RNA SPLICING SPECIFICITY DETERMINED BY THE COORDINATED ACTION OF RNA RECOGNITION MOTIFS IN SR PROTEINS

Pre-mRNA splicing requires a large number of RNA-binding proteins that have one or more RNA-recognition motifs (RRMs). Among these is the SR protein family, whose members are essential for splicing and are able to commit pre-mRNAs to the splicing pathway with overlapping but dist inct substrate specificity. Some SR proteins, such as SC35, contain an N-terminal RRM and a C-terminal arginine/serine-rich (RS) domain, whe reas others, such as SF2/ASF, also contain a second, atypical RRM. Alt hough both the RRMs and the RS domain of SR proteins are required for constitutive splicing, it is unclear which domain(s) defines their sub strate specificity, and whether two RRMs in a given SR protein functio n independently or act coordinately. Using domain swaps between SC35 a nd SF2/ASF and a functional commitment assay, we demonstrate that indi vidual domains are functional modules, RS domains are interchangeable, and substrate specificity is defined by the RRMs. The atypical RRM of SF2/ASF does not appear to function alone in splicing, hut can either activate or suppress the splicing specificity of an N-terminal RRM. T herefore, multiple RRMs in SR proteins act coordinately to achieve a u nique spectrum of pre-mRNA substrate specificity.