Fission Yeast Myosin-I, Myo1p, Stimulates Actin Assembly by Arp2/3 Complex and Shares Functions with Wasp

Abstract

Fission yeast myo1 + encodes a myosin-I with all three tail homology domains (TH1, 2, 3) found in typical long-tailed myosin-Is. Myo1p tail also contains a COOH-terminal acidic region similar to the A-domain of WASp/Scar proteins and other fungal myosin-Is. Our analysis shows that Myo1p and Wsp1p, the fission yeast WASp-like protein, share functions and cooperate in controlling actin assembly. First, Myo1p localizes to cortical patches enriched at tips of growing cells and at sites of cell division. Myo1p patches partially colocalize with actin patches and are dependent on an intact actin cytoskeleton. Second, although deletion of myo1 + is not lethal, Δmyo1 cells have actin cytoskeletal defects, including loss of polarized cell growth, delocalized actin patches, and mating defects. Third, additional disruption of wsp1 + is synthetically lethal, suggesting that these genes may share functions. In mapping the domains of Myo1p tail that share function with Wsp1p, we discovered that a Myo1p construct with just the head and TH1 domains is sufficient for cortical localization and to rescue all Δmyo1 defects. However, it fails to rescue the Δmyo1 Δwsp1 lethality. Additional tail domains, TH2 and TH3, are required to complement the double mutant. Fourth, we show that a recombinant Myo1p tail binds to Arp2/3 complex and activates its actin nucleation activity.

Myosin I mutants with only 1% of wild-type actin-activated MgATPase activity retain essentialin vivofunction(s)

“…Deletion of Myo1p causes similar actin cytoskeletal defects in S. pombe (13). Although deletion of MYOA is lethal for A. nidulans (8), thus giving no clue to MYOA's function, partial rescue by MYOA mutants (37) suggests roles for MYOA in cell polarity, septal wall formation, hyphal branching patterns, and hyphal size and shape.…”

“…Insight into this question might be gained from the general similarities, and specific differences, between our results for A. nidulans MYOA and recent studies of the two class I myosins, Myo3p and Myo5p (33,34), of budding yeast, S. cerevisiae, and the single class I myosin, Myo1p (13), of fission yeast, S. pombe. Like MYOA, which localizes to actin patches at hyphal tips and sites of septum formation (8,9), the yeast class I myosins partially colocalize with actin patches at sites of polarized growth at the tips of cells and sites of cell division (13,34,35). Double deletion of functionally redundant Myo3p and Myo5p is nearly lethal for some strains of S. cerevisiae, resulting in severe defects in growth and actin cytoskeleton organization (33) and lethality in other strains (36).…”

“…Conidia of MYOA null cells swell and undergo nuclear division but do not initiate hyphal growth and ultimately die (8). Importantly, MYOA is one of a small subset of myosins (class I myosins of A. nidulans, Acanthamoeba castellanii, Dictyostelium discoideum, Saccharomyces cerevisiae, Schizosaccharomyces pombe, and the eight class VI myosins that have been sequenced) that have either a serine or threonine residue in the motor domain at a position [the TEDS site (10), Ser-371 in MYOA] in an actinbinding surface loop (11) where all other myosins contain either an aspartate or glutamate residue (10,12,13). The actindependent MgATPase and in vitro motility activities of A. castellanii (14) and D. discoideum (15) class I myosins are regulated by phosphorylation of the TEDS-site serine or threonine by p21-activated kinases (16,17).…”

Take five: A myosin class act in fission yeast

“…Myo1 is present at the poles of growing cells and relocates to the equator during cell division. Its localisation overlaps that of actin but the two proteins do not colocalise and myo1⌬ cells have aberrant cell walls and septa [Lee et al, 2000;Toya et al, 2001]. The C-terminal domain of the Myo1 tail is acidic and shares homology to the A-domain of the WASp/Scar family of proteins that stimulate the Arp2/3 complex-mediated assembly of actin filaments.…”

Transcript Profiles of Candida albicans Cortical Actin Patch Mutants Reflect Their Cellular Defects: Contribution of the Hog1p and Mkc1p Signaling Pathways

“…Indeed, cortical actin patches are the actual sites of endocytosis (29). S. cerevisiae myosin I (Myo3/5p) and Sla2p have been shown to play important roles in organizing the actin cytoskeleton and mediating endocytosis by the cortical actin patches (18,28,33,34). In addition, C. albicans SLA2 and MYO5 are required for hyphal formation, MYO5 being the unique gene encoding myosin I, hereby designated Myo5p (5,41).…”