal) membranes of the cells lining distal tubuli in kidney and the glandular part of the stomach. Biochemical analysis of plasma membranes isolated from spinal cord and kidney demonstrated that rMAL is a proteolipid that is present in detergent insoluble complexes typical for proteins associated with glycosphingolipids. Lipid and protein analysis showed a co-enrichment of glycosphingolipids and rMAL protein within these complexes, indicating a close association of rMAL to glycosphingolipids in myelin and in kidney in vivo. We conclude that specific rMAL-glycosphingolipid interactions may lead to the formation and maintenance of stable protein-lipid microdomains in myelin and apical epithelial membranes. They may contribute to specific properties of these highly specialized plasma membranes.