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Regulation of G Protein-Coupled Receptor Function by the G Protein-Coupled Receptor Kinase Family

Dr Julie A. Pitcher

Senior Lecturer

Tel: +44 (0)20 7679 7950 (lab)

Tel: +44 (0)20 7679 7270 (office)

Email: julie.pitcher@ucl.ac.uk

Dr Julie A Pitcher graduated with a degree in Biochemistry from Oxford University and completed her PhD with Prof. Philip Cohen in the Department of Biochemistry, University of Dundee. During her Post-Doctoral training she worked with Prof. Robert J Lefkowitz, Duke University, North Carolina, on the molecular mechanisms underlying the phenomenon of G protein-coupled receptor desensitisation. In 1999 she joined the MRC Laboratory of Molecular Cell Biology and Cell Biology Unit at UCL as a Wellcome Senior Research Fellow. She joined the faculty at UCL as a lecturer in 2006.

G-protein-coupled receptors (GPCRs) represent the largest family of cell surface receptors. They share a common 7 transmembrane domain configuration and act to transduce extracellular stimuli to the interior of the cell. GPCRs are expressed in virtually every cell type and modulate an extremely wide range of physiological processes. They represent a rich source of targets for the pharmaceutical industry with approximately 30-50% of current pharmaceuticals targeting these proteins.

Following repeated or continuous exposure to agonists GPCRs exhibit diminished responsiveness, a phenomenon termed receptor desensitisation. For many GPCRs rapid desensitisation is mediated by a family of serine/threonine kinases, the G protein-coupled receptor kinases (GRKs). GRKs phosphorylate agonist-occupied GPCRs and the phosphorylated receptor serves as a binding site for a member of the arrestin family. Arrestin binding uncouples the GPCR from G proteins and targets the phosphorylated receptor for internalisation via clathrin-coated pits. Given their role in regulating GPCR function it is not surprising that changes in the expression levels, or activities, of the GRKs are associated with a number of disease states. For example, GRK2 and 5 upregulation are responsible for alterations in myocardial function in chronic heart failure.

In addition to their role in mediating GPCR desensitisation several lines of evidence suggest the GRKs have other cellular functions. It is our aim to elucidate at the molecular level the cellular functions of the GRKs, work which may aid in the design of therapeutically useful GRK inhibitors.

Current projects include:

1) Investigating potential nuclear functions of the GRKs The GRK family is divided into three subfamilies based on sequence similarity. The GRK1 (GRKs 1 and 7); GRK2 (GRKs 2 and 3); and GRK4 (GRKs 4, 5 and 6) subfamilies. We have discovered that members of the GRK4, but not GRK2, subfamilies contain functional nuclear localisation and nuclear export sequences (NLSs and NESs). Furthermore, the location of the NLS, its DNA-binding properties and the mechanisms regulating NES function, varies between GRK4 subfamily members. We are currently investigating the hypothesis that GRK4 family members regulate gene transcription using a variety of model systems.

2) GRKs as G protein effectors A common structural feature of the GRKs is an RGS homology (RH) domain in their amino-termini. The ligand for the RH domain of GRK2 has been identified as GaqGTP. Notably, in contrast to most other RGS-containing proteins GRK2 fails to GAP GaqGTP. We are currently investigating the ability of the GRKs to act as effectors for a variety of both heterotrimeric and small G proteins. The elucidation of G protein effector functions for the GRKs may provide a molecular explanation for the chemotactic defects observed in cells derived from GRK knockout mice.

Recent Publications:

• Cotton M, Boulay PL, Houndolo T, Vitale N, Pitcher JA, Claing A. (2007). Endogenous ARF6 Interacts with Rac1 upon Angiotensin II Stimulation to Regulate Membrane Ruffling and Cell Migration. Mol Biol Cell. 18: 501-11.
• Scott M.G.H., Pierotti V., Storez H., Lindberg E., Thuret A., Muntaner O., Labbe-Julli C., Pitcher J.A.* and Marullo S.* (2006). Cooperative regulation of ERK activation and cell shape change by filamin A and β-arrestins. Mol Cell Biol. 26: 3432-45. * Both authors contributed equally to this work
• Cant S.H., and Pitcher J.A. (2005). G Protein-coupled receptor kinase 2-mediated phosphorylation of ezrin is required for G protein-coupled receptor-dependent reorganization of the actin cytoskeleton. Mol Biol Cell. 16: 3088-99.
• Johnson L.J., Scott M.G.H., and Pitcher J.A. (2004). G protein-coupled receptor kinase 5 contains a DNA-binding nuclear localization sequence. Mol Cell Biol. 24: 10169-79.
• Fairfax B.P., Pitcher J.A., Scott M.G., Calver A.R., Pangalos M.N., Moss S.J., and Couve A. (2004). Phosphorylation and chronic agonist treatment atypically modulate GABAB receptor cell surface stability. J Biol Chem. 279: 12565-73.
• Lodowski D.T., Pitcher J.A., Capel W.D., Lefkowitz R.J., and Tesmer J.J. (2003). Keeping G proteins at bay: a complex between G protein-coupled receptor kinase 2 and Gbetagamma. Science. 300: 1256-62.