Myelin membrane protein biosynthesis : an in vitro study
The sites of biosynthesis and incorporation of the abundant CNS myelin proteins 2' , 3' -cyclic nucleotide-3'-phosphodiesterase (CNPase) and P2 protein into the growing myelin membrane were investigated. Cell-free translation systems programmed with mRNA from rat brain, rabbit spinal cord, free and bound polysomes and purified myelin demonstrated conclusively that both CNPase and P2 are synthesized on free polysomes like the myelin basic proteins (MBPs) but unlike the proteolipid protein (PLP), the major intrinsic membrane protein of CNS myelin, which is known to be synthesized at the oligodendrocyte endoplasmic reticulum on bound polysomes (Colman et al., 1982) . These observations were supported by labelling studies on rats in vivo during the period of maximal myelin deposition. Newly synthesized CNPase associated with the myelin membrane very rapidly after labelling (~2 minutes) and this is consistent with the view that there is only a brief delay between synthesis and incorporation into their target membrane for extrinsic-type plasma membrane proteins. An RNA fraction isolated from purified CNS myelin was not enriched in mRNAs coding for CNPase and P2 but a considerable enrichment of mRNAs coding for MBPs was observed. This phenomenon has important implications for the cell biology of myelination since it suggests that although MBPs, CNPase and P2 are all basic extrinsic membrane proteins, and synthesized on free polysomes, different mechanisms for their transport to the myelin membrane exist. The addition of dog pancreatic microsomes (DPM) during translation showed no membrane association for CNPase however, at least 50% of MBPs were observed to non-specifically associate with these membranes. When newly synthesized MBP and P2 were incubated post-translationally with DPM or rabbit spinal cord myelin P2 only associated with myelin whereas MBP showed an equal affinity for both types of membranes. The segregation of MBP free polysomes at the myelin membrane during synthesis ensures that the nascent MBP polypeptides associate with the correct membrane. Recent evidence has shown that the free polysome-mRNA complex is bound to the cytoskeleton during protein synthesis. After extensive characterization of the purified rat brain oligodendrocyte and myelin-associated cytoskeletons it was shown that the synthesis of MBPs and CNPase only occurs from mRNA that is associated with the cytoskeleton and not when it is part of the cytoplasmic mRNA pool. Lipid analysis of the purified rat brain myelin-associated cytoskeleton revealed the presence of tightly bound lipid with a considerable enrichment of cerebroside and sphingomyelin (the latter at the expense of phosphatidylethanolamine). These studies on the cytoskeletal involvement in myelinogenesis suggest that extrinsic CNS myelin proteins are synthesized on the cytoskeleton and that post-translational cytoskeletal transport of these proteins to the growing myelin membrane may take place.