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J. Cell Biol., Volume 140, Number 5, March 9, 1998 1125-1136

Corequirement of Specific Phosphoinositides and Small GTP-binding Protein Cdc42 in Inducing Actin Assembly in Xenopus Egg Extracts

Le Ma,* Lewis C. Cantley,*Dagger Paul A. Janmey,§ and Marc W. Kirschner*

* Department of Cell Biology, Harvard Medical School, Boston, Massachusetts 02115; Dagger  Division of Signal Transduction, Beth Israel Hospital, Boston, Massachusetts 02115; and § Division of Experimental Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115

Both phosphoinositides and small GTP-binding proteins of the Rho family have been postulated to regulate actin assembly in cells. We have reconstituted actin assembly in response to these signals in Xenopus extracts and examined the relationship of these pathways. We have found that GTPgamma S stimulates actin assembly in the presence of endogenous membrane vesicles in low speed extracts. These membrane vesicles are required, but can be replaced by lipid vesicles prepared from purified phospholipids containing phosphoinositides. Vesicles containing phosphatidylinositol (4,5) bisphosphate or phosphatidylinositol (3,4,5) trisphosphate can induce actin assembly even in the absence of GTPgamma S. RhoGDI, a guanine-nucleotide dissociation inhibitor for the Rho family, inhibits phosphoinositide-induced actin assembly, suggesting the involvement of the Rho family small G proteins. Using various dominant mutants of these G proteins, we demonstrate the requirement of Cdc42 for phosphoinositide-induced actin assembly. Our results suggest that phosphoinositides may act to facilitate GTP exchange on Cdc42, as well as to anchor Cdc42 and actin nucleation activities. Hence, both phosphoinositides and Cdc42 are required to induce actin assembly in this cell-free system.


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