RalB Mobilizes the Exocyst To Drive Cell Migration

Abstract

The Ras family GTPases RalA and RalB have been defined as central components of the regulatory machinery supporting tumor initiation and progression. Although it is known that Ral proteins mediate oncogenic Ras signaling and physically and functionally interact with vesicle trafficking machinery, their mechanistic contribution to oncogenic transformation is unknown. Here, we have directly evaluated the relative contribution of Ral proteins and Ral effector pathways to cell motility and directional migration. Through loss-of-function analysis, we find that RalA is not limiting for cell migration in normal mammalian epithelial cells. In contrast, RalB and the Sec6/8 complex or exocyst, an immediate downstream Ral effector complex, are required for vectorial cell motility. RalB expression is required for promoting both exocyst assembly and localization to the leading edge of moving cells. We propose that RalB regulation of exocyst function is required for the coordinated delivery of secretory vesicles to the sites of dynamic plasma membrane expansion that specify directional movement.

Supplemental material for this article may be found at http://mcb.asm.org/.

This work was supported by grant CT-99-00875 from the EU and grant 5440 from Association de Recherche sur le Cancer (ARC) to J.C., by a grant Equipe labelisée from the Ligue Nationale contre le Cancer to P.C., a grant from the Association Christelle Bouillot to Inserm U528, and grant I-1414 from the Robert Welch Foundation to M.A.W. C.R. was supported by a fellowship from the Ministère de la Recherche, by ARC, and by the Fondation de la Recherche Médicale (FRM).

We are grateful to S. Hsu for providing anti-Exo70 monoclonal antibodies and to L. Feig, C. Marshall, P. Frankel, L. Quilliam, and M. Katan for reagents used during this work. We thank Dominique Morineau for his expert artwork, Chloé Camonis for film editing, and Nathalie Brandon, Marie Bourgeois, and Carole Gomez for skillful technical help. Fanny Momboisse was an efficient intern during the course of this work. We are very indebted to Franck Perez for stimulating discussions and suggestions, as well as to Jean de Gunzburg and to Julie Plastino for critical reading of the manuscript.