Fast and Slow Actin Nucleation by WASP Proteins

Abstract

Members of the Wiscott-Aldrich syndrome protein (WASP) and the Scar family of actin binding proteins stimulate actin filament formation in response to signals from the Rho family of guanosine triphosphatases. Zalevsky et al. measured actin nucleation in vitro of Acanthamoeba extracts in the presence of peptides containing the actin-binding domains (WH2) and the acidic residues of the COOH-terminals of three members of the family: N-WASP, Scar1, and WASP. N-WASP with two WH2 domains was able to induce actin nucleation with the fastest kinetics, whereas Scar1 was the slowest at inducing actin nucleation. Analysis of chimeric proteins showed that the difference was not due to the presence of two WH2 domains, but rather was attributed to differences in the acidic domain. The N-WASP peptide induced the highest number of branches in the actin filaments. Although both N-WASP and Scar1 induced conformational changes in the actin-nucleating protein Arp2/3; they appear to mediate their effects through interactions with different subunits of the Arp2/3 complex as seen in chemical crosslinking experiments. Thus, members of the same family can stimulate actin nucleation through different molecular mechanisms, allowing differences in cellular response to stimuli that promote actin filament formation. Higgs discusses the various models for actin nucleation.J. Zalevsky, L. Lempert, H. Kranitz, R. D. Mullins, Different WASP family proteins stimulate different Apr2/3 complex-dependent actin-nucleating activities. Curr. Biol. 11, 1903-1913 (2001). [Online Journal]H. N. Higgs, Actin nucleation: Nucleation-promoting factors are not all equal. Curr. Biol. 11, R1009-R1012 (2001). [Online Journal]