Regulation of the anaphase promoting complex/cyclosome by the degradation of its unassembled catalytic subunit, Apc1

One of the challenges encountered by the protein quality control machinery is the need to ensure that members of multiprotein complexes are available in the correct proportions. In this study, we demonstrate that the ubiquitin-proteasome system (UPS) mediates the degradation of Apc11, the catalytic core subunit of the anaphase-promoting complex/cyclosome (APC/C). In vitro studies have shown that Apc11, together with its E2 enzyme, is sufficient to ubiquitinate substrates independently of the APC/C. Here, we establish that this can occur in living yeast cells. We show that the tight controls regulating the function of the fully assembled APC/C can be circumvented when its substrates are ubiquitinated by the excess levels of Apc11 independently of the assembled complex. We thus suggest that the UPS-mediated degradation of Apc11 is an overlooked mechanism ensuring that proper function of the APC/C is limited to suitably delimited holoenzymes and that an imbalance in protein expression may result in detrimental gain-of-function activity, rather than merely the disruption of protein complex stoichiometry.