Tsr4, the dedicated chaperone of ribosomal protein S2

Ribosomes are cellular nano-machines responsible for the synthesis of proteins. Each cell contains 100,000s of ribosomes. In our research group, we are investigating how these ribosomes are synthesized in the cell. One part of this process is the production of the individual building blocks of a ribosome, namely ~80 different ribosomal proteins and 4 different ribosomal RNAs. While the RNAs are produced in the nucleus, the ribosomal proteins are made in the cytoplasm and are then transported into the nucleus where they are joined together with the RNAs. In proliferating cells, numerous ribosomal proteins have to be produced each minute and need to be transported efficiently to the nucleus. However, ribosomal proteins have some unfavorable features that can prevent their delivery at their destination site. In particular, they have many positive charges, which can lead to their aggregation, making the proteins infunctional. Such aggregation can be prevented by dedicated chaperones, private bodyguards that protect individual ribosomal proteins from aggregation and help them to become efficiently incorporated into new ribosomes. The goal of this project is to gain a better understanding of how dedicated chaperones function. As a paradigm, we want to examine one of these dedicated chaperones in more depth, Tsr4. We have recently identified Tsr4 to be a dedicated chaperone for the ribosomal protein S2. We found that Tsr4 protects S2 already as soon as it is being synthesized. Cells lacking Tsr4 are inviable, indicating that the function of Tsr4 is very important. In this project, we are investigating how Tsr4 binds to S2 and how it can thereby protect the ribosomal protein. Moreover, we will assess how Tsr4 and S2 are transported into the nucleus together and how Tsr4 helps to incorporate S2 into nascent ribosomes. Dedicated ribosomal protein chaperones are a so far only poorly characterized protein group. Therefore, our research is expected to yield many novel insights into the functions of these proteins. This will give insights into how dedicated chaperones manage to make the central cellular process of ribosome biogenesis so efficient. Notably, mutations in dedicated ribosomal protein chaperones can cause the bone marrow disease Diamond Blackfan Anemia. Therefore, our work is also important to understand the molecular causes for diseases caused by defective dedicated chaperones.

How ribosomebuilding blocks safely reach the cellnucleus Ribosomes are tiny molecular machines in our cells that produce every protein we need to live. Because proteins are in constant demand, each cell houses hundreds of thousands of ribosomes. Every ribosome is built from four ribosomal RNAs (rRNAs) plus about 80 distinct ribosomal proteins.While rRNAs are produced inside the nucleus, the ribosomal proteins originate in the cytoplasm and must then travel into the nucleus, where ribosome assembly takes place. That journey is anything but straightforward. Ribosomal proteins have tricky biochemical properties that make them prone to clumping or sticking to the wrong partners-much like puzzle pieces gluing together before reaching the puzzle board. To prevent such gluing, cells employ escort proteins called chaperones. Acting as personal bodyguards, they stabilize individual ribosomal proteins and guide them safely into the nucleus. Our project investigated how one particular ribosomal protein, Rps2, completes this trip and what role its dedicated chaperone, Tsr4, plays. We discovered that Rps2 is ferried into the nucleus by the transport protein Pse1. Pse1 recognizes and grips two specific regions of Rps2-one in the middle and one right at the protein's very beginning (termed N-terminus). Fascinatingly, the N-terminal "landing pad" of Rps2 can be chemically modified inside the cell by the addition of a small methyl group. This methylation weakens Pse1's hold on Rps2. Methylation occurs especially at low temperatures, and may serve as a brake that slows ribosome production when it is cold. Intriguingly, Tsr4 binds to exactly the same N terminal region of Rps2. Our data show that Tsr4 and Pse1 compete for this single site-they cannot attach simultaneously. We therefore propose a relay model: freshly made Rps2 is first shielded by Tsr4 and then handed over to Pse1, which completes the journey into the nucleus. Methylation likely acts as a molecular switch that fine tunes this hand off. The mechanisms we uncovered ensure that cells can build new ribosomes efficiently and at the right pace. Understanding these fundamental steps also sheds light on diseases in which ribosome production is disturbed-such as certain bone marrow disorders or cancers where ribosome formation is abnormally increased.