An intact SH3 domain is required for myosin I-induced actin polymerization

The yeast type I myosins (MYO3 and MYO5) are involved in endocytosis and in the polarization of the actin cytoskeleton, The tail of these proteins con tains a Tail Homology 2 (TH2) domain that constitutes a putative actin-bind ing site. Because of the important mechanistic implications of a second ATP -independent actin-binding site, we analyzed its functional relevance in vi vo. Even though the myosin tail interacts with actin, and this interaction seems functionally important, deletion of a major portion of the TH2 domain did not abolish interaction. In contrast, we found that the SH3 domain of Myo5p significantly contributes to this interaction, implicating other prot eins. We found that Vrp1p, the yeast homolog of WIP [Wiskott-Aldrich syndro me protein (WASP)interacting protein], seems necessary to sustain the Myo5p tail-F-actin interaction. Consistent with recent results implicating the y east type I myosins in regulating actin polymerization in vivo, we demonstr ate that the C-terminal domain of Myo5p is able to induce cytosol-dependent actin polymerization in vitro, and that this activity requires both an int act Myo5p SH3 domain and Vrp1p.