Polypeptides in Frog and Rat: Evolutionary Changes in Rapidly Transported and Abundant Nerve Proteins

Dorsal root ganglion (DRG) neurons from rat and frog were labeled in vitro with [³⁵S]methionine, and the newly synthesized, rapidly transported proteins were collected at ligatures on the sciatic nerves. The proteins were extracted and separated by two-dimensional polyacrylamide gel electrophoresis. Exposure of x-ray film to dried gels allowed comparison of the labeled, rapidly transported proteins from frog and rat. The gel staining patterns of abundant proteins in the sciatic nerves were also compared. Triplets of gels were examined; one gel from frog, one from rat, and one from frog plus rat combined. Among the transported proteins, some (including A2, A17 and/or A18 B6, B14(a-i), C1 C22, and some members of A1(a-i) and B3(a-g)) co-migrated on the gels, suggesting that these proteins have been well conserved during evolution. The gel staining patterns of abundant proteins in the sciatic nerves also show some similarities: two forms of actin, serum albumin, and α- and β-tubulin are each in identical positions on the frog and rat gels. Other sciatic nerve and rapidly transported proteins had similar, but not identical, positions on the gels. A number of the rat and frog proteins had no obvious counterpart. We have calculated the magnitude of expected changes in charge and molecular weight of proteins due to accumulation of point mutations during evolution. We conclude that many of the differences between rat and frog protein patterns on the two-dimensional gels could be the result of such point mutations, but we cannot rule out radical changes in polypeptide sequence of abundance between frog and rat for some of these proteins.