cDNA encoding a 59 kDa homolog of ribosomal protein S6 kinase from rabbit liver

Abstract

We have isolated cDNA molecules encoding a protein with the characteristic sequence elements that are conserved between the catalytic domains of protein kinases. This protein is apparently a serine/threonine kinase and is most closely related to the amino‐terminal half of the ribosomal protein S6 kinase II first characterized in Xenopus eggs (42% overall identity and 56% identity in the predicted catalytic domain). However, it clearly differs from S6 kinase II in that it has only one, rather than two predicted catalytic domains and a deduced molecular mass of 59109 Da. We propose that is may be more related to, or identical with, the mitogen‐inducible S6 kinase of molecular mass 65–70 kDa described in mammalian liver, mouse 3T3 cells and chicken embryos. Remarkable structural features of the cDNA‐encoded polypeptide are a section rich in proline, serine and threonine residues that resemble the multiphosphorylation domains of glycogen synthase and phosphorylase kinase a subunit, and a characteristic tyrosine residue in the putative nucleotide‐binding glycine cluster which, by analogy to cdc2 kinase, is a potential tyrosine phosphorylation site.