Fission yeast myosin-I, Myo1p, stimulates actin assembly by Arp2/3 complexand shares functions with WASp

Fission yeast myol(+) encodes a myosin-I with all three tail homology domai ns (TH1, 2, 3) found in typical long-tailed myosin-Is. Myo1p tail also cont ains a COOH-terminal acidic region similar to the A-domain of WASp/Scar pro teins and other fungal myosin-Is. Our analysis shows that Myo1p and Wsp1p, the fission yeast WASp-like protein, share functions and cooperate in contr olling actin assembly. First, Myo1p localizes to cortical patches enriched at tips of growing cells and at sites of cell division. Myo1p patches parti ally colocalize with actin patches and are dependent on an intact actin cyt oskeleton. Second, although deletion of myol(+) is not lethal, Delta myo1 c ells have actin cytoskeletal defects, including loss of polarized cell grow th, delocalized actin patches, and mating defects. Third, additional disrup tion of wsp1(+) is synthetically lethal, suggesting that these genes may sh are functions. In mapping the domains of Myo1p tail that share function wit h Wsp1p, we discovered that a Myo1p construct with just the head and TH1 do mains is sufficient for cortical localization and to rescue all Delta myo1 defects. However, it fails to rescue the Delta myo1 Delta wsp1 lethality. A dditional tail domains, TH2 and TH3, are required to complement the double mutant. Fourth, we show that a recombinant Myo1p tail binds to Arp2/3 compl ex and activates its actin nucleation activity.