U2AF35 Background Information SF3b is a U2 snRNP-associated protein complex essential for spliceosome assembly (1). SF3b contains the spliceosomal proteins SAPs 49, 130, 145 and 155 (1,2). SAPs 130, 145 and 155 associate with one another to form a complex that is present in HeLa nuclear extracts (1,2). SAPs 49 and 145 are known to interact directly with each other (1,3,4). Unexpectedly, the SAP 49-SAP 145 protein-protein interaction requires the amino-terminus of SAP 49, which contains two RNA-recognition motifs (5). SAP 49 and SAP 145 interact directly with both U2 snRNP and the pre-mRNA, which suggests that this protein complex plays a role in tethering U2 snRNP to the branch site (5). U2AF recruits SAP 49 to the branch point sequence during the initial steps of spliceosome assembly (6). U2AF exists as a heterodimer consisting of U2AF65 and U2AF35 and is required for splicing in vivo (7,8).
