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The 26S proteasome consists of the 20S core particle and 19S regulatory caps. The 20S core is a hollow barrel-shaped cylinder containing stacked rings with proteolytic active sites. The 19S caps are ring-shaped protein structures attached at one or both ends that regulate proteasome activity and recognize ubiquitin-tagged proteins destined for degradation.

The proteasome recognizes target proteins through ubiquitin, a small protein covalently linked to proteins marked for destruction. The 19S cap contains ubiquitin receptors that specifically identify and bind ubiquitin-tagged proteins. This ubiquitin modification allows the proteasome to distinguish between target proteins and healthy proteins that should remain in the cell.

The unfoldase protein, an ATP-dependent enzyme in the 19S cap, unfolds target proteins as they enter the proteasome core. Using energy from ATP hydrolysis, it pulls the folded protein substrate through the cap's ring structure into the 20S core. Highly stable substrates may require hundreds of ATP hydrolysis cycles before complete unfolding and entry into the core.

Deubiquitinase, an enzyme located in the 19S cap, removes ubiquitin from the target protein before it enters the 20S core. This enzymatic removal, called de-ubiquitylation, occurs after the protein has been recognized and unfolded. Once the ubiquitin tag is cleaved, the unfolded protein is exposed to proteases inside the core for complete digestion.

Unlike free cellular proteases that dissociate quickly after acting on target proteins, the proteasome core retains the target protein until it is fully digested into smaller peptides. The hollow cylindrical structure of the 20S core, lined with proteolytic active sites, contains the substrate throughout degradation. These small peptides are then released into the cytosol for further processing or recycling.

The ubiquitin-proteasome pathway is a cellular mechanism that removes misfolded, damaged, or unnecessary proteins from eukaryotic cells. Proteins are tagged with ubiquitin chains at specific lysine residues, marking them for degradation by the proteasome. This ATP-driven process maintains cellular protein quality and prevents accumulation of dysfunctional proteins that could harm cell function.

The 19S caps function as gatekeepers by containing three key regulatory components: ubiquitin receptors that recognize tagged proteins, deubiquitinase that removes ubiquitin tags, and unfoldase that unfolds substrates. Together, these components ensure only properly marked and prepared proteins enter the 20S core. This selective regulation prevents random protein degradation and maintains cellular protein homeostasis.