mRNA-DEG-RIBOSOME · Understanding molecular principles of regulated mRNA degradation on translating ribosomes

Understanding molecular principles of regulated mRNA degradation on translating ribosomes

Cellular function fundamentally hinges on maintaining defined protein levels, a process tightly regulated by messenger RNA (mRNA) synthesis and decay. Excess protein production is not only costly to cells but can also be dangerous, because certain surplus proteins tend to misfold, aggregate, and cause disease. Furthermore, novel RNA-based therapeutics such as mRNA vaccines that helped curb the global COVID-19 pandemic (Nobel Prize in Physiology or Medicine 2023) require a deep mechanistic understanding of cellular mRNA regulation to improve innovative drug design. Cells tightly regulate mRNA processing, localisation and stability to ensure accurate gene expression in diverse cellular states and conditions. Most of these mRNA regulation steps have traditionally been thought to primarily happen before translation. However, recent discoveries highlight the role of nascent polypeptides on translating ribosomes in the active regulation of mRNAs. As the nascent protein emerges from the ribosome, it marks the identity of the encoding mRNA by its unique amino acid sequence. This allows cotranslational engagement by specific polypeptide recognition factors on the ribosome, providing them access to the associated mRNA. A striking example for such regulation is the negative feedback loop in the expression of tubulins known as “tubulin autoregulation”. In this case, tubulin mRNA is degraded cotranslationally when cells sense excess free tubulin, which is critical during mitosis to ensure faithful chromosome segregation. Recently, the first specific factor in this pathway has been identified: TTC5 recognises the N-terminus of nascent tubulin emerging from the translating ribosome and triggers degradation of the associated mRNA. However, the events leading to tubulin mRNA decay downstream of TTC5 remained elusive before this project. In mRNA-DEG-RIBOSOME, the tubulin autoregulation pathway was used as a model system to study how the nascent protein chain on translating ribosomes directs mRNA degradation. Specifically, the aims were to: (1) identify factors acting downstream of TTC5 required for tubulin mRNA degradation using cutting-edge mass spectrometry and genetic screening methods; (2) understand the mechanistic role of each factor in TTC5-directed mRNA degradation by using a powerful in vitro reconstitution approach; and (3) expand the understanding of nascent chain-dependent mRNA degradation by investigating related feedback systems. mRNA-DEG-RIBOSOME aimed to establish a conceptual framework for how cells have evolved to exploit nascent polypeptide recognition to direct mRNA fate.

Data: CORDIS, © European Union