SEC3P IS A SPATIAL LANDMARK FOR POLARIZED SECRETION IN BUDDING YEAST

Exocytosis in yeast occurs at plasma membrane subdomains whose locatio ns vary with the cell cycle, but the primary protein determinants of t hese sites are unknown. A functional fusion of Sec3 protein with green fluorescent protein (Sec3-GFP) localizes to the site of polarized exo cytosis for each cell-cycle stage, where it colocalizes with Sec4p and Sec8p. Sec3-GFP localization is independent of secretory pathway func tion, of the actin and septin cytoskeletons, and of the polarity estab lishment proteins. We propose that Sec3p is a spatial landmark definin g sites of exocytosis. Polarized secretion would result from the coupl ing of actin-dependent vesicle targeting with Sec3p-dependent establis hment of the vesicle fusion site.