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J. Biol. Chem., Vol. 283, Issue 16, 10232-10240, April 18, 2008

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Rap1 Activation in Response to cAMP Occurs Downstream of Ras Activation during Dictyostelium Aggregation^*

From the Department of Microbiology and Immunology, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada

We have used a doubly disrupted rasC^–/rasG^– strain of Dictyostelium discoideum, which ectopically expresses the carA gene, to explore the relationship between the activation of RasC and RasG, the two proteins that are necessary for optimum cAMP signaling, and the activation of Rap1, a Ras subfamily protein, that is also activated by cAMP. The ectopic expression of carA restored early developmental gene expression to the rasC^–/rasG^– strain, rendering it suitable for an analysis of cAMP signal transduction. Because there was negligible signaling through both the cAMP chemotactic pathway and the adenylyl cyclase activation pathway in the rasC^–/rasG^–/[act15]:carA strain, it is clear that RasG and RasC are the only two Ras subfamily proteins that directly control these pathways. The position of Rap1 in the signal transduction cascade was clarified by the finding that Rap1 activation was totally abolished in rasC^–/rasG^–/[act15]:carA and rasG^– cells but only slightly reduced in rasC^– cells. Rap1 activation, therefore, occurs downstream of the Ras proteins and predominantly, if not exclusively, downstream of RasG. The finding that in vitro guanylyl cyclase activation is also abolished in the rasC^–/rasG^–/[act15]:carA strain identifies RasG/RasC as the presumptive monomeric GTPases required for this activation.

Received for publication, September 5, 2007 , and in revised form, December 7, 2007.

^* This research was funded in part by a grant from the Canadian Institute of Health Research (to G. W.). 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 Recipient of a scholarship from the Ministry of Science Research and Technology of Iran.

² To whom correspondence should be addressed: Life Sciences Centre, 2350 Health Sciences Mall, University of British Columbia, Vancouver, British Columbia V6T 1Z3, Canada. Tel.: 604-822-6649; Fax: 604-822-6041; E-mail: gerwee{at}interchange.ubc.ca.

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				K. Parkinson, P. Bolourani, D. Traynor, N. L. Aldren, R. R. Kay, G. Weeks, and C. R. L. Thompson Regulation of Rap1 activity is required for differential adhesion, cell-type patterning and morphogenesis in Dictyostelium J. Cell Sci., February 1, 2009; 122(3): 335 - 344. [Abstract] [Full Text] [PDF]