Rho-family GTPases require the Arp2/3 complex to stimulate actin polymerization in Acanthamoeba extracts.
BACKGROUND
Actin filaments polymerize in vivo primarily from their fast-growing barbed ends. In cells and extracts, GTPgammaS and Rho-family GTPases, including Cdc42, stimulate barbed-end actin polymerization; however, the mechanism responsible for the initiation of polymerization is unknown. There are three formal possibilities for how free barbed ends may be generated in response to cellular signals: uncapping of existing filaments; severing of existing filaments; or de novo nucleation. The Arp2/3 complex localizes to regions of dynamic actin polymerization, including the leading edges of motile cells and motile actin patches in yeast, and in vitro it nucleates the formation of actin filaments with free barbed ends. Here, we investigated actin polymerization in soluble extracts of Acanthamoeba.


RESULTS
Addition of actin filaments with free barbed ends to Acanthamoeba extracts is sufficient to induce polymerization of endogenous actin. Addition of activated Cdc42 or activation of Rho-family GTPases in these extracts by the non-hydrolyzable GTP analog GTPgammaS stimulated barbed-end polymerization, whereas immunodepletion of Arp2 or sequestration of Arp2 using solution-binding antibodies blocked Rho-family GTPase-induced actin polymerization.


CONCLUSIONS
For this system, we conclude that the accessibility of free barbed ends regulates actin polymerization, that Rho-family GTPases stimulate polymerization catalytically by de novo nucleation of free barbed ends and that the primary nucleation factor in this pathway is the Arp2/3 complex.
Mullins
R D
RD
The Salk Institute for Biological Studies, 10010 N. Torrey Pines Road, La Jolla, California 92037, USA. dyche@sbl.salk.edu
Pollard
T D
TD
D000048:Acanthamoeba / Q000737:chemistry*; D051377:Actin-Related Protein 2; D051378:Actin-Related Protein 3; D000199:Actins / Q000187:drug effects / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002457:Cell Extracts / Q000737:chemistry; D003598:Cytoskeletal Proteins; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism / Q000502:physiology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D011108:Polymers; D015800:Protozoan Proteins / Q000378:metabolism
D051377:Actin-Related Protein 2; D051378:Actin-Related Protein 3; D000199:Actins; D002457:Cell Extracts; D003598:Cytoskeletal Proteins; D011108:Polymers; D015800:Protozoan Proteins; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000255:Adenosine Triphosphate; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins