Regulation of Ribosomal S6 Kinase 2 by Effectors of the Phosphoinositide 3-Kinase Pathway

Kathleen A Martin,Stefanie S Schalm,Celeste Richardson,Angela Romanelli,Kristen L Keon,John Blenis

doi:10.1074/jbc.m006969200

Kathleen A Martin, Stefanie S Schalm + Show 4 more

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https://doi.org/10.1074/jbc.m006969200

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Abstract

Ribosomal S6 kinase (S6K1), through phosphorylation of the 40 S ribosomal protein S6 and regulation of 5'-terminal oligopyrimidine tract mRNAs, is an important regulator of cellular translational capacity. S6K1 has also been implicated in regulation of cell size. We have recently identified S6K2, a homolog of S6K1, which phosphorylates S6 in vitro and is regulated by the phosphatidylinositide 3-kinase (PI3-K) and mammalian target of rapamycin pathways in vivo. Here, we characterize S6K2 regulation by PI3-K signaling intermediates and compare its regulation to that of S6K1. We report that S6K2 is activated similarly to S6K1 by the PI3-K effectors phosphoinositide-dependent kinase 1, Cdc42, Rac, and protein kinase Czeta but that S6K2 is more sensitive to basal activation by myristoylated protein kinase Czeta than is S6K1. The C-terminal sequence of S6K2 is divergent from that of S6K1. We find that the S6K2 C terminus plays a greater role in S6K2 regulation than does the S6K1 C terminus by functioning as a potent inhibitor of activation by various agonists. Removal of the S6K2 C terminus results in an enzyme that is hypersensitive to agonist-dependent activation. These data suggest that S6K1 and S6K2 are similarly activated by PI3-K effectors but that sequences unique to S6K2 contribute to stronger inhibition of its kinase activity. Understanding the regulation of the two S6K homologs may provide insight into the physiological roles of these kinases.

Highlights

The 70-kDa ribosomal S6 kinase 1 (S6K1)1 is a ubiquitously expressed serine/threonine protein kinase that phosphorylates the 40 S ribosomal protein S6 in response to mitogen stimula
We report that S6K2 is activated to S6K1 by the phosphatidylinositide 3-kinase (PI3-K) effectors phosphoinositide-dependent kinase 1, Cdc42, Rac, and protein kinase C␨ but that S6K2 is more sensitive to basal activation by myristoylated protein kinase C␨ than is S6K1
We find that the S6K2 C terminus plays a greater role in S6K2 regulation than does the S6K1 C terminus by functioning as a potent inhibitor of activation by various agonists

Summary

Introduction

The 70-kDa ribosomal S6 kinase 1 (S6K1) is a ubiquitously expressed serine/threonine protein kinase that phosphorylates the 40 S ribosomal protein S6 in response to mitogen stimula-. S6 phosphorylation up-regulates translation of mRNAs with 5Ј-terminal oligopyrimidine tracts, many of which encode ribosomal proteins and translation elongation factors [2]. S6 phosphorylation and 5Ј-terminal oligopyrimidine tract mRNA translation were found to be normal in fibroblasts derived from mice lacking S6K1, suggesting a compensatory mechanism for these S6K1 functions. S6K2 is a good candidate kinase that may supply some but not all of the functions of S6K1 in the knockout mouse, because the small animal phenotype persists despite the presence of S6K2, S6 phosphorylation, and 5Ј-terminal oligopyrimidine tract mRNA translation. The p70S6K1␣II isoform is cytosolic, whereas p85 S6K1␣I is nuclear [8] Both isoforms of S6K2 (p54 S6K2␤II, p60 S6K2␤I) [5, 6] are primarily nuclear, because of the presence of a C-terminal putative nuclear localization signal sequence (NLS) [7] not found in S6K1. The S6K2 ␤I and ␤II isoforms may reside in distinct nuclear compartments based on subcellular fractionation studies [6]

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