Nop10 Is a Conserved H/ACA snoRNP Molecular Adaptor

The H/ACA class of small nucleolar ribonucleoproteins (snoRNPs) is primarily responsible for catalyzing the isomerization of uridine to pseudouridine (Ψ) in ribosomal and other cellular RNAs. Each H/ACA snoRNP consist of four conserved proteins, Cbf5 (the Ψ-synthase), Gar1, Nhp2 (L7Ae in archaea) and Nop10, that assemble onto a unique RNA component (the snoRNA). The smallest of these proteins, Nop10 (~7 kDa), has an essential role in the assembly and activity of these particles and binds directly to the Ψ-synthase to form the minimal active enzyme in archaea. To better understand the conserved function of this protein, we characterized the NMR structure and dynamics of Nop10 proteins from both archaea and yeast. We show that archaeal Nop10 contains a highly stable Zn2+ binding motif that is replaced in eukaryotes by a smaller meta-stable ß-hairpin, while a highly conserved and conformationally dynamic linker connects these motifs to a nascent R-helical structure. Our structural analysis and NMR relaxation data show that these motifs do not interact with each other and tumble independently in solution. Several residues within the archaeal Nop10 Zn2+ binding motif have clear structural and functional roles and are conserved in eukaryotes, yet remain disordered in the free yeast Nop10. We propose that the dynamic structure of Nop10 facilitates an induced-fit recognition with the H/ACA Ψ-synthase and allows it to act as a molecular adaptor for guiding snoRNP assembly in similar fashion in all archaea and eukaryotic organisms