ACTIN-DEPENDENT LOCALIZATION OF AN RNA-ENCODING A CELL-FATE DETERMINANT IN YEAST

The cytoplasmic localization of messenger RNA creates an asymmetric di stribution of proteins that specify cell fate during development in mu lticellular eukaryotes(1,2). The protein Ash1 is a cell-fate determina nt in budding yeast which localizes preferentially to the presumptive daughter nucleus, where it inhibits mating-type switching(3'4). Here w e show that Ash1 mRNA is localized to the distal tip of daughter buds in post-anaphase cells. Three-dimensional imaging reveals that Ash1 mR NA is assembled into particles that associate with the cell cortex, To achieve this localization, Ash1 mRNA must have its 3' untranslated re gion and the actin cytoskeleton must be intact, Ash1 mRNA is not local ized correctly in the absence of a myosin (Myo4) and is mislocalized t o the mother-bud neck in the absence of a regulator of the actin cytos keleton known as Bni1. We propose that Ash1 mRNA particles are transpo rted into the daughter bud along actin filaments and are anchored at t he distal tip. Thus, as in higher eukaryotes, Saccharomyces cerevisiae employs RNA localization to generate an asymmetric distribution of pr oteins and hence to determine cell fate.