A well-connected and conserved nucleoplasmic helicase is required for production of box C/D and H/ACA snoRNAs and localization of snoRNP proteins

Biogenesis of small nucleolar RNA-protein complexes (snoRNPs) consists of s ynthesis of the snoRNA and protein components, snoRNP assembly, and localiz ation to the nucleolus. Recently, two nucleoplasmic proteins from mice were observed to bind to a model box C/D snoRNA in vitro, suggesting that they function at an early stage in snoRNP biogenesis. Both proteins have been de scribed in other contexts. The proteins, called p50 and p55 in the snoRNA b inding study, are highly conserved and related to each other. Both have Wal ker A and B motifs characteristic of ATP- and GTP-binding and nucleoside tr iphosphate-hydrolyzing domains, and the mammalian orthologs have DNA helica se activity in vitro. Here, we report that the Saccharomyces cerevisiae ort holog of p50 (Rvb2, Tih2p, and other names) is required for production of C /D snoRNAs in vivo and, surprisingly, H/ACA snoRNAs as well. Point mutation s in the Walker A and B motifs cause temperature-sensitive or lethal growth phenotypes and severe defects in snoRNA accumulation. Notably, depletion o f p50 (called Rvb2 in this study) also impairs localization of CID and H/AC A core snoRNP proteins Nop1p and Gar1p, suggesting a defect(s) in snoRNP as sembly or trafficking to the nucleolus. Findings from other studies link Rv b2 orthologs with chromatin remodeling and transcription. Taken together, t he present results indicate that Rvb2 is involved in an early stage of snoR NP biogenesis and may play a role in coupling snoRNA synthesis with snoRNP assembly and localization.