As the degradation machinery that is responsible for ~70% of intracellular proteolysis, the proteasome complex (26S proteasome) plays critical roles in maintaining the homeostasis of the cellular proteome. Misfolded proteins and damaged protein need to be continuously removed to recycle amino acids for new synthesis; in addition, some key regulatory proteins fulfil their biological functions via selective degradation; furthermore, proteins are digested into peptides for MHC class I antigen presentation. To meet such complicated demands in biological processes via spatial and temporal proteolysis, protein substrates have to be recognized, recruited, and eventually hydrolyzed in a controlled fashion. Thus, the 19S regulatory particle has a series of important capabilities to address these functional challenges. To recognize proteins as designated substrates, the 19S complex has subunits that are capable of recognizing proteins with a special degradative tag, ubiquitination. It also has subunits that can bind to nucleotides (e.g., ATPs) in order to facilitate the association between the 19S and 20S particles, as well as to cause conformational changes to the alpha subunit C-terminals that form the substate entrance of the 20S complex. Rpn11 drives metalloprotease activity to hydrolyze the ubiquitin molecules from the poly-ubiquitin chain before protein substrates are unfolded and degraded.