19S regulatory particle The 19S particle in eukaryotes consists of 19 individual proteins and is divisible into two subassemblies, a 9-subunit base that binds directly to the α ring of the 20S core particle, and a 10-subunit lid. Six of the nine base proteins are ATPase subunits from the AAA Family, and an evolutionary homolog of these ATPases exists in archaea, called PAN (proteasome-activating nucleotidase). 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. Note that 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). 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.
19s REguLatoRy ParticLe THE 19s pARtiCle in eukaryotEs ConSiSts Of 19 INDiViDUal pRoTeINs AnD is dIvIsiBle INtO two SUbasSEmbLies, a 9-subuNit BASe tHat biNdS direcTly To THE α rInG Of The 20S CorE ParTicLe, ANd a 10-subuNit lid. sIx of ThE nine BAse pRoteiNS are atPasE suBUNitS From The aaa fAmily, aND an evOlutiONARy homoLog of thESE AtpaSes exiSts iN arcHaea, CalLeD pan (PRotEAsOMe-aCtiVaTing nucleoTIdAsE). 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. note that 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). 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.