Protein kinase C phosphorylates ribosomal protein S6 kinase beta II and regulates its subcellular localization

Valovka, Taras, Verdier, Frederique, Cramer, Rainer, Zhyvoloup, Alexander, Fenton, Timothy, Rebholz, Heike, Wang, Mong-Lien, Gzhegotsky, Miechyslav, Lutsyk, Alexander, Matsuka, Genadiy, Filonenko, Valeriy, Wang, Lijun, Proud, Christopher G., Parker, Peter J. and Gout, Ivan T. (2003) Protein kinase C phosphorylates ribosomal protein S6 kinase beta II and regulates its subcellular localization. Molecular and Cellular Biology, 23 (3), 852-863. (doi:10.1128/MCB.23.3.852-863.2003).

Abstract

The ribosomal protein S6 kinase (S6K) belongs to the AGC family of Ser/Thr kinases and is known to be involved in the regulation of protein synthesis and the G1/S transition of the cell cycle. There are two forms of S6K, termed S6K{alpha} and S6Kß, which have cytoplasmic and nuclear splice variants. Nucleocytoplasmic shuttling has been recently proposed for S6K{alpha}, based on the use of the nuclear export inhibitor, leptomycin B. However, the molecular mechanisms regulating subcellular localization of S6Ks in response to mitogenic stimuli remain to be elucidated. Here we present data on the in vitro and in vivo phosphorylation of S6Kß, but not S6K{alpha}, by protein kinase C (PKC). The site of phosphorylation was identified as S486, which is located within the C-terminal nuclear localization signal. Mutational analysis and the use of phosphospecific antibodies provided evidence that PKC-mediated phosphorylation at S486 does not affect S6K activity but eliminates the function of its nuclear localization signal and causes retention of an activated form of the kinase in the cytoplasm. Taken together, this study uncovers a novel mechanism for the regulation of nucleocytoplasmic shuttling of S6KßII by PKC-mediated phosphorylation.

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Contributors

Author: Timothy Fenton

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