A novel U2 and U11/U12 snRNP protein that associates with the pre-mRNA branch site

Previous UV cross-linking studies demonstrated that, upon integration of th e U2 snRNP into the spliceosome, a 14 kDa protein (p14) interacts directly with the branch adenosine, the nucleophile for the first transesterificatio n step of splicing. We have identified the cDNA encoding this protein by mi crosequencing a 14 kDa protein isolated from U2-type spliceosomes. This pro tein contains an RNA recognition motif and is highly conserved across speci es. Antibodies raised against this cDNA-encoded protein precipitated the 14 kDa protein cross-linked to the branch adenosine, confirming the identity of the p14 cDNA. A combination of immunoblotting, protein microsequencing a nd immunoprecipitation revealed that p14 is a component of both 17S U2 and 18S U11/U12 snRNPs, suggesting that it contributes to the interaction of th ese snRNPs with the branch sites of U2- and U12-type pre-mRNAs, respectivel y. p14 was also shown to be a subunit of the heteromeric splicing factor SF 3b and to interact directly with SF3b155. Immunoprecipitations indicated th at p14 is present in U12-type spliceosomes, consistent with the idea that b ranch point selection is similar in the major and minor spliceosomes.