Previous studies have shown that temperature-sensitive, myo2-66 yeast arrest as large, unbudded cells that accumulate vesicles within their cytoplasm (Johnston, G. C., J. A. Prendergast, and R. A. Singer. 1991. J. Cell Biol. 113:539-551). In this study we show that myo2-66 is synthetically lethal in combination with a subset of the late-acting sec mutations. Thin section electron microscopy shows that the post-Golgi blocked secretory mutants, sec1-1 and sec6-4, rapidly accumulate vesicles in the bud, upon brief incubations at the restrictive temperature. In contrast, myo2-66 cells accumulate vesicles predominantly in the mother cell. Double mutant analysis also places Myo2 function in a post-Golgi stage of the secretory pathway. Despite the accumulation of vesicles in myo2-66 cells, pulse-chase studies show that the transit times of several secreted proteins, including invertase and alpha factor, as well as the vacuolar proteins, carboxy-peptidase Y and alkaline phosphatase, are normal. Therefore the vesicles which accumulate in this mutant may function on an exocytic pathway that transports a set of cargo proteins that is distinct from those analyzed. Our observations are consistent with a role for Myo2 in transporting a class of secretory vesicles from the mother cell along actin cables into the bud.

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