A Role for Myosin-I in Actin Assembly through Interactions with Vrp1p, Bee1p, and the Arp2/3 Complex

Abstract

Type I myosins are highly conserved actin-based molecular motors that localize to the actin-rich cortex and participate in motility functions such as endocytosis, polarized morphogenesis, and cell migration. The COOH-terminal tail of yeast myosin-I proteins, Myo3p and Myo5p, contains an Src homology domain 3 (SH3) followed by an acidic domain. The myosin-I SH3 domain interacted with both Bee1p and Vrp1p, yeast homologues of human WASP and WIP, adapter proteins that link actin assembly and signaling molecules. The myosin-I acidic domain interacted with Arp2/3 complex subunits, Arc40p and Arc19p, and showed both sequence similarity and genetic redundancy with the COOH-terminal acidic domain of Bee1p (Las17p), which controls Arp2/3-mediated actin nucleation. These findings suggest that myosin-I proteins may participate in a diverse set of motility functions through a role in actin assembly.

Interaction of the Endocytic Scaffold Protein Pan1 with the Type I Myosins Contributes to the Late Stages of Endocytosis

“…To determine whether the interaction with Pan1 was through the SH3 domain, we made single point mutations at a conserved tryptophan residue in the myosin SH3 domain: W1157S in Myo3 and W1123S in Myo5. This mutation causes Myo3 mislocalization in vivo and reduces SH3 domain-dependent physical interactions of Myo3 with its other known partners in yeast (Evangelista et al, 2000;Geli et al, 2000). We have observed a similar mislocalization with the corresponding mutation in Myo5 (Barker and Wendland, unpublished data).…”

“…Actin NPF activity is unique to fungal type I myosins, as type I myosins from other species lack an A domain (Evangelista et al, 2000). In fungi, this actin activity of the myosins could supplant the requirement of a dynamin-like protein in neck elongation/ vesicle scission, and simultaneously provide an actin component to the process as well.…”

“…It has been previously shown that type I myosins localize to the membrane in an SH3-dependent manner (Anderson et al, 1998;Evangelista et al, 2000). To determine whether the PRD of Pan1 plays a role in the localization of the myosins, a Myo5-GFP plasmid was transformed into wild-type and pan1⌬PRD cells.…”

“…Both the motor and Arp2/3 activities of Myo5 have been implicated in completing the late stages of endocytosis (Sun et al, 2006). In support of their endocytic roles, the C-terminal tails of Myo3 and Myo5 interact with multiple endocytic proteins: Las17, a yeast homologue of Wiskcott Aldrich Syndrome protein (WASP) (Evangelista et al, 2000); Vrp1, a yeast homologue of human WASP-interacting protein (Anderson et al, 1998); and Arc19 and Arc40, components of the Arp2/3 actin nucleating complex (Evangelista et al, 2000;Lechler et al, 2000). Vrp1 is an activator of the Arp2/3 actin polymerization activity of type I myosins in both S. cerevisiae and Schizosaccharomyces pombe (Sirotkin et al, 2005;Sun et al, 2006).…”

“…Typically, myosins have N-terminal motor domains that bind actin in an ATP-dependent cycle and C-terminal tails that bind cargo through protein interaction domains. Type I myosins have an SH3 domain in their C-terminal tails, and fungal type I myosins also have an A domain at the extreme C-terminus that binds and activates the Arp2/3 complex (Geli and Riezman, 1996;Goodson et al, 1996;Evangelista et al, 2000;Lechler et al, 2000). MYO3 and MYO5 are the only genes encoding type I myosins in the Saccharomyces cerevisiae genome.…”