Accumulation of H/ACA snoRNPs depends on the integrity of the conserved central domain of the RNA-binding protein Nhp2p

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Abstract
Box H/ACA small nucleolar ribonucleoprotein particles (H/ACA snoRNPs) play key roles in the synthesis of eukaryotic ribosomes. How box H/ACA snoRNPs are assembled remains unknown. Here we show that yeast Nhp2p, a core component of these particles, directly binds RNA. In vitro, Nhp2p interacts with high affinity with RNAs containing irregular stem–loop structures but shows weak affinity for poly(A), poly(C) or for double-stranded RNAs. The central region of Nhp2p is believed to function as an RNA-binding domain, since it is related to motifs found in various RNA-binding proteins. Removal of two amino acids that shortens a putative β-strand element within Nhp2p central domain impairs the ability of the protein to interact with H/ACA snoRNAs in cell extracts. In vivo, this deletion prevents cell viability and leads to a strong defect in the accumulation of H/ACA snoRNAs and Gar1p. These data suggest that proper direct binding of Nhp2p to H/ACA snoRNAs is required for the assembly of H/ACA snoRNPs and hence for the stability of some of their components. In addition, we show that converting a highly conserved glycine residue (G59) within Nhp2p central domain to glutamate significantly reduces cell growth at 30 and 37°C. Remarkably, this modification affects the steady-state levels of H/ACA snoRNAs and the strength of Nhp2p association with these RNAs to varying degrees, depending on the nature of the H/ACA snoRNA. Finally, we show that the modified Nhp2p protein whose interaction with H/ACA snoRNAs is impaired cannot accumulate in the nucleolus, suggesting that only the assembled H/ACA snoRNP particles can be efficiently retained in the nucleolus.