CONSERVATION OF TOPOLOGY, BUT NOT CONFORMATION, OF THE PROTEOLIPID PROTEINS OF THE MYELIN SHEATH

The proteolipid protein gene products DM-20 and PLP are adhesive intri nsic membrane proteins that make up greater than or equal to 50% of th e protein in myelin and serve to stabilize compact myelin sheaths at t he extracellular surfaces of apposed membrane lamellae. To identify wh ich domains of DM-20 and PLP are positioned topologically in the extra cellular space to participate in adhesion, we engineered N-glycosylati on consensus sites into the hydrophilic segments and determined the ex tent of glycosylation. In addition, we assessed the presence of two tr anslocation stop-transfer signals and, finally, mapped the extracellul ar and cytoplasmic dispositions of four antibody epitopes. We find tha t the topologies of DM-20 and PLP are identical, with both proteins po ssessing four transmembrane domains and N and C termini exposed to the cytoplasm. Consistent with this notion, DM-20 and PLP contain within their N- and C-terminal halves independent stop-transfer signals for i nsertion into the bilayer of the rough endoplasmic reticulum during de novo synthesis. Surprisingly, the conformation (as opposed to topolog y) of DM-20 and PLP may differ, which has been inferred from the diver gent effects that many missense mutations have on the intracellular tr afficking of these two isoforms. The 35 amino acid cytoplasmic peptide in PLP, which distinguishes this protein from DM-20, imparts a sensit ivity to mutations in extracellular domains, This peptide may normally function during myelinogenesis to detect conformational changes origi nating across the bilayer from extracellular PLP interactions in trans and trigger intracellular events such as membrane compaction in the c ytoplasmic compartment.