Partial purification of the yeast U2 snRNP reveals a novel yeast pre-mRNA splicing factor required for pre-spliceosome assembly

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Abstract
We have partially purified the U2 snRNP of Saccharomyces cerevisiae . Identification of some proteins consistently found in the purified fractions by nanoelectrospray mass spectrometry indicated the presence of a novel splicing factor named Rse1p. The RSE1 gene is essential and codes for a 148.2 kDa protein. We demonstrated that Rse1p associates specifically with U2 snRNA at low salt concentrations. In addition, we showed that Rse1p is a component of the pre‐spliceosome. Depletion of Rse1p and analysis of a conditional mutant indicated that Rse1p was required for efficient splicing in vivo . In vitro Rse1p is required for the formation of pre‐spliceosomes. Database searches revealed that Rse1p is conserved in humans and that it belongs to a large protein family that includes polyadenylation factors and DNA repair proteins. The characteristics of Rse1p suggest that its human homologue could be a subunit of the SF3 splicing factor.