Late cytoplasmic maturation of the small ribosomal subunit requires RIO proteins in Saccharomyces cerevisiae.
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.
Vanrobays
Emmanuel
E
Laboratoire de Biologie Moléculaire Eucaryote du CNRS, 118 route de Narbonne, 31062 Toulouse Cedex, France.
Gelugne
Jean-Paul
JP
Gleizes
Pierre-Emmanuel
PE
Caizergues-Ferrer
Michele
M
D001692:Biological Transport; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm; D015966:Gene Expression Regulation, Fungal; D017403:In Situ Hybridization; D005810:Multigene Family; D009687:Nuclear Proteins / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000502:physiology*; D012322:RNA Precursors / Q000378:metabolism; D012331:RNA, Fungal / Q000096:biosynthesis; D012337:RNA, Ribosomal, 18S / Q000096:biosynthesis; D012270:Ribosomes / Q000378:metabolism*; D012441:Saccharomyces cerevisiae / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000302:isolation & purification / Q000502:physiology*
D009687:Nuclear Proteins; D012322:RNA Precursors; D012331:RNA, Fungal; D012337:RNA, Ribosomal, 18S; D029701:Saccharomyces cerevisiae Proteins; D017346:Protein Serine-Threonine Kinases; C462948:Rio1 protein, S cerevisiae; C473025:Rio2 protein, S cerevisiae