Question

What signals induce the major subunits of the ribosome to be recruited and the full complex to assemble?

Answer 1

The ribosome is a large two-subunit RNA–protein complex responsible for protein translation in all organisms. In Saccharomyces cerevisiae, the 40S small subunit is composed of 18S rRNA and 33 Small sub unit r-proteins and the 60S large subunit (LSU) contains 25S, 5.8S and 5S rRNAs and 46 LSU r-proteins. Ribosome assembly is a complicated and dynamic process that requires transcription, modification and processing of rRNAs and association of ribosomal proteins (1,2). More than 200 protein assembly factors and numerous small nucleolar RNAs (snoRNAs) function in ribosome assembly. Each subunit matures by forming a series of pre-ribosomal particles that transit from the nucleolus through the nucleoplasm to the cytoplasm.

The 18S, 5.8S and 25S rRNAs are co-transcribed as a long 35S precursor rRNA (pre-rRNA) that additionally contains four external and internal transcribed spacers (ETS and ITS). Electron microscopy images of rDNA chromatin spreads have visualized the early assembly process of ribosomes on the nascent pre-rRNA. The 5΄ growing end of the pre-rRNA transcript is progressively packed into a large knot, which is the earliest precursor of SSU known as the 90S pre-ribosome or SSU processome (3–6). After cleavage of the pre-rRNA at sites A0, A1 and A2, 90S is transformed into a pre-40S ribosomal particle, which is rapidly exported to the cytoplasm for final maturation. Although the co-transcriptional assembly process of the SSU is well established (5,7,8), whether the LSU is assembled co-transcriptionally is less evident. The 90S particle can be cleaved off co-transcriptionally under favorable growth conditions. New terminal knot structures, indicative of nascent pre-60S particles, were only occasionally observed on the remnant transcript harboring 5.8S and 25S rRNAs (5). The SSU is composed of four structurally distinct domains, whereas the LSU is a monolithic structure with six highly intertwined domains. This feature raises a major question as to whether the LSU assembles post-transcriptionally in a cooperative manner on the full length pre-rRNA.

Model of cotranslational folding and assembly of complex subunits

a, Nascent chains emerging from the ribosome exit tunnel are first engaged by ribosome-associated chaperones. Upon emergence of a complete interaction domain the nascent chain is engaged by its complex partner subunit. This engagement remains stable throughout the rest of the ORF translation.

b, The nascent-chain amino acid composition at the ribosome exit tunnel may direct the interplay between Ssb and partner-subunit association. High hydrophobicity and positively charged amino acids (aa) are engaged by Ssb; low hydrophobicity disfavours binding of Ssb at the onset of subunit association, allowing for folding of the interaction domain and subunit joining.

c, Modes of cotranslational assembly: most complexes are assembled in a unidirectional manner, in which one dedicated, fully synthesized subunit engages its nascent partner.

d, Diverging misfolding propensities of complex subunits: subunits engaged as nascent chains are prone to misfolding, whereas their partner subunits are generally more stable.

Co-translational folding and assembly of complex subunits: RAC-Ssb Hsp40/70 1. Ribosome-associated chaperones 2. Partner subunit in complex 3. Chaperones rebinding to the emerging chain 4. Assembled complex release Low Hydrophobicity e Ssb V RAC-Ssb Hsp40/70 Hydrophobic, positively charged aa Hydrophobic, positively charged aa 1. Ribosome-associated chaperones 2. Partner subunit in complex Bi-directional Uni-directional Unassembled subunit misfolding