An exclusively nuclear RNA-binding protein affects asymmetric localizationof ASH1 mRNA and Ash1p in yeast

The localization ofASHI mRNA to the distal tip of budding yeast cells is es sential for the proper regulation of mating type switching in Saccharomyces cerevisiae. A localization element that is predominantly in the 3 ' -untra nslated region (UTR) can direct this mRNA to the bud. Using this element in the three-hybrid in vivo RNA-binding assay, we identified a protein, Loc1p , that binds in vitro directly to the wildtype ASH1 3 ' -UTR RNA, but not t o a mutant RNA incapable of localizing to the bud nor to several other mRNA s. LOC1 codes for a novel protein that recognizes double-stranded RNA struc tures and is required for efficient localization of ASH1 mRNA. Accordingly, Ash1p gets symmetrically distributed between daughter and mother cells in a loci strain. Surprisingly, Loc1p was found to be strictly nuclear, unlike other known RNA-binding proteins involved in mRNA localization which shutt le between the nucleus and the cytoplasm. We propose that efficient cytopla smic ASH1 mRNA localization requires a previous interaction with specific n uclear factors.