NUCLEOLAR KKE D REPEAT PROTEINS NOP56P AND NOP58P INTERACT WITH NOP1PAND ARE REQUIRED FOR RIBOSOME BIOGENESIS/

Different point mutations in the nucleolar protein fibrillarin (Nop1p in Saccharomyces cerevisiae) can inhibit different steps in ribosome s ynthesis. A screen for mutations that are synthetically lethal (sl) wi th the nop1-5 allele, which inhibits prc-rRNA processing, identified N OP56. An independent sl mutation screen with nop1-3, which inhibits pr e-rRNA methylation, identified a mutation in NOP58. Strikingly, Nop56p and Nop58p are highly homologous (45% identity). Both proteins were f ound to be essential and localized to the nucleolus. A temperature-sen sitive lethal mutant allele, nop56-2, inhibited many steps in pre-rRNA processing, particularly on the pathway of 25S/5.8S rRNA synthesis, a nd led to defects in 60S subunit assembly. Epitope-tagged constructs s how that both Nop56p and Nop58p are associated with Nop1p in complexes , Nop56p and Nop1p exhibiting a stoichiometric association. These phys ical interactions presumably underlie the observed sl phenotypes. Well -conserved homologs are present in a range of organisms, including hum ans (52% identity between human hNop56p and yeast Nop56p), suggesting that these complexes have been conserved in evolution.