The UPS is a multistep process involving the tagging of proteins with ubiquitin, a small regulatory protein. Ubiquitination is mediated by three main enzymes: E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase). Once tagged, proteins are recognized by the proteasome, a large protease complex, which degrades the ubiquitinated proteins into peptides. This system is essential for regulating the concentrations of specific proteins and thus plays a critical role in various cellular processes.
