The small GTPase RalA targets filamin to induce filopodia

The small GTPase RalA targets filamin to induce filopodia

^*Division of Biochemistry and Cellular Biology, National Institute of Neuroscience, National Center of Neurology and Psychiatry, 4-1-1 Ogawahigashi, Kodaira, Tokyo 187, Japan; and ^‡Division of Hematology, Brigham and Women’s Hospital, Department of Medicine, Harvard Medical School, 221 Longwood Avenue, Boston, MA 02115

Contributed by Thomas P. Stossel

Abstract

The Ras-related small GTPases Rac, Rho, Cdc42, and RalA bind filamin, an actin filament-crosslinking protein that also links membrane and other intracellular proteins to actin. Of these GTPases only RalA binds filamin in a GTP-specific manner, and GTP-RalA elicits actin-rich filopods on surfaces of Swiss 3T3 cells and recruits filamin into the filopodial cytoskeleton. Either a dominant negative RalA construct or the RalA-binding domain of filamin 1 specifically block Cdc42-induced filopod formation, but a Cdc42 inhibitor does not impair RalA’s effects, which, unlike Cdc42, are Rac independent. RalA does not generate filopodia in filamin-deficient human melanoma cells, whereas transfection of filamin 1 restores the functional response. RalA therefore is a downstream intermediate in Cdc42-mediated filopod production and uses filamin in this pathway.

Footnotes

↵ † To whom reprint requests should be addressed. e-mail: ohta{at}ncnaxp.ncnp.go.jp.
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected on April 29, 1997.
ABBREVIATIONS:

GST,

glutathione S-transferase;

GTPγS,

guanosine 5′-γ-thiotriphosphate;

FITC,

fluorescein isothiocyanate