Splicing functions of SR proteins. (a) SR proteins (green) bound to an exonic splicing enhancer (ESE) may function in constitutive splicing by interacting with the splicing factors U2AF bound at the upstream 3' splice site and U1 snRNP bound to the downstream 5' splice site. Py represents the polypyrimidine tract, the binding site for U2AF. (b) Exon-independent functions of SR proteins. SR proteins may have two exon-independent functions. SR proteins facilitate splice-site pairing by simultaneously interacting with U1 snRNP and U2AF across the intron. SR proteins also assist in recruiting the U4/U6U5 tri-snRNP. (c) Splicing repression is mediated when SR proteins associate with intronic sequences close to the splice sites. Recruitment of spliceosomal components is inhibited through steric hindrance or nonproductive spliceosomal assembly. Adapted with permission from .