The initial events in myelin synthesis: orientation of proteolipid protein in the plasma membrane of cultured oligodendrocytes

Proteolipid protein (PLP) is the most abundant transmembrane protein in myelin of the central nervous system. Conflicting models of PLP topology have been generated by computer predictions based on its primary sequence and experiments with purified myelin. We have examined the initial events in myelin synthesis, including the insertion and orientation of PLP in the plasma membrane, in rat oligodendrocytes which express PLP and the other myelin-specific proteins when cultured without neurons (Dubois-Dalcq, M., T. Behar, L. Hudson, and R. A. Lazzarini. 1986. J. Cell Biol. 102:384-392). These cells, identified by the presence of surface galactocerebroside, the major myelin glycolipid, were stained with six anti-peptide antibodies directed against hydrophilic or short hydrophobic sequences of PLP. Five of these anti-peptide antibodies specifically stained living oligodendrocytes. Staining was only seen approximately 10 d after PLP was first detected in the cytoplasm of fixed and permeabilized cells, suggesting that PLP is slowly transported from the RER to the cell surface. The presence of PLP domains on the extracellular surface was also confirmed by cleavage of such domains with proteases and by antibody-dependent complement-mediated lysis of living oligodendrocytes. Our results indicate that PLP has only two transmembrane domains and that the great majority of the protein, including its amino and carboxy termini, is located on the extracellular face of the oligodendrocyte plasma membrane. This disposition of the PLP molecule suggests that homophilic interactions between PLP molecules of apposed extracellular faces may mediate compaction of adjacent bilayers in the myelin sheath.