Human H/ACA small nucleolar RNPs and telomerase share evolutionarily conserved proteins NHP2 and NOP10

The H/ACA small nucleolar RNAs (snoRNAs) are involved in pseudouridylation of pre-rRNAs. In the yeast Saccharomyces cerevisiae, four common proteins a re associated with H/ACA snoRNAs: Gar1p, Cbf5p, Nhp2p, and Nop10p. In vitro reconstitution studies showed that four proteins also specifically interac t with H/ACA snoRNAs in mammalian cell extracts. Two mammalian proteins, NA P57/dyskerin (the ortholog of Cbf5p) and hGAR1, have been characterized. In this work we describe properties of hNOP10 and hNHP2, human orthologs of y east Nop10p and Nhp2p, respectively, and further characterize hGAR1. hNOP10 and hNHP2 complement yeast cells depleted of Nhp2p and Nop10p, respectivel y. Immunoprecipitation experiments with extracts from transfected HeLa cell s indicated that epitope-tagged hNOP10 and hNHP2 specifically associate wit h hGAR1 and H/ACA RNAs; they also interact with the RNA subunit of telomera se, which contains an H/ACA-like domain in its 3' moiety. Immunofluorescenc e microscopy experiments showed that hGAR1, hNOP10, and hNHP2 are localized in the dense fibrillar component of the nucleolus and in Cajal (coiled) bo dies. Deletion analysis of hGAR1. indicated that its evolutionarily conserv ed core domain contains all the signals required for localization, but prog ressive deletions from either the N or the C terminus of the core domain ab olish localization in the nucleolus and/or the Cajal bodies.