In the presence of Zn2+, the Drosophila 26 S proteasome disassembles into RP (regulatory particle) and CP (catalytic particle), this process being accompanied by the dissociation of subunit Rpn10/p54, the ubiquitin receptor subunit of the proteasome. The dissociation of Rpn10/p54 induces extensive rearrangements within the lid subcomplex of the RP, while the structure of the ATPase ring of the base subcomplex seems to be maintained. As a consequence of the dissociation of the RP, the peptidase activity of the 26 S proteasome is lost. The Zn2+-induced structural and functional changes are fully reversible; removal of Zn2+ is followed by reassociation of subunit Rpn10/p54 to the RP, reassembly of the 26 S proteasome and resumption of the peptidase activity. After the Zn2+-induced dissociation, Rpn10/p54 interacts with a set of non-proteasomal proteins. Hsp82 (heat-shock protein 82) has been identified by MS as the main Rpn10/p54-interacting protein, suggesting its role in the reassembly of the 26 S proteasome after Zn2+ removal. The physiological relevance of another Rpn10/p54-interacting protein, the Smt3 SUMO (small ubiquitin-related modifier-1)-activating enzyme, detected by chemical cross-linking, has been confirmed by yeast two-hybrid analysis. Besides the Smt3 SUMO-activating enzyme, the Ubc9 SUMO-conjugating enzyme also exhibited in vivo interaction with the 5'-half of Rpn10/p54 in yeast cells. The mechanism of 26 S proteasome disassembly after ATP depletion is clearly different from that induced by Zn2+. Rpn10/p54 is permanently RP-bound during the ATP-dependent assembly-disassembly cycle, but during the Zn2+ cycle it reversibly shuttles between the RP-bound and free states.