Analysis of eukaryotic ribosome biogenesis in Saccharomyces cerevisiae

Assembly of eukaryotic ribosomes

Ribosomes are the molecular machines devoted to the synthesis of all cellular proteins from mRNA templates. The process of ribosome biogenesis is evolutionarily conserved among eukaryotes and constitutes a main cellular activity. Most of our current knowledge about this complicated process comes from studies with the yeast Saccharomyces cerevisiae. Research over the last 30 years has revealed that numerous biogenesis factors (>200), including many energy-consuming enzymes and quality-controlling checkpoint factors, are required for the accurate and efficient maturation of pre-ribosomal particles as they travel from the nucleolus to the cytoplasm. Recent advances in cryo-electron microscopy have started to provide a structural view of ribosome assembly by visualizing several assembly intermediates at near-atomic resolution. Moreover, we have only recently learnt that dedicated chaperones, which in many cases already capture their client in a co-translational manner, selectively protect and facilitate the assembly of individual ribosomal proteins. Despite the enormous progress in understanding how this gigantic molecular jigsaw puzzle is pieced together, the precise role of many biogenesis factors and the molecular mechanisms driving ribosome assembly remain in many instances to be determined.

• Investigation of early pre-60S maturation events

  The aim of our research is to provide molecular insight into selected aspects of eukaryotic ribosome biogenesis. The early steps of pre-60S maturation are particularly poorly understood. We have previously shown that the AAA-type ATPase Rix7 is required for the release of Nsa1 from a nucleolar pre-60S particle. By performing an ‘in vivo structure probing’ approach, we have identified mutations in several early-acting pre-60S biogenesis factors that bypass the requirement for the essential Nsa1. We expect that the careful dissection of this interaction network will unveil the assembly alterations that compensate for the lack of Nsa1 recruitment and illuminate the earliest phase of pre-60S maturation.

• Hold on to your friends – Dedicated chaperones of ribosomal proteins

  A detailed knowledge of eukaryotic ribosome assembly is instrumental to eventually understand and treat ribosomopathies, diseases frequently caused by altered functionalities of ribosomal proteins (r-proteins). Eukaryotic ribosomes, whose assembly takes primarily place in the nucleolus, are composed of four ribosomal RNAs and around 80 r-proteins. Actively growing yeast cells must produce more than 160’000 r-proteins per minute in order to meet the cellular demand for new ribosomes. Most of these r-proteins have to travel from the cytoplasm to their incorporation site on pre-ribosomes within the nucleus. Due to their difficult physicochemical properties, r-proteins are especially prone to aggregation and, hence, the synthesis of assembly-competent r-proteins represents a major challenge. Recent evidence from our and other laboratories has revealed that r-proteins, besides relying on the general chaperone and transport systems, may also employ specialized binding partners, termed dedicated chaperones, in order to ensure their soluble expression, nuclear import and/or correct assembly into pre-ribosomes.

  Notably, our work has highlighted that several r-proteins are captured by their dedicated chaperones in a co-translational manner. Such a co-translational capturing mechanism appears to be an advantageous strategy in order to provide sufficient amounts of assembly-competent r-proteins.

  Our ongoing studies are aimed at identifying novel dedicated chaperones and subsequently at defining their relevance for the life cycle of their r-protein client. By determining the co-translational capturing potential and the structural basis of the interaction, we expect to obtain insight into the timing and mode of r-protein recognition as well as the mechanism of r-protein assembly into pre-ribosomes.