Role of actin and Myo2p in polarized secretion and growth of Saccharomycescerevisiae

We examined the role of the actin cytoskeleton in secretion in Saccharomyce s cerevisiae with the use of several quantitative assays, including time-la pse video microscopy of cell surface growth in individual living cells. In latrunculin, which depolymerizes filamentous actin, cell surface growth was completely depolarized but still occurred, albeit at a reduced level. Thus , filamentous actin is necessary for polarized secretion but not for secret ion per se. Consistent with this conclusion, latrunculin caused vesicles to accumulate at random positions throughout the cell. Cortical actin patches cluster at locations that correlate with sites of polarized secretion. How ever, we found that actin patch polarization is not necessary for polarized secretion because a mutant, bee1 Delta(las17 Delta), which completely lack s actin patch polarization, displayed polarized growth. In contrast, a muta nt lacking actin cables, tpm1-2 tpm2 Delta, had a severe defect in polarize d growth. The yeast class V myosin Myo2p is hypothesized to mediate polariz ed secretion. A mutation in the motor domain of Myo2p, myo2-66, caused grow th to be depolarized but with only a partial decrease in the level of overa ll growth. This effect is similar to that of latrunculin, suggesting that M yo2p interacts with filamentous actin. However, inhibition of Myo2p functio n by expression of its tail domain completely abolished growth.