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J. Biol. Chem., Vol. 282, Issue 34, 25168-25176, August 24, 2007

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The RACK1 Ortholog Asc1 Functions as a G-protein Subunit Coupled to Glucose Responsiveness in Yeast^*

From the Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, North Carolina 27599

According to the prevailing paradigm, G-proteins are composed of three subunits, an subunit with GTPase activity and a tightly associated subunit complex. In the yeast Saccharomyces cerevisiae there are two known G proteins (Gpa1 and Gpa2) but only one G, which binds only to Gpa1. Here we show that the yeast ortholog of RACK1 (receptor for activated protein kinase C1) Asc1 functions as the G for Gpa2. As with other known G proteins, Asc1 has a 7-WD domain structure, interacts directly with the G in a guanine nucleotide-dependent manner, and inhibits G guanine nucleotide exchange activity. In addition, Asc1 binds to the effector enzyme adenylyl cyclase (Cyr1), and diminishes the production of cAMP in response to glucose stimulation. Thus, whereas Gpa2 promotes glucose signaling through elevated production of cAMP, Asc1 has opposing effects on these same processes. Our findings reveal the existence of an unusual G subunit, one having multiple functions within the cell in addition to serving as a signal transducer for cell surface receptors and intracellular effectors.

Received for publication, March 26, 2007 , and in revised form, June 6, 2007.

^* This work was supported National Institutes of Health Grant GM080739 (to H. G. D.) and American Heart Association Grant 0515277U (to C. E. Z.). 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: 405 Mary Ellen Jones Bldg., Campus Box 7260, Chapel Hill, NC 27599-7260. Tel.: 919-843-6894; E-mail: henrik_dohlman{at}med.unc.edu.

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