LOSl Gene Involved in Pre-tRNA Splicing Encodes a Nuclear Protein That Behaves as a Component of the Nuclear Matrix*

Mutations of the Saccharomyces cerevisiae LOSl gene cause the accumulation of end matured introncontaining pre-tRNAs at elevated temperatures. In an effort to decipher the role of the LOSl protein in pretRNA splicing, we have analyzed the LOSl gene and its protein product. The LOSl gene is located on the left arm of chromosome XI and the order of genes in this area of the chromosome is . . . . URAl . . .SAC1 TRP3 UBAl STE6 LOSl . . . . F A S l . . , . . The LOSl open reading frame encodes a putative protein of 1100 amino acids that shows no significant homology to other genes. The LOSl open reading frame was tagged with the influenza virus hemagglutinin epitope recognized by the 12CA5 antibody. The 12CA5 antibody recognizes an epitope-tagged protein of the size predicted by the LOSl open reading frame. Using this antibody for indirect immunofluorescence and cell fractionation studies we show that the LOSl protein is located in nuclei. Loslp cannot be extracted from nuclei by treatment with nucleases, salts, or Triton X100. This insolubility suggests that Loslp is a component of the nucleoskelton. We propose that LOSl mutations may affect pre-tRNA processing via alteration of the nuclear matrix.