A COOPERATIVE INTERACTION BETWEEN U2AF65 AND MBBP SF1 FACILITATES BRANCHPOINT REGION RECOGNITION/

During the early events of pre-mRNA splicing, intronic cis-acting sequ ences are recognized and interact through a network of RNA-RNA, RNA-pr otein, and protein-protein contacts. Recently, rye identified a branch point sequence binding protein in yeast (BBP). The mammalian ortholog (mBBP/SF1) also binds specifically to branchpoint sequences and intera cts with the well studied mammalian splicing factor U2AF65, which bind s to the adjacent polypyrimidine (PY) tract. In this paper we demonstr ate that the mBBP/SF1-U2AF65 interaction promotes cooperative binding to a branchpoint sequence-polyprimidine tract-containing RNA, and we s uggest that this cooperative RNA binding contributes to initial recogn ition of the branchpoint sequence (BPS) during pre-mRNA splicing. We a lso demonstrate the essential nature of the third RED of U2AF65 for th e interaction between the two proteins, both in the presence and absen ce of RNA.