SNT309P, A COMPONENT OF THE PRP19P-ASSOCIATED COMPLEX THAT INTERACTS WITH PRP19P AND ASSOCIATES WITH THE SPLICEOSOME SIMULTANEOUSLY WITH ORIMMEDIATELY AFTER DISSOCIATION OF U4 IN THE SAME MANNER AS PRP19P

The yeast protein Prp19p is essential for pre-mRNA splicing and is ass ociated with the spliceosome concurrently with or just after dissociat ion of U4 small nuclear RNA. in splicing extracts, Prp19p is associate d with several other proteins in a large protein complex of unknown fu nction, hut at least one of these proteins is also essential for splic ing (W.-Y. Tarn, C.-H. Hsu, K.-T. Huang, H.-R. Chen, H.-Y. Kao, K.-R. Lee, and S.-C. Cheng, EMBO J. 13:2421-2431, 1994). To identify protein s in the Prp19p-associated complex, we have isolated trans-acting muta tions that exacerbate the phenotypes of conditional alleles of prp19, using the ade2-ade3 sectoring system, A novel splicing factor, Snt309p , was identified through such a screen, Although the SNT309 gene,vas n ot essential for growth of Saccharomyces cerevisiae under normal condi tions, yeast cells containing a null allele of the SNT309 gene were te mperature sensitive and accumulated pre-mRNA at the nonpermissive temp erature, Far-Western blot analysis revealed direct interaction between Prp19p and Snt309p. Snt309p was shown to be a component of the Prp19p -associated complex by Western blot analysis. Immunoprecipitation stud ies demonstrated that Snt309p was also a spliceosomal component and as sociated with the spliceosome in the same manner as Prp19p during spli ceosome assembly. These results suggest that the functions of Prp19p a nd Snt309p in spicing may require coordinate action of these two prote ins.