Identification of the S6 kinase activity stimulated in quiescent brine shrimp embryos upon entry to preemergence development as p70 ribosomal proteinS6 kinase: Isolation of Artemia franciscana p70(S6k) cDNA

We previously demonstrated that a protein kinase responsible for phosphoryl ating 40S ribosomal subunits is activated in quiescent Artemia franciscana embryos within 15 min of restoration of normal tonicity and incubation at 3 0 degreesC. Here, we identify the activated S6 kinase as A. franciscana p70 ribosomal S6 kinase (p70(S6k)) subsequent to the isolation of an Artemia p 70(S6k) cDNA. The protein conceptually translated from cDNA has 70% similar ity and 64% identity to both Drosophila melanogaster and human p70(S6k). So uthern blot analysis is consistent with presence of a single p70(S6k) gene. Two transcripts of 5.4 and 2.7 kb were found. Abundance of both mRNAs incr eased dramatically around 4 h of preemergence development, and exhibited di fferent steady-state level variation thereafter. Stimulated S6 kinase activ ity, partially purified by Superose 6 chromatography, correlated best with the slowest migrating, similar to 65 kDa, form detected by Western analysis using a specific polyclonal antibody made to a peptide from the predicted p70(S6k) NH2-terminus. Furthermore, the A. franciscana p70(S6k) was immunop recipitated with the same antibody, showing in parallel an S6 kinase activi ty similar to peak profiles. We conclude that the stimulated S6 kinase acti vity is that of an ortholog of human p70(S6k) that may be involved in the r egulation of protein synthesis during preemergence development in A. franci scana species.