Cloning and characterization of p70 S6 kinase-related kinase, SRK

Abstract

p70 S6 kinase (p70S6K) plays a critical role in cell cycle progression. Studies on p70S6K knockout mice strongly suggested the existence of p70S6K homologs in the cell. By EST database searching and consequent library screening, cDNA for human p70 S6 kinase-related kinase (SRK) was isolated. The catalytic domain of SRK was highly homologous to that of p70S6K. However, the N- and C-terminal domains of SRK were quite different from those of p70S6K. Treatments of rapamycin or wortmannin, the inhibitors of p70S6K, strongly inhibited the phosphorylation and activation of SRK. In immunolocalization analyses, a constitutive nuclear localization of SRK was observed in the cell. However, p70S6K was translocated to the plasma membrane in response to EGF stimulation. In vitro S6 kinase activities of SRK were also stimulated with a slower kinetics by the agonists of p70S6K including serum, epidermal growth factor (EGF), a phorbol ester, and cycloheximide. Differences in their structure, biochemical activities, and intracellular localization between SRK and p70S6K suggest the presence of complex cell signaling pathways to regulate these diverse in vitro S6 kinases.