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

The mammalian 70K protein, a component of the U1 small nuclear ribonucleopr otein involved in pre-mRNA splicing, interacts with a number of proteins im portant for regulating constitutive and alternative splicing. Similar prote ins that interact with the yeast homolog of the 70K protein, Snp1p, have ye t to be identified. We used the two-hybrid system to find four (U) under ba r1-(S) under bar np1 (a) under bar ssociating (Usa) proteins. Two of these proteins physically associate with Snp1p as assayed by coimmunoprecipitatio n. One is Prp8p, a known, essential spliceosomal component. This interactio n suggests some novel functions for Snp1p and the U1 small nuclear ribonucl eoprotein late in spliceosome development. The other, Exo84p, is a conserve d subunit of the exocyst, an eight-protein complex functioning in secretion . We show here that Exo84p is also involved in pre-mRNA splicing. A tempera ture-sensitive exo84 mutation caused increased ratios of pre-mRNA to mRNA f or the Rp130 and actin transcripts in cells incubated at the non-permissive temperature. The mutation also led to a defect in splicing and prespliceos ome formation in vitro; an indication that Exo84p has a direct role in spli cing. The results elucidate a surprising link between splicing and secretio n.