PUB1 IS A MAJOR NUCLEAR AND CYTOPLASMIC POLYADENYLATED RNA-BINDING PROTEIN IN SACCHAROMYCES-CEREVISIAE

Proteins that directly associate with nuclear polyadenylated RNAs, or heterogeneous nuclear RNA-binding proteins (hnRNPs), and those that as sociate with cytoplasmic mRNAs, or mRNA-binding proteins (mRNPs), play important roles in regulating gene expression at the posttranscriptio nal level. Previous work with a variety of eukaryotic cells has demons trated that hnRNPs are localized predominantly within the nucleus wher eas mRNPs are cytoplasmic. While studying proteins associated with pol yadenylated RNAs in Saccharomyces cerevisiae, we discovered an abundan t polyuridylate-binding protein, PUB1, which appears to be both an hnR NP and an mRNP. PUB1 and PAB1, the polyadenylate tail-binding protein, are the two major proteins cross-linked by UV light to polyadenylated RNAs in vivo. The deduced primary structure of PUB1 indicates that it is a member of the ribonucleoprotein consensus sequence family of RNA -binding proteins and is structurally related to the human hnRNP M pro teins. Even though the PUB1 protein is a major cellular polyadenylated RNA-binding protein, it is nonessential for cell growth. Indirect cel lular immunofluorescence combined with digital image processing allowe d a detailed comparison of the intracellular distributions of PUB1 and PAB1. While PAB1 is predominantly, and relatively uniformly, distribu ted within the cytoplasm, PUB1 is localized in a nonuniform pattern th roughout both the nucleus and the cytoplasm. The cytoplasmic distribut ion of PUB1 is considerably more discontinuous than that of PAB1. Furt hermore, sucrose gradient sedimentation analysis demonstrates that PAB 1 cofractionates with polyribosomes whereas PUB1 does not. These resul ts suggest that PUB1 is both an hnRNP and an mRNP and that it may be s tably bound to a translationally inactive subpopulation of mRNAs withi n the cytoplasm.