New roles for the Snp1 and Exo84 proteins in yeast pre-mRNA splicing.

The mammalian 70K protein, a component of the U1 small nuclear ribonucleoprotein involved in pre-mRNA splicing, interacts with a number of proteins important for regulating constitutive and alternative splicing. Similar proteins that interact with the yeast homolog of the 70K protein, Snp1p, have yet to be identified. We used the two-hybrid system to find four U1-Snp1 associating (Usa) proteins. Two of these proteins physically associate with Snp1p as assayed by coimmunoprecipitation. One is Prp8p, a known, essential spliceosomal component. This interaction suggests some novel functions for Snp1p and the U1 small nuclear ribonucleoprotein late in spliceosome development. The other, Exo84p, is a conserved subunit of the exocyst, an eight-protein complex functioning in secretion. We show here that Exo84p is also involved in pre-mRNA splicing. A temperature-sensitive exo84 mutation caused increased ratios of pre-mRNA to mRNA for the Rpl30 and actin transcripts in cells incubated at the non-permissive temperature. The mutation also led to a defect in splicing and prespliceosome formation in vitro; an indication that Exo84p has a direct role in splicing. The results elucidate a surprising link between splicing and secretion.