G Protein-Coupled Receptor Kinase 5 Contains a DNA-Binding Nuclear Localization Sequence

Abstract

G protein-coupled receptor kinases (GRKs) mediate desensitization of agonist-occupied G protein-coupled receptors (GPCRs). Here we report that GRK5 contains a DNA-binding nuclear localization sequence (NLS) and that its nuclear localization is regulated by GPCR activation, results that suggest potential nuclear functions for GRK5. As assessed by fluorescence confocal microscopy, transfected and endogenous GRK5 is present in the nuclei of HEp2 cells. Mutation of basic residues in the catalytic domain of GRK5 (between amino acids 388 and 395) results in the nuclear exclusion of the mutant enzyme (GRK5_ΔNLS), demonstrating that GRK5 contains a functional NLS. The nuclear localization of GRK5 is subject to dynamic regulation. Calcium ionophore treatment or activation of Gq-coupled muscarinic-M3 receptors promotes the nuclear export of the kinase in a Ca²⁺/calmodulin (Ca²⁺/CaM)-dependent fashion. Ca²⁺/CaM binding to the N-terminal CaM binding site of GRK5 mediates this effect. Furthermore, GRK5, but not GRK5_ΔNLS or GRK2, binds specifically and directly to DNA in vitro. Consistent with their presence in the nuclei of transfected cells, all the GRK4, but not GRK2, subfamily members contain putative NLSs. These results suggest that the GRK4 subfamily of GRKs may play a signaling role in the nucleus and that GRK4 and GRK2 subfamily members perform divergent cellular functions.

We thank Marcia A. Kaetzel, John Dedman, and David Sacks for the CaM binding protein and cDNAs, Andrew Tobin for the anti-muscarinic M3 antibody and CHO-M3 stable cell line, and Robert J. Lefkowitz for the GRK5_K215R construct.

This work was funded by grants from the Wellcome Trust (J.A.P. and M.G.H.S.) and Medical Research Council (L.R.J.).