A U3 snoRNP protein with homology to splicing factor PRP4 and Gβ domains is required for ribosomal RNA processing

Yeast fibrillarin (NOP1) is an evolutionarily conserved, nucleolar protein necessary for multiple steps in ribosome biogenesis. Yeast mutants lacking a functional NOP1 gene can be complemented by human Fibrillarin but are temperature sensitive for growth and impaired in pre-rRNA processing. In order to identify components which interact functionally with human Fibrillarin in yeast, we isolated extragenic suppressors of this phenotype. One dominant suppressor, sof1-56, which is allele-specific for human Fibrillarin and restores growth and pre-RNA processing at 35°C, was cloned by in vivo complementation. The wild-type allele of SOF1 is essential for cell growth and encodes a novel 56 kDa protein. In its central domain, SOF1 contains a repeated sequence also found in β-subunits of trimeric G-proteins and the splicing factor PRP4. A single amino acid exchange in the G(β)-like repeat domain is responsible for the suppressing activity of sof1-56. Indirect immunofluorescence shows that SOF1 is located within the yeast nucleolus. Co-immunoprecipitation demonstrates the physical association of SOF1 with U3 small nucleolar RNA and NOP1. In vivo depletion of SOF1 leads to impaired pre-rRNA processing and inhibition of 18S rRNA production. Thus, SOF1 is a new component of the nucleolar rRNA processing machinery.