RS domains contact the pre-mRNA throughout spliceosome assembly.

SR proteins are essential metazoan splicing factors that contain an RNA-binding domain and an arginine/serine-rich domain that functions to promote assembly of the spliceosome. The prevailing model over the past several years suggests that the RS domains function as protein-interaction domains. However, two new papers from Green et al. demonstrate that these RS domains directly contact the pre-mRNA within the functional spliceosome. The sequential character of these contacts suggests that RS domain interactions with RNA promote spliceosome assembly.