Generation of DM-20 splice site in myelin proteolipid protein gene: a hypothesis based on analysis of the amphibian protein

Abstract

The DM-20 isoform of proteolipid protein (PLP) of central nervous system myelin is undetectable in amphibia, but readily detectable in reptiles, birds and mammals. To explain the phylogenetic origin of DM-20, it has been proposed that a donor-acceptor RNA splice site was generated by the introduction of a single base change in the codon encoding amino acid 116 of PLP. We tested this hypothesis by isolating and sequencing peptides from bullfrog PLP. One of the peptides corresponded to mammalian residues 110-122, which contain the N-terminal boundary region for the domain excluded from DM-20 in higher vertebrates. We found that the Thr115-Val116 sequence is conserved between frog and mammal. Therefore, we propose that a single base change in the third position of the codon for Thr115 would account for the appearance of the new donor-acceptor splice site. Three amino acids elsewhere in this thirteen-residue peptide were found to differ between bullfrog and mammal, which could account for the previously reported weak recognition of amphibian PLP by an antiserum specific for peptide 109-128. A second peptide from bullfrog PLP had a sequence identical to that of residues 45-52 in mammalian PLP. Our findings explain how a specific change in the myelin PLP gene could generate a new form of the PLP protein.