Myelin proteolipid DM20: evidence for function independent of myelination

DM20 is a proteolipid protein that has been extensively studied for its role in central nervous system myelination. We demonstrate that DM20 expression is widespread and independent of myelination. In the Schwann cells and neurons of the peripheral nervous system, DM20 is not incorporated into the membrane as it is in the central nervous system (CNS), but remains cytoplasmic. Mutations that severely reduce the amount of DM20 mRNA in CNS myelinating cells have little effect on DM20 expression in nonmyelinating cells of the peripheral nervous system and embryonic CNS. Most importantly, the combination of wild-type DM20 from the endogenous X-linked gene and mutant DM20 expressed from an autosomal transgene results in embryonic lethality. We propose a function for DM20 to explain these diverse findings based on the ability of DM20 to form multimeric complexes, and hypothesize that the DM20 complex participates in intracellular molecular transport.