A specific chaperone for the ribosomal protein S3

Ribosomes are the universal protein synthesis machines consisting of RNA and ribosomal proteins. The highly complex structure of ribosomes is sustained by ribosomal proteins, which interact with the negatively charged ribosomal RNA mainly via ionic interactions. Every minute, 2000 ribosomes have to be synthesized in yeast, therefore the fast production of ribosomal proteins, their efficient delivery to the nucleus and correct incorporation into ribosomal subunits are prerequisites for optimal growth rates. However, due to their high content in positive charges, free ribosomal proteins are prone to aggregation; hence mechanisms have to exist to stabilize these proteins until their incorporation into ribosomes. Our preliminary data show, that the yeast ribosomal protein S3 (Rps3) forms a stable complex with the ankyrin repeat protein Yar1 before its incorporation into ribosomes. This interaction is important for the solubility of Rps3 and consequently likely required for efficient assembly of Rps3 into the ribosome. The deletion of Yar1 leads to pronounced growth defects, probably as a result of delays in ribosome biogenesis. These data suggest that Yar1 acts as a specific stabilizer of soluble Rps3, thereby ensuring efficient delivery of Rps3 to emerging ribosomal subunits. Beside this stabilizing function, Yar1 might also promote import of Rps3 into the nucleus and/or contribute to the incorporation of Rps3 into 40S subunits. The aim of this project is to explore on a mechanistic and functional basis, how newly translated Rps3 is kept soluble and how it is transferred to the rRNA to allow the generation of functional ribosomes. To address these questions, we will employ a combination of structural biological, biochemical, cell biological and genetic approaches. Very likely, beside Rps3, also other ribosomal proteins require specific non-ribosomal binding partners that act as chaperones or assembly factors. Hence, the mechanistic and functional understanding gained from the investigation of the interplay between Yar1 and Rps3 will contribute to a better general understanding of mechanisms how ribosomal proteins are kept stable along their path from translation to assembly into ribosomal subunits.

Ribosomes are highly complex cellular factories, which are responsible for the production of all proteins in a cell. They are composed of a large and a small subunit, which are each built up of two different types of components, ribosomal RNA (rRNA) and ribosomal proteins (r-proteins). While rRNA is negatively charged, r-proteins have many positive charges, allowing them to strongly bind to the rRNA.In our research, we are interested in ribosome biogenesis, the process in which new ribosomes are produced. Ribosome biogenesis is a very important process in every growing cell, as the whole ribosome population of a cell (at least 100,000 ribosomes) has to be duplicated with every cell division. The constituents of ribosomes are produced in two different cellular compartments. While rRNA is synthesized in the nucleus, r-proteins are produced in the cytoplasm and have to be transported into the nucleus, where they bind to the rRNA. Disadvantageously however, r-proteins are unstable before they have bound to their rRNA binding site, suggesting that mechanisms have to exist to protect r-proteins on their path into the nucleus.In this project, we discovered how the r-protein Rps3 is protected and transported to the nucleus. In the cytoplasm, Rps3 is bound by its own specific "bodyguard" the protein Yar1. Yar1 binds to one end of the protein, and shields exposed positive charges in this region. The other end of Rps3 forms a dimer with a second Rps3 protein. Thereby, also this region can be protected. This second Rps3 is then bound by a so called import factor, which is responsible for the transport of the Rps3/Yar1 complex into the nucleus, where Rps3 can be transferred from Yar1 to the rRNAPreviously, ribosome biogenesis was mainly investigated beginning with the stage in which r-proteins bind to rRNA, however, important processes take place before this can happen.By unraveling the path of Rps3 from its synthesis site in the cytoplasm to its binding to rRNA in the nucleus, we paved the way for a new area in ribosome biogenesis research, the study of dedicated protection factors (so called chaperones) for r-proteins.