THE ROLE OF MYO2, A YEAST CLASS-V MYOSIN, IN VESICULAR TRANSPORT

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 syn thetically lethal in combination with a subset of the late-acting sec mutations. Thin section electron miscoscopy shows that the post-Golgi blocked secretory mutants, secl-1 and sec6-4, rapidly accumulate vesic les in the bud, upon brief incubations at the restrictive temperature. In contrast, myo2-66 cells accumulate vesicles predominantly in the m other cell. Double mutant analysis also places Myo2 function in a post -Golgi stage of the secretory pathway. Despite the accumulation of ves icles in myo2-66 cells, pulse-chase studies show that the transit time s of several secreted proteins, including invertase and alpha factor, as well as the vacuolar proteins, carboxy-peptidase Y and alkaline pho sphatase, are normal. Therefore the vesicles which accumulate in this mutant may function on an exocytic pathway that transports a set of ca rgo proteins that is distinct from those analyzed. Our observations ar e consistent with a role for Myo2 in transporting a class of secretory vesicles from the mother cell along actin cables into the bud.