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

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