THE LA PROTEIN IN SCHIZOSACCHAROMYCES-POMBE - A CONSERVED YET DISPENSABLE PHOSPHOPROTEIN THAT FUNCTIONS IN TRANSFER-RNA MATURATION

Most RNA polymerase III transcripts are bound immediately after synthe sis by an abundant nuclear phosphoprotein known as the La autoantigen. Experiments performed in the budding yeast Saccharomyces cerevisiae h ave revealed that binding of the La protein to tRNA precursors is requ ired for the endonucleolytic maturation of the 3' terminus of many tRN As. In the absence of this protein, the 3' ends of these tRNAs are tri mmed by exonucleases (Yoo CJ, Wolin SL, 1997, Cell 89:393-402). Here w e report the characterization of the La protein in the fission yeast S chizosaccharomyces pombe. As was described for budding yeast, S. pombe cells lacking the La protein are viable and exhibit alterations in th e pathway of pre-tRNA maturation. Introduction of either the human, S. cerevisiae, or S. pombe La protein into these cells restores the dete cted pattern of tRNA processing intermediates to that of wild-type cel ls. By performing immunoprecipitations from cells that were metabolica lly labeled with P-32-orthophosphate, we demonstrate that the S. pombe and S. cerevisiae La proteins, like the human La protein, are phospho rylated in vivo. Thus, although the La protein is dispensable for grow th in these yeasts, both the structure of the protein and its function in pre-tRNA maturation have been highly conserved throughout evolutio n.