Myelin glycosphingolipid/cholesterol-enriched microdomains selectively sequester the non-compact myelin proteins CNP and MOG

Plasma membranes are complex arrays of protein and lipid subdomains. Deterg ent-insoluble, glycosphingolipid/cholesterol-enriched micro-domains (DIGCEM s) have been implicated in protein sorting and/or as sites for signaling ca scades in the plasma membrane. We previously identified the presence of DIG CEMs in oligodendrocytes in culture and purified myelin and characterized a novel DIGCEM-associated tetraspan protein, MVP17/rMAL (Kim et al. (1995) J ournal of Neuroscience Research 42, 413-422). We have now analyzed the asso ciation of known myelin proteins with DIGCEMs in order to provide a better understanding of their roles during myelin biogenesis. We used four well-es tablished criteria to identify myelin DIGCEM-associated proteins: insolubil ity in a non-ionic detergent Triton X-100 at low temperature (4 degrees C), flotation of the insoluble complexes to low density fractions in sucrose g radients, and TX-100 solubilization at 37 degrees C, or at 4 degrees C foll owing treatment with the cholesterol-binding detergent saponin. We demonstr ate that these proteins fall into four distinct groups. Although all tested proteins could be floated to a low-density fraction, proteolipid protein ( PLP), myelin basic protein (MBP) and myelin associated glycoprotein (MAG) w ere solubilized by the detergent extraction, and connexin32 (Cx32) and olig odendrocyte-specific protein (OSP) met only some of the criteria for DIGCEM s. Only the non-compact myelin proteins 2',3'-cyclic nucleotide 3'-phosphod iesterase (CNP) and myelin/oligodendrocyte glycoprotein (MOG) satisfied all four criteria for DIGCEM-associated proteins. Significantly, only similar to 40% of CNP and MOG were selectively associated with DIGCEMs. This sugges ts that they may have both non-active "soluble", and functionally active DI GCEM-associated, forms in the membrane, consistent with current views that DIGCEMs provide platforms for bringing together and activating components o f the signal transduction apparatus. We therefore propose that CNP and MOG may have unique roles among the major myelin proteins in signaling pathways mediated by lipid-protein microdomains formed in myelin.