Splicing factor SUP-12 and the molecular complexity of apparent cooperativity

The splicing factor SUP-12 from C. elegans, in combination with either ASD-1 or FOX-1 from the Fox-1 (RBFOX) family, is required for generating a muscle-specific isoform of the fibroblast growth factor receptor EGL-15. Biophysical techniques have revealed the sequence preference for the RNA Recognition Motif (RRM) domain from SUP-12 as well as the structural details of the RNA-bound complex. Detailed genetics have identified a requisite need for the presence of both SUP-12 and ASD-1/FOX-1 to regulate the alternative splicing event, prompting speculation of a cooperative mechanism between these proteins on binding RNA. In contrast, the interplay between SUP-12 and ASD-1 suggests that although the RRM domains from each protein are in direct contact on the egl-15 pre-mRNA, there is no simple contribution of binding cooperativity. Evidence for an independent binding mechanism by SUP-12 and ASD-1 will be discussed, including a model in which both positive and negative contributions are balanced during complex assembly. The ability to monitor tissue-specific alternative splicing in live nematodes will continue to provide a powerful method to test in vivo mechanistic models derived from atomic-level investigation.