Two yeast La motif-containing proteins are RNA-binding proteins that associate with polyribosomes

We have characterized two Saccharomyces cerevisiae proteins, Sro9p and Slf1 p, which contain a highly conserved motif found in all known La proteins. O riginally described as an autoantigen in patients with rheumatic disease, t he La protein binds to newly synthesized RNA polymerase III transcripts. In yeast, the La protein homologue Lhp1p is required for the normal pathway o f tRNA maturation and also stabilizes newly synthesized U6 RNA. We show tha t deletions in both SXO9 and SLF1 are not synthetically lethal with a delet ion in LHP1, indicating that the three proteins do not function in a single essential process. Indirect immunofluorescence microscopy reveals that alt hough Lhp1p is primarily localized to the nucleus, Sro9p is cytoplasmic. We demonstrate that Sro9p and Slf1p are RNA-binding proteins that associate p referentially with translating ribosomes. Consistent with a role in transla tion, strains lacking either Sro9p or Slf1p are less sensitive than wild-ty pe strains to certain protein synthesis inhibitors. Thus, Sro9p and Slf1p d efine a new and possibly evolutionarily conserved class of La motif-contain ing proteins that may function in the cytoplasm to modulate mRNA translatio n.