Yvh 1 is required for a late maturation step in the 60 S biogenesis pathway

Eukaryotic cells expend a substantial amount of cellular energy and resources to manufacture ribosomal subunits (Warner, 1999). To produce a ribosome, eukaryotic cells must assemble >70 ribosomal proteins (r-proteins) with four different ribosomal RNA (rRNA) species (Venema and Tollervey, 1999; Tschochner and Hurt, 2003). The concerted actions of all three transcription machineries (RNA polymerases I, II, and III) need to be coordinated to ensure the high efficiency and accuracy of ribosome production: Pol I produces 25S/28S, 5.8S, and 18S rRNA, Pol III synthesizes 5S rRNA, and Pol II transcribes the mRNAs for the r-proteins. Additionally, the process of ribosome assembly requires numerous nonribosomal factors, termed trans-acting factors (Venema and Tollervey, 1999; Tschochner and Hurt, 2003). The final outcome of this spatially and temporally coordinated effort is that every second, a growing yeast cell delivers up to 40 nascent ribosomal subunits from the nucleolus, the primary site of assembly, to the cytoplasm (Tschochner and Hurt, 2003). Most of our current understanding of this highly dynamic, regulated multistep process comes from studies in budding yeast. The combined use of genetic, cell biological, and proteomic methods has uncovered >150 trans-acting factors that are required for the assembly and maturation of preribosomal particles as they travel from the nucleolus to the cytoplasm (Venema and Tollervey, 1999; Fatica and Tollervey, 2002; FromontRacine et al., 2003; Tschochner and Hurt, 2003; Granneman and Baserga, 2004). Moreover, tandem affinity purification (TAP; Rigaut et al., 1999; Puig et al., 2001) and sensitive mass spectrometric methods have allowed refinement of the ribosome assembly scheme, which was originally proposed upon identification of 90S, 66S, and 43S preribosomal particles in the mid-70s (Trapman et al., 1975). These approaches have uncovered changes in protein and rRNA composition of several distinct and successive pre-40S and -60S ribosomal particles (Bassler et al., 2001; Harnpicharnchai et al., 2001; Fatica et al., 2002; Grandi et al., 2002; Nissan et al., 2002; Saveanu et al., 2003; Schäfer et al., 2003). Nevertheless, the functions of the majority of identified trans-acting factors are only starting to be unraveled. Early pre-60S particles associate with 100 trans-acting factors along their biogenesis pathway (Fatica and Tollervey, 2002; Fromont-Racine et al., 2003; Tschochner and Hurt, 2003). As pre-60S particles travel through the nucleoplasm toward the Before entering translation, preribosomal particles undergo sequential late maturation steps. In the case of pre-60S particles, these steps involve the release of shuttling maturation factors and transport receptors. In this study, we report a new maturation step in the 60S biogenesis pathway in budding yeast. We show that efficient release of the nucleolar/nuclear ribosomallike protein Mrt4 (homologous to the acidic ribosomal P-protein Rpp0) from pre-60S particles requires the highly conserved protein Yvh1, which associates only with late pre-60S particles. Cell biological and biochemical analyses reveal that Mrt4 fails to dissociate from late pre-60S particles in yvh1∆ cells, inducing a delay in nuclear pre– ribosomal RNA processing and a pre-60S export defect in yvh1∆ cells. Moreover, we have isolated gain of function alleles of Mrt4 that specifically bypass the requirement for Yvh1 and rescue all yvh1∆-associated phenotypes. Together, our data suggest that Yvh1-mediated release of Mrt4 precedes cytoplasmic loading of Rpp0 on pre-60S particles and is an obligatory late step toward construction of translation-competent 60S subunits. Yvh1 is required for a late maturation step in the 60S biogenesis pathway