Glucocorticoid-Dependent Phosphorylation of the Transcriptional Coregulator GRIP1

Abstract

Much of the regulatory diversity in eukaryotic transcription is provided by coregulators, which are recruited by DNA-binding factors to propagate signaling to basal machinery or chromatin. p160 family members, including the glucocorticoid receptor (GR)-interacting protein 1 (GRIP1), function as coactivators for GR, a ligand-dependent transcription factor of the nuclear receptor superfamily. Unlike other p160s, GRIP1 also potentiates GR-mediated repression of AP1 and NF-κB targets and, surprisingly, transcriptional activation by interferon regulatory factors. What enables GRIP1 activating or repressing properties or discrimination between physiologically antagonistic pathways is unknown. We found that endogenous GRIP1 in mammalian cells undergoes glucocorticoid-induced, GR interaction-dependent phosphorylation and identified one constitutive and six inducible phosphorylation sites and two putative GRIP1 kinases, casein kinase 2 and cyclin-dependent kinase 9. We raised phosphospecific antibodies to the four closely spaced sites in a previously uncharacterized part of GRIP1 which, combined with mutagenesis, revealed the conservation of GRIP1 phosphorylation across several cell types and species and its functional relevance to GR-activated transcription and to response element-specific recruitment of phospho-GRIP1 to native GR targets. We propose that cofactor engagement by GR is neither passive nor stochastic; rather, GR actively imparts modifications that dictate GRIP1 function in a subset of complexes, adding a layer of specificity to GR transcriptional control.

ACKNOWLEDGMENTS

We thank J. Fernandez and H. Deng at the Rockefeller U Proteomics Resource Center for MS data analysis, M. Reily for technical assistance, M. Garabedian (NYU), S. Logan (NYU), L. Ivashkiv (HSS), S. Chen-Kiang (Weill Cornell), and R. Fisher (Mount Sinai) for providing kinase inhibitors, J.-C. Wang (UC Berkeley) for the GILZ-luc reporter construct, H. Samuels (NYU) for providing T3, and K. Zarember and R. Gupte for critical comments on the manuscript.

This work was supported by the grants from the NIH-NIAID and Kirkland Center to I.R. and from the American Heart Association to Y.C.