19S regulatory particle

The 19S particle in eukaryotes consists of 19 individual proteins and is divisible into two subassemblies, a 10-protein base that binds directly to the α ring of the 20S core particle, and a 9-protein lid where polyubiquitin is bound. Six of the ten base proteins are ATPase subunits from the AAA Family, and an evolutionary homolog of these ATPases exists in archaea, called PAN (Proteasome Activating Nucleotidase).[17] The association of the 19S and 20S particles requires the binding of ATP to the 19S ATPase subunits, and ATP hydrolysis is required for the assembled complex to degrade folded and ubiquitinated proteins. Interestingly, only the step of substrate unfolding requires energy from ATP hydrolysis, while ATP-binding alone can support all the other steps required for protein degradation (e.g. complex assembly, gate opening, translocation and proteolysis).[18][19] In fact, ATP binding to the ATPases by itself supports the rapid degradation of unfolded proteins. However, while ATP hydrolysis is required for unfolding only it is not yet clear whether this energy may be used in the coupling of some of these steps.[19][20] As of 2008, the atomic structure of the 26S proteasome has not been solved, despite massive efforts to do so. Nevertheless, it is understood generally how the 19S associates with and regulates the 20S core particle.[14][21] In fact, the 19S and 11S particles bind to the same sites in the α rings of the 20S core particle although, they each induce gate opening by different mechanism.[14]


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