Structure and Function of the Intracellular Region of the Plexin-B1 Transmembrane Receptor*
Yufeng Tong
1,
Prasanta K. Hota
12,
Junia Y. Penachioni¶13,
Mehdi B. Hamaneh12,
SoonJeung Kim,
Rebecca S. Alviani,
Limin Shen,
Hao He,
Wolfram Tempel,
Luca Tamagnone¶34,
Hee-Won Park||5, , and
Matthias Buck**6
From the Departments of Physiology and Biophysics,
**Neuroscience, and
Pharmacology, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106,
the Structural Genomics Consortium and
||Department of Pharmacology, University of Toronto, Toronto, Ontario M5G 1L7, Canada, and
the ¶Institute for Cancer Research and Treatment, University of Torino, I-10060 Candiolo (Torino), Italy
Abstract:
Members of the plexin family are unique transmembrane receptors in that they interact directly with Rho family small GTPases; moreover, they contain a GTPase-activating protein (GAP) domain for R-Ras, which is crucial for plexin-mediated regulation of cell motility. However, the functional role and structural basis of the interactions between the different intracellular domains of plexins remained unclear. Here we present the 2.4 Å crystal structure of the complete intracellular region of human plexin-B1. The structure is monomeric and reveals that the GAP domain is folded into one structure from two segments, separated by the Rho GTPase binding domain (RBD). The RBD is not dimerized, as observed previously. Instead, binding of a conserved loop region appears to compete with dimerization and anchors the RBD to the GAP domain. Cell-based assays on mutant proteins confirm the functional importance of this coupling loop. Molecular modeling based on structural homology to p120GAP·H-Ras suggests that Ras GTPases can bind to the plexin GAP region. Experimentally, we show that the monomeric intracellular plexin-B1 binds R-Ras but not H-Ras. These findings suggest that the monomeric form of the intracellular region is primed for GAP activity and extend a model for plexin activation.
Received for publication August 17, 2009.
Revision received October 14, 2009.
1 These authors contributed equally to this work.
2 Postdoctoral fellow of the American Heart Association, Ohio Valley/Great Rivers Affiliate.
3 Supported by grants from the Italian Association for Cancer Research and Regione Piemonte.
4 To whom correspondence may be addressed; IRCC, University of Torino, Strada Prov. 142, I-10060 Candiolo, Italy. Tel.: 39-011-993-3204; E-mail: luca.tamagnone{at}ircc.it.
5 To whom correspondence may be addressed: Structural Genomics Consortium, MaRS South Tower, 101 College St., Toronto, Ontario M5G 1L7, Canada. Tel.: 416-946-3867; E-mail: heewon.park{at}utoronto.ca.
6 To whom correspondence may be addressed: Case Western Reserve University, School of Medicine, 10900 Euclid Ave., Cleveland, OH 44106. Tel.: 216-36-8651; E-mail: matthias.buck{at}case.edu.
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