Through a genetic screen to search for factors that interact with Prp17/Cdc40p, a protein involved in both cell cycle progression and pre-mRNA splicing, we identify three novel factors, which we call Syf1p, Syf2p, and Syf3 (SYnthetic lethal with cdc Forty). Here we present evidence that all three proteins are spliceosome associated, that they associate weakly or transiently with U6 and U5 snRNAs, and that Syf1p and Syf3p (also known as Clf1p) are required for pre-mRNA splicing. In addition we show that depletion of Syf1p or Syf3p results in cell cycle arrest at the G2/M transition. Thus, like Prp17/Cdc40p, Syf1p and Syf3p are involved in two distinct cellular processes. We discuss the likelihood that Syf1p, Syf2p, and Syf3p are components of a protein complex that assembles into spliceosomes and also regulates cell cycle progression.
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