The Dedicated Chaperone Acl4 Escorts Ribosomal Protein Rpl4 to Its Nuclear Pre-60S Assembly Site

The Roles of Puf6 and Loc1 in 60S Biogenesis Are Interdependent, and Both Are Required for Efficient Accommodation of Rpl43

Puf6 and Loc1 have two important functional roles in the cells, asymmetric mRNA distribution and ribosome biogenesis. Puf6 and Loc1 are localized predominantly in the nucleolus. They bind ASH1 mRNA, repress its translation, and facilitate the transport to the daughter cells. Asymmetric mRNA distribution is important for cell differentiation. Besides their roles in mRNA localization, Puf6 and Loc1 have been shown to be involved in 60S biogenesis. In puf6Δ or loc1Δ cells, pre-rRNA processing and 60S export are impaired and 60S subunits are underaccumulated. The functional studies of Puf6 and Loc1 have been focused on ASH1 mRNA pathway, but their roles in 60S biogenesis are still not clear. In this study, we found that Puf6 and Loc1 interact directly with each other and both proteins interact with the ribosomal protein Rpl43 (L43e). Notably, the roles of Puf6 and Loc1 in 60S biogenesis are interdependent, and both are required for efficient accommodation of Rpl43. Loc1 is further required to maintain the protein level of Rpl43. Additionally, the recruitment of Rpl43 is required for the release of Puf6 and Loc1. We propose that Puf6 and Loc1 facilitate Rpl43 loading and are sequentially released from 60S after incorporation of Rpl43 into ribosomes in yeast.

The eukaryote-specific N-terminal extension of ribosomal protein S31 contributes to the assembly and function of 40S ribosomal subunits

The archaea-/eukaryote-specific 40S-ribosomal-subunit protein S31 is expressed as an ubiquitin fusion protein in eukaryotes and consists of a conserved body and a eukaryote-specific N-terminal extension. In yeast, S31 is a practically essential protein, which is required for cytoplasmic 20S pre-rRNA maturation. Here, we have studied the role of the N-terminal extension of the yeast S31 protein. We show that deletion of this extension partially impairs cell growth and 40S subunit biogenesis and confers hypersensitivity to aminoglycoside antibiotics. Moreover, the extension harbours a nuclear localization signal that promotes active nuclear import of S31, which associates with pre-ribosomal particles in the nucleus. In the absence of the extension, truncated S31 inefficiently assembles into pre-40S particles and two subpopulations of mature small subunits, one lacking and another one containing truncated S31, can be identified. Plasmid-driven overexpression of truncated S31 partially suppresses the growth and ribosome biogenesis defects but, conversely, slightly enhances the hypersensitivity to aminoglycosides. Altogether, these results indicate that the N-terminal extension facilitates the assembly of S31 into pre-40S particles and contributes to the optimal translational activity of mature 40S subunits but has only a minor role in cytoplasmic cleavage of 20S pre-rRNA at site D.

Role of the yeast ribosomal protein L16 in ribosome biogenesis

Most ribosomal proteins play essential roles in ribosome synthesis and function. In this study, we have analysed the contribution of yeast ribosomal protein L16 to ribosome biogenesis. We show that in vivo depletion of the essential L16 protein results in a deficit in 60S subunits and the appearance of half-mer polysomes. This phenotype is likely due to the instability and rapid turnover of early and intermediate pre-60S particles, as evidenced by the reduced steady-state levels of 27SBS and 7SL /S pre-rRNA, and the low amounts of de novo synthesized 27S pre-rRNA and 25S rRNA. Additionally, depletion of L16 blocks nucleocytoplasmic export of pre-60S particles. Moreover, we show that L16 assembles in the nucleolus and binds to early 90S preribosomal particles. Many evolutionarily conserved ribosomal proteins possess extra eukaryote-specific amino- or carboxy-terminal extensions and/or internal loops. Here, we have also investigated the role of the eukaryote-specific carboxy-terminal extension of L16. Progressive truncation of this extension recapitulates, albeit to a lesser extent, the growth and ribosome biogenesis defects of the L16 depletion. We conclude that L16 assembly is a prerequisite to properly stabilize rRNA structures within early pre-60S particles, thereby favouring efficient 27S pre-rRNA processing within the internal transcribed spacer 1 at sites A3 and B1 . Upon depletion of L16, the lack of this stabilization aborts early pre-60S particle assembly and subjects these intermediates to turnover.

Studies on the Coordination of Ribosomal Protein Assembly Events Involved in Processing and Stabilization of Yeast Early Large Ribosomal Subunit Precursors

Cellular production of ribosomes involves the formation of highly defined interactions between ribosomal proteins (r-proteins) and ribosomal RNAs (rRNAs). Moreover in eukaryotic cells, efficient ribosome maturation requires the transient association of a large number of ribosome biogenesis factors (RBFs) with newly forming ribosomal subunits. Here, we investigated how r-protein assembly events in the large ribosomal subunit (LSU) rRNA domain II are coordinated with each other and with the association of RBFs in early LSU precursors of the yeast Saccharomyces cerevisiae. Specific effects on the pre-ribosomal association of RBFs could be observed in yeast mutants blocked in LSU rRNA domain II assembly. Moreover, formation of a cluster of r-proteins was identified as a downstream event in LSU rRNA domain II assembly. We analyzed in more detail the functional relevance of eukaryote specific bridges established by this r-protein cluster between LSU rRNA domain II and VI and discuss how they can support the stabilization and efficient processing of yeast early LSU precursor RNAs.