The proteasome is a large protein complex responsible for the degradation of intracellular proteins in eukaryotic cells. It plays a crucial role in maintaining cellular homeostasis by removing damaged, misfolded, or unwanted proteins. The proteasome recognizes and degrades specific protein substrates through a highly regulated process involving ubiquitination and recognition by the proteasome subunits.1. Ubiquitination: The first step in the degradation process is the covalent attachment of a small protein called ubiquitin to the target protein. This process is mediated by a series of enzymes: E1 ubiquitin-activating enzyme , E2 ubiquitin-conjugating enzyme , and E3 ubiquitin ligase . The E3 ligase is responsible for substrate recognition and transfers the ubiquitin from the E2 enzyme to the target protein. This results in the formation of a polyubiquitin chain on the target protein, which serves as a signal for proteasomal degradation.2. Recognition by the proteasome: The 26S proteasome is composed of two main subcomplexes: the 20S core particle CP and the 19S regulatory particle RP . The 19S RP recognizes the polyubiquitinated protein and binds to it. The RP contains several subunits with ubiquitin-binding domains, such as Rpn1, Rpn10, and Rpn13, which facilitate the recognition of the ubiquitin chain on the target protein.3. Unfolding and translocation: Before the target protein can be degraded, it must be unfolded and translocated into the 20S CP. The 19S RP contains ATPase subunits Rpt1-6 that use the energy from ATP hydrolysis to unfold the target protein and translocate it into the 20S CP.4. Proteolysis: Once inside the 20S CP, the target protein is degraded by the proteolytic active sites located within the -subunits. The proteasome cleaves the protein into small peptides, which are then released from the proteasome and further degraded by cellular peptidases into amino acids that can be reused for protein synthesis.5. Deubiquitination and recycling: The ubiquitin chains attached to the target protein are removed by deubiquitinating enzymes DUBs associated with the 19S RP before the protein is degraded. This process allows the recycling of ubiquitin molecules for further rounds of ubiquitination and degradation.In summary, the proteasome recognizes and degrades specific protein substrates through a highly regulated process involving ubiquitination, recognition by the proteasome subunits, unfolding and translocation of the target protein, proteolysis, and recycling of ubiquitin molecules. This ensures that only proteins marked for degradation are targeted, maintaining cellular homeostasis and protein quality control.