A proteolipid protein gene family: Expression in sharks and rays and possible 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 developments, which arose de novo in the antecedents of early tetrapods. However, PCR primed with degenerate oligonu-cleotides corresponding to common 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 DM20. We term the novel proteolipid proteins DM α, DM β, and DM γ. At least DM α and DM γ are highly expressed in white matter in myelinating shark brain. The DMs not only are highly homologous to each other, but also contain regions bearing similarities with segments of channel-forming regions of the nicotinic acetylcholine receptor and the glutamate receptor macromolecular complexes. Significantly, we find that across these segments, DM α and DM γ are more similar to the channel proteins than the two channel proteins are to each other.