A PROTEOLIPID PROTEIN GENE FAMILY - EXPRESSION IN SHARKS AND RAYS ANDPOSSIBLE EVOLUTION FROM AN ANCESTRAL GENE ENCODING A PORE-FORMING POLYPEPTIDE

The myelin proteolipid proteins (PLP and DM20) are believed to act as ''adhesive struts'' in the extracellular apposition of the CNS myelin sheath. These proteins have been considered late evolutionary developm ents, which arose de novo in the antecedents of early tetrapods. Howev er, PCR primed with degenerate oligonucleotides corresponding to commo n segments of rat PLP/DM20 revealed three novel mRNAs in the brains of two elasmobranchs. These mRNAs are closely related to each other and to mammalian DM20, but lack the sequence that distinguishes PLP from D M20. We term the novel proteolipid proteins DM(alpha), DM(beta), and D M(gamma). At least DM(alpha) and DM(gamma) are highly expressed in whi te matter in myelinating shark brain. The DMs not only are highly homo logous to each other, but also contain regions bearing similarities wi th segments of channel-forming regions of the nicotinic acetylcholine receptor and the glutamate receptor macromolecular complexes. Signific antly, we find that across these segments, DM(alpha) and DM(gamma) are more similar to the channel proteins than the two channel proteins ar e to each other.