Identification of the S6 kinase activity stimulated in quiescent brine shrimp embryos upon entry to preemergence development as p70 ribosomal protein S6 kinase: Isolation ofArtemia franciscanap70S6kcDNA

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Abstract
We previously demonstrated that a protein kinase responsible for phosphorylating 40S ribosomal subunits is activated in quiescent Artemia franciscana embryos within 15 min of restoration of normal tonicity and incubation at 30°C. Here, we identify the activated S6 kinase as A. franciscana p70 ribosomal S6 kinase (p70S6k) subsequent to the isolation of an Artemia p70S6kcDNA. The protein conceptually translated from cDNA has 70% similarity and 64% identity to both Drosophila melanogaster and human p70S6k. Southern blot analysis is consistent with presence of a single p70S6kgene. Two transcripts of 5.4 and 2.7 kb were found. Abundance of both mRNAs increased dramatically around 4 h of preemergence development, and exhibited different steady-state level variation thereafter. Stimulated S6 kinase activity, partially purified by Superose 6 chromatography, correlated best with the slowest migrating, ~65 kDa, form detected by Western analysis using a specific polyclonal antibody made to a peptide from the predicted p70S6kNH2-terminus. Furthermore, the A. franciscana p70S6kwas immunoprecipitated with the same antibody, showing in parallel an S6 kinase activity similar to peak profiles. We conclude that the stimulated S6 kinase activity is that of an ortholog of human p70S6kthat may be involved in the regulation of protein synthesis during preemergence development in A. franciscana species.Key Words: p70 ribosomal S6 kinase cDNA, brine shrimp, development.