partner of proteasome complex than a constitutive subunit. However, emerging evidences suggested that Rpn13 is a novel subunit of 19S. A recent study provided new evidences of 19S complex structure via an integrative approach combining data from cryoelectron microscopy, X-ray crystallography, residue-specific chemical cross-linking, and several proteomics techniques. In the newly established sub complex model of 19S base, Rpn2 is rigid protein located on the side of ATPase ring, supporting as the connection between the lid and base. Rpn1 is conformationally variable, positioned at the periphery of the ATPase ring. The ubiquitin receptors Rpn10 and Rpn13 are located further in the distal part of the 19S complex, indicating that they were recruited to the complex late during the assembly process. Function As the degradation machinery that is responsible for ~70% of intracellular proteolysis, proteasome complex (26S proteasome) plays a critical roles in maintaining the homeostasis of cellular proteome. Accordingly, misfolded proteins and damaged protein need to be continuously removed to recycle amino acids for new synthesis; in parallel, some key regulatory proteins fulfill 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 process via spatial and temporal proteolysis, protein substrates have to be recognized, recruited, and eventually hydrolyzed in a well controlled fashion. Thus, 19S regulatory particle pertains a series of important capabilities to address these functional challenges. To recognize protein as designated substrate, 19S complex has subunits that are capable to recognize proteins with a special degradative tag, the ubiquitinylation. It also have subunits that can bind with nucleotides (e.g., ATPs) in order to facilitate the association between 19S and 20S particles, as well as to cause confirmation changes of alpha subunit C-terminals that form the substate entrance of 20S complex. Rpn13 is one essential subunit of 19S regulatory particle and it contributes to the assembly of the "base" subcomplex. In the base sub complex, Rpn13, as a ubiquitin receptor, offers a docking position for ubiquitinated substrate. Evidence showed that ubiquitination of Rpn13 subunit can significantly reduced the proteasome's ability to bind and degrade ubiquitin-conjugated proteins. Investigation employing biochemical and unbiased AQUA-MS methodologies offered evidences showing that, although the vast majority (if not all) of the double-capped 26S proteasomes, both 19S complexes, contain the ubiquitin receptor Rpn10, only one of these 19S particles contains the additional ubiquitin receptor Rpn13, thereby defining asymmetry in