CONSERVATION OF STRUCTURE AND SUBUNIT INTERACTIONS IN YEAST HOMOLOGS OF SPLICING FACTOR 3B (SF3B) SUBUNITS

Human SAP49, a subunit of the multimeric splicing factor 3b (SF3b), co ntains two RNA recognition motifs (RRMs) and binds another SF3b subuni t called SAP 145, whose yeast homologue is CUS1. Here we show that the predicted yeast open reading frame YOR319w (HSH49) encodes an essenti al yeast splicing factor. Using bacterially expressed proteins, we fin d that yeast HSH49 binds CUS1. Mutations that alter putative RNA-bindi ng residues of either HSH49 RRM are lethal in vivo, but do not prevent binding to CUS1 in vitro, suggesting that the predicted RNA-binding s urfaces of HSH49 are not required for interaction with CUS1. In vivo i nteraction tests show that HSH49 and CUS1 associate primarily through the N-terminal RRM of HSH49. Recombinant HSH49 protein has a general R NA-binding activity that does not require CUS1. The parallels in struc ture and interaction between two SF3b subunits from yeast implies that the mechanism of SF3b action is highly conserved.