Conservation in budding yeast of a kinase specific for SR splicing factors

SR protein kinases (SRPKs) and their substrates, the SR family of serine/ar ginine-rich pre-mRNA splicing factors, appear to be key regulators of alter native splicing. Although SR proteins have been well characterized through biochemical experiments in metazoans, their functions in vivo are unclear. Because of the strict splice site consensus and near absence of alternative splicing in Saccharomyces cerevisiae, it had been thought that budding yea st would lack an SRPK and its substrates. Here, we present structural, bioc hemical, and cell-biological evidence that directly demonstrates an SR prot ein kinase, Sky1p, as well as a number of SRPK substrates in S. cerevisiae. One of these substrates is Np13p, an SR-like protein involved in mRNA expo rt. This finding raises the provocative possibility that Sky1p, and by exte nsion metazoan SRPKs, regulates mRNA export or the nucleocytoplasmic shuttl ing of RS domain proteins. The unexpected discovery of an SR protein kinase in budding yeast provides a foundation for genetic dissection of the biolo gical functions of SR proteins and their kinases.