Michal Sharon

Weizmann Institute of Science, Rehovot, Israel

Beyond ubiquitin: The autonomous power of the 20S proteasome in protein degradation

For years, proteasomal degradation was predominantly attributed to the ubiquitin-26S pathway. However, it is now evident that the core 20S proteasome can independently target proteins for degradation. Despite nearly half of cellular proteasomes existing as free 20S complexes, understanding of the regulatory mechanisms and substrate landscape of the 20S proteasome lags behind that of the extensively studied 26S proteasome. In my talk, I will discuss our recent findings on this ubiquitin-independent degradation pathway, shedding light on its substrate landscape and its tight regulation by a novel family of proteins we termed Catalytic Core Regulators (CCRs) (1-3). CCRs employ an allosteric mechanism, wherein the physical interaction with the PSMB4 β-subunit leads to the attenuation of the complex's three proteolytic activities (4). Additionally, through the dissection of structural properties essential for CCR-like function, we successfully replicated this activity using a designed protein half the size of natural CCRs. We also provide insights into the 20S proteasome substrate repertoire, identifying proteins undergoing complete degradation and others experiencing specific proteolytic processing at distinct disordered regions (5). Furthermore, our exploration extends to mapping substrates undergoing proteolysis under basal and oxidative conditions, allowing a comparison of the functionality between naïve and oxidized 20S proteasome complexes. Collectively, our results challenge the notion of 20S proteasome-mediated degradation as a simple and random process, revealing it as a highly regulated and coordinated mechanism.
Selected references
1. Moscovitz O, Tsvetkov P, Hazan N, Michaelevski I, Keisar H, Ben-Nissan G, Shaul Y and M Sharon. (2012). A mutually inhibitory feedback loop between the 20S proteasome and its regulator, NQO1. Mol Cell, 47(1):76-86
2. Moscovitz O, Ben-Nissan G, Polack D, Zaroock O and M Sharon. (2015). The Parkinson's-associated protein DJ-1 regulates the 20S proteasome. Nat Commun, 6:6609
3. Olshina M, Deshmukh FK, Arkind G, Taranavsk M, Hayat D, Ben-Dor, S, Ben-Nissan G and M Sharon. (2020). Regulation of the 20S proteasome by a novel family of inhibitory proteins. ‎Antioxid Redox Signal, 32(9):636-655
4. Deshmukh FK, Ben-Nissan G, Olshina MA, Füzesi-Levi MG, Polkinghorn C, Arkind G, Leushkin Y, Fainer I, Fleishman SJ, Tawfik, D, and M Sharon. (2023). Allosteric regulation of the 20S proteasome by the Catalytic Core Regulators (CCRs) family. Nat Commun, 14, 3126
5. Pepelnjak, M., Rogawski, R., Arkind, G., Leushkin, Y., Fainer, I., Ben-Nissan, G., Picotti, P., and Sharon, M. (2024). Systematic identification of 20S proteasome substrates Mol Syst Biol In Press