CUS1, A SUPPRESSOR OF COLD-SENSITIVE U2 SNRNA MUTATIONS, IS A NOVEL YEAST SPLICING FACTOR HOMOLOGOUS TO HUMAN SAP-145

The function of U2 snRNA in splicing is mediated by the proteins of th e U2 small nuclear ribonucleoprotein. To identify proteins that influe nce the function of U2 snRNA we carried out a screen for mutations in Saccharomyces cerevisiae that suppress the cold-sensitive growth defec t of a mutation in U2 stem loop IIa, a structure important for the sta ble association of the U2 snRNP with pre-mRNA. The screen identified t hree dominant suppressor genes, one of which, CUS1-54, encodes an esse ntial splicing protein required for U2 snRNP addition to the spliceoso me. The suppressor protein rescues the spliceosome assembly defect of the mutant U2 in vitro, indicating that suppression is direct. Allele specificity tests show that the suppressor does not simply bypass the requirement for U2 stem loop IIa. Extra copies of wild-type CUS1, but not CUS1-54, suppress the temperature-sensitive prp11 and prp5 mutatio ns, linking CUS1 protein to a subset of other factors required at the same step of spliceosome assembly. CUS1 is homologous to SAP 145, a co mponent of the mammalian U2 snRNP that interacts with pre-mRNA. The ye ast genome also encodes a homolog of human SAP 49, a protein that inte racts strongly with both SAP 145 and pre-mRNA, underscoring the conser vation of U2 snRNP proteins that function in spliceosome assembly.