A biochemical function for the Sm complex

Within the yeast commitment complex, SmB, SmD1, and SmD3 make direct contact with the pre-mRNA substrate, close to the 5' splice site. Only these three 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 overlapping functions, and cross-linking assays show that the tails make direct contact with the pre-mRNA in a largely sequence-independent manner. Other biochemical assays indicate that they function at least in part to stabilize the U1 snRNP-pre-mRNA interaction. We speculate that this role may be general, and may have even evolved to aid weak intermolecular nucleic acid interactions of only a few base pairs.