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J. Biol. Chem., Vol. 282, Issue 24, 17855-17865, June 15, 2007

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Identification of the Sites for CaMK-II-dependent Phosphorylation of GABA_A Receptors^*

Catriona M. Houston, Henry H. C. Lee, Alastair M. Hosie, Stephen J. Moss, and Trevor G. Smart¹

From the Department of Pharmacology, University College London, Gower Street, London WC1E 6BT, United Kingdom and the Department of Neuroscience, University of Pennsylvania, Philadelphia, Pennsylvania 19104

Phosphorylation can affect both the function and trafficking of GABA_A receptors with significant consequences for neuronal excitability. Serine/threonine kinases can phosphorylate the intracellular loops between M3-4 of GABA_A receptor and subunits thereby modulating receptor function in heterologous expression systems and in neurons (1, 2). Specifically, CaMK-II has been demonstrated to phosphorylate the M3-4 loop of GABA_A receptor subunits expressed as GST fusion proteins (3, 4). It also increases the amplitude of GABA_A receptor-mediated currents in a number of neuronal cell types (5-7). To identify which substrate sites CaMK-II might phosphorylate and the consequent functional effects, we expressed recombinant GABA_A receptors in NG108-15 cells, which have previously been shown to support CaMK-II modulation of GABA_A receptors containing the 3 subunit (8). We now demonstrate that CaMK-II mediates its effects on 13 receptors via phosphorylation of Ser³⁸³ within the M3-4 domain of the subunit. Ablation of 3 subunit phosphorylation sites for CaMK-II revealed that for receptors, CaMK-II has a residual effect on GABA currents that is not mediated by previously identified sites of CaMK-II phosphorylation. This residual effect is abolished by mutation of tyrosine phosphorylation sites, Tyr³⁶⁵ and Tyr³⁶⁷, on the 2S subunit, and by the tyrosine kinase inhibitor genistein. These results suggested that CaMK-II is capable of directly phosphorylating GABA_A receptors and activating endogenous tyrosine kinases to phosphorylate the 2 subunit in NG108-15 cells. These findings were confirmed in a neuronal environment by expressing recombinant GABA_A receptors in cerebellar granule neurons.

Received for publication, December 18, 2006 , and in revised form, April 17, 2007.

^* This work was supported in part by the MRC and the Wellcome Trust. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Pharmacology, UCL, Gower St., London WC1E 6BT, United Kingdom. Tel.: 0044-207-679-2013; E-mail: t.smart{at}ucl.ac.uk.

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