Identification of RNAs that can accept assembly of an Sm-protein ring

Sm-ring assembly is important for the biogenesis, stability, and function of uridine-rich small nuclear RNAs (U snRNAs) involved in pre-mRNA splicing and histone pre-mRNA 3’ end processing. Assembly of Sm-rings occurs in the cytoplasm and is dependent on a specific sequence and structure motif (Sm-site), ATP, and the Survival motor neuron (SMN) protein complex. The following study informatically investigates the occurence of Sm-sites within the mouse and human transcriptomes and biochemically assesses whether these sites can accept Sm-rings. Sm-sites were found on snRNAs, but are highly prevalent in the 3’ untranslated regions (3’UTR) of long mRNAs. RNA immunoprecipitation experiments confirm that Sm-site containing mRNAs associate with Sm-proteins in the cytoplasm. Established Sm-ring assembly assays were modified to identify polyA-RNAs that specifically associate with Sm-proteins in an ATP-dependent manner, modeling newly assembled Sm-rings. Sm-rings were then specifically assembled onto candidate Sm-site containing mRNAs in an ATP and Sm-site dependent manner. mRNAs containing Sm-sites are down-regulated in models of SMA, suggesting reduced Sm-ring assembly on these mRNAs may contribute to the mechanism of SMA pathogenesis. Together, this study establishes that Sm-site containing mRNAs can accept Sm-rings and identifies a novel mechanism for Sm-proteins in regulation of cytoplasmic mRNAs.