Numerous nonribosomal trans-acting factors involved in pre-rRNA processing have been characterized, but few of them are specifically required for the last cytoplasmic steps of 18S rRNA maturation. We have recently demonstrated that Rrp10p/Rio1p is such a factor. By BLAST analysis, we identified the product of a previously uncharacterized essential gene, YNL207W/RIO2, called Rio2p, that shares 43% sequence similarity with Rrp10p/Rio1p. Rio2p homologues were identified throughout the Archaea and metazoan species. We show that Rio2p is a cytoplasmic-nuclear protein and that its depletion blocks 18S rRNA production, leading to 20S pre-rRNA accumulation. In situ hybridization reveals that in Rio2p-depleted cells, 20S pre-rRNA localizes in the cytoplasm, demonstrating that its accumulation is not due to an export defect. We also show that both Rio1p and Rio2p accumulate in the nucleus of crm1-1 cells at the nonpermissive temperature. Nuclear as well as cytoplasmic Rio2p and Rio1p cosediment with pre-40S particles. These results strongly suggest that Rio2p and Rrp10p/Rio1p are shuttling proteins which associate with pre-40S particles in the nucleus and they are not necessary for export of the pre-40S complexes but are absolutely required for the cytoplasmic maturation of 20S pre-rRNA at site D, leading to mature 40S ribosomal subunits.Seen from a home economics point of view, making ribosomes is the main task of a living cell (36, 37). Synthesis of the ribosome constituents, their processing and assembly into mature particles, and the regulation of the entire process require the participation of numerous factors. Surprisingly, until relatively recently, few players involved in these processes had been identified. However, in the past decade, by means of genetic and biochemical tricks, and thanks to the tractability of the yeast Saccharomyces cerevisiae and, more recently, with in silico searches, dozens of factors (snoRNAs or protein factors) implicated in ribosome biogenesis have been characterized. For most of them, however, their molecular function in this process is not known.During the past year, systematic analyses of protein complexes in S. cerevisiae by tandem affinity chromatography purification (3,7,11,14,16) as well as proteomic analysis of the nucleolus of human cells (1) and definition of putative modular transcriptional networks by computer assisted analysis of the transcriptional expression patterns (18,20) led to a burst of new putative players. Through these approaches, new factors associated with the pre-60S particle, precursor of the large ribosomal subunit (LSU) and thus putatively involved in the maturation-assembly pathway of the LSU, have been identified. Likewise, factors associated with the early processing complexes containing the large 35S pre-rRNA have been characterized (11) and shown to include mostly proteins specifically required for small ribosomal subunit (SSU) production. In contrast, the nonribosomal proteins found in late nucleoprotein complex precursors of the SSU have not been ide...

Numerousnon-ribosomal trans-acting factors involved in pre-ribosomal RNA processing have been characterized, but none of them is speci®cally required for the last cytoplasmic steps of 18S rRNA maturation. Here we demonstrate that Rio1p/Rrp10p is such a factor. Previous studies showed that the RIO1 gene is essential for cell viability and conserved from archaebacteria to man. We isolated a RIO1 mutant in a screen for mutations synthetically lethal with a mutant allele of GAR1, an essential gene required for 18S rRNA production and rRNA pseudouridylation. We show that RIO1 encodes a cytoplasmic non-ribosomal protein, and that depletion of Rio1p blocks 18S rRNA production leading to 20S pre-rRNA accumulation. In situ hybridization reveals that, in Rio1p depleted cells, 20S pre-rRNA localizes in the cytoplasm, demonstrating that its accumulation is not due to an export defect. This strongly suggests that Rio1p is involved in the cytoplasmic cleavage of 20S pre-rRNA at site D, producing mature 18S rRNA. Thus, Rio1p has been renamed Rrp10p (ribosomal RNA processing #10). Rio1p/Rrp10p is the ®rst non-ribosomal factor characterized speci®cally required for 20S pre-rRNA processing.

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