Ribosomal S6 Kinase (RSK) Regulates Phosphorylation of Filamin A on an Important Regulatory Site

Abstract

The Ras-mitogen-activated protein (Ras-MAP) kinase pathway regulates various cellular processes, including gene expression, cell proliferation, and survival. Ribosomal S6 kinase (RSK), a key player in this pathway, modulates the activities of several cytoplasmic and nuclear proteins via phosphorylation. Here we report the characterization of the cytoskeletal protein filamin A (FLNa) as a membrane-associated RSK target. We show that the N-terminal kinase domain of RSK phosphorylates FLNa on Ser²¹⁵² in response to mitogens. Inhibition of MAP kinase signaling with UO126 or mutation of Ser²¹⁵² to Ala on FLNa prevents epidermal growth factor (EGF)-stimulated phosphorylation of FLNa in vivo. Furthermore, phosphorylation of FLNa on Ser²¹⁵² is significantly enhanced by the expression of wild-type RSK and antagonized by kinase-inactive RSK or specific reduction of endogenous RSK. Strikingly, EGF-induced, FLNa-dependent migration of human melanoma cells is significantly reduced by UO126 treatment. Together, these data provide substantial evidence that RSK phosphorylates FLNa on Ser²¹⁵² in vivo. Given that phosphorylation of FLNa on Ser²¹⁵² is required for Pak1-mediated membrane ruffling, our results suggest a novel role for RSK in the regulation of the actin cytoskeleton.

We thank F. Nakamura for generously sharing unpublished data and reagents and M. Chou, P. James, and S. Hollenberg for providing reagents. We are grateful to members of the Blenis lab for helpful discussions and critical reading of the manuscript.

This work was supported in part by grant CA09595 from NCI/NIH (M.S.W.), grants RO1 CA46595 and RO1 GM51405 from the NIH (J.B.), USPHS grant HL19429, and the American Cancer Society (T.P.S.).