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Molecular and Cellular Biology, February 2007, p. 1380-1393, Vol. 27, No. 4
0270-7306/07/$08.00+0     doi:10.1128/MCB.01608-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Asef2 Functions as a Cdc42 Exchange Factor and Is Stimulated by the Release of an Autoinhibitory Module from a Concealed C-Terminal Activation Element{triangledown}

Michael J. Hamann, Casey M. Lubking, Doris N. Luchini, and Daniel D. Billadeau*

Division of Oncology Research, Mayo Clinic College of Medicine, Rochester, Minnesota 55905

Received 28 August 2006/ Returned for modification 17 October 2006/ Accepted 27 November 2006

Asef (herein called Asef1) was identified as a Rac1-specific exchange factor stimulated by adenomatous polyposis coli (APC), contributing to colorectal cancer cell metastasis. We investigated Asef2, an Asef1 homologue having a similar N-terminal APC binding region (ABR) and Src-homology 3 (SH3) domain. Contrary to previous reports, we found that Asef1 and Asef2 exchange activity is Cdc42 specific. Moreover, the ABR of Asef2 did not function independently but acted in tandem with the SH3 domain to bind APC. The ABRSH3 also bound the C-terminal tail of Asef2, allowing it to function as an autoinhibitory module within the protein. Deletion of the C-terminal tail did not constitutively activate Asef2 as predicted; rather, a conserved C-terminal segment was required for augmented Cdc42 GDP/GTP exchange. Thus, Asef2 activation involves APC releasing the ABRSH3 from the C-terminal tail, resulting in Cdc42 exchange. These results highlight a novel exchange factor regulatory mechanism and establish Asef1 and Asef2 as Cdc42 exchange factors, providing a more appropriate context for understanding the contribution of APC in establishing cell polarity and migration.

* Corresponding author. Mailing address: Mayo Clinic, Division of Oncology Research, 200 First Street SW, Rochester, MN 55905. Phone: (507) 266-4334. Fax: (507) 266-5146. E-mail: billadeau.daniel{at}mayo.edu.

{triangledown} Published ahead of print on 4 December 2006.

Molecular and Cellular Biology, February 2007, p. 1380-1393, Vol. 27, No. 4
0270-7306/07/$08.00+0     doi:10.1128/MCB.01608-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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