The proteome of cells is synthesized by ribosomes, complex ribonucleoproteins that in eukaryotes contain 79–80 proteins and four ribosomal RNAs (rRNAs) more than 5,400 nucleotides long. How these molecules assemble together and how their assembly is regulated in concert with the growth and proliferation of cells remain important unanswered questions. Here, we review recently emerging principles to understand how eukaryotic ribosomal proteins drive ribosome assembly in vivo. Most ribosomal proteins assemble with rRNA cotranscriptionally; their association with nascent particles is strengthened as assembly proceeds. Each subunit is assembled hierarchically by sequential stabilization of their subdomains. The active sites of both subunits are constructed last, perhaps to prevent premature engagement of immature ribosomes with active subunits. Late-assembly intermediates undergo quality-control checks for proper function. Mutations in ribosomal proteins that affect mostly late steps lead to ribosomopathies, diseases that include a spectrum of cell type–specific disorders that often transition from hypoproliferative to hyperproliferative growth.

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The translation initiation factor eIF4E mediates the binding of the small ribosomal subunit to the cap structure at the 5′ end of the mRNA. In Saccharomyces cerevisiae , the cap-binding protein eIF4E is mainly associated with eIF4G, forming the cap-binding complex eIF4F. Other proteins are detected upon purification of the complex on cap-affinity columns. Among them is p20, a protein of unknown function encoded by the CAF20 gene. Here, we show a negative regulatory role for the p20 protein in translation initiation. Deletion of CAF20 partially suppresses mutations in translation initiation factors. Overexpression of the p20 protein results in a synthetic enhancement of translation mutation phenotypes. Similar effects are observed for mutations in the DED1 gene, which we have isolated as a multicopy suppressor of a temperature-sensitive eIF4E mutation. The DED1 gene encodes a putative RNA helicase of the DEAD-box family. The analyses of its suppressor activity, of polysome profiles of ded1 mutant strains, and of synthetic lethal interactions with different translation mutants indicate that the Ded1 protein has a role in translation initiation in S. cerevisiae .

Yeast cell wall proteins, including Cwp1p and α-agglutinin, could be released by treating the cell wall with either β-1,3- or β-1,6-glucanases, indicating that both polymers are involved in anchoring cell wall proteins. It was shown im-munologically that both β-1,3- and β-1,6-glucan were linked to yeast cell wall proteins, including Cwp1p and α-agglutinin. It was further shown that β-1,3-glucan was linked to the wall protein through a β-1,6-glucan moiety. The β-1,6-glucan moiety could be removed from Cwp1p and other cell wall proteins by cleaving phosphodiester bridges either enzymatically using phosphodiesterases or chemically using ice-cold aqueous hydrofluoric acid. These observations are consistent with the notion that cell wall proteins in Saccharomyces cerevisiae are linked to a β-1,3-/β-1,6-glucan heteropolymer through a phosphodiester linkage and that this polymer is responsible for anchoring cell wall proteins. It is proposed that this polymer is identical to the alkali-soluble β-1,3-/β-1,6-glucan heteropolymer characterized by Fleet and Manners (1976,1977).