A biochemical function for the Sm complex

Within the yeast commitment complex, SmB, SmD1, and SmD3 make direct contac t with the pre-mRNA substrate, close to the 5' splice site. Only these thre e Sm proteins have long and highly charged C-terminal tails, in metazoa as well as in yeast. We replaced these proteins with tail-truncated versions. Genetic assays demonstrate that the tails contribute to similar and overlap ping functions, and cross-linking assays show that the tails make direct co ntact with the pre-mRNA in a largely sequence-independent manner. Other bio chemical assays indicate that they function at least in part to stabilize t he U1 snRNP-pre-mRNA interaction. We speculate that this role may be genera l, and may have even evolved to aid weak intermolecular nucleic acid intera ctions of only a few base pairs.