A dual role for BBP/ScSF1 in nuclear pre-mRNA retention and splicing

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Abstract
The MSL5 gene, which codes for the splicing factor BBP/ScSF1, is essential in Saccharomyces cerevisiae, yet previous analyses failed to reveal a defect in assembly of (pre)‐spliceosomes or in vitro splicing associated with its depletion. We generated 11 temperature‐sensitive (ts) mutants and one cold‐sensitive (cs) mutant in the corresponding gene and analyzed their phenotypes. While all mutants were blocked in the formation of commitment complex 2 (CC2) at non‐permissive and permissive temperature, the ts mutants showed no defect in spliceosome formation and splicing in vitro. The cs mutant was defective in (pre)‐spliceosome formation, but residual splicing activity could be detected. In vivo splicing of reporters carrying introns weakened by mutations in the 5′ splice site and/or in the branchpoint region was affected in all mutants. Pre‐mRNA leakage to the cytoplasm was strongly increased (up to 40‐fold) in the mutants. A combination of ts mutants with a disruption of upf1, a gene involved in nonsense‐mediated decay, resulted in a specific synthetic growth phenotype, suggesting that the essential function of SF1 in yeast could be related to the retention of pre‐mRNA in the nucleus.