K. Kitagawa et al., A PROTEOLIPID PROTEIN GENE FAMILY - EXPRESSION IN SHARKS AND RAYS ANDPOSSIBLE EVOLUTION FROM AN ANCESTRAL GENE ENCODING A PORE-FORMING POLYPEPTIDE, Neuron, 11(3), 1993, pp. 433-448
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.