Unbalanced proteasome function has been associated with cardiovascular disease. However, regulation of the function of the proteasome and its impact on the pathogenesis of cardiovascular disease are largely unknown. Previously, we have identified three distinct mechanisms regulating proteasome function during cardiac remodeling. In this study, the composition of 20S proteasome subpopulations and their impact on cardiac remodeling was investigated. Proteasome subpopulations differ in their assembly by incorporation of up to six inducible subunits. We have discovered that the myocardial concentration of proteasome subpopulations containing inducible subunits increased by up to 55% during cardiac remodeling. Cardiac remodeling was induced by chronic b-adrenergic stimulation of mice for 7 days (30mg/kg/d isoproterenol), a condition that promotes the pathogenesis of cardiac disease in humans. During treatment, the heart weight to body weight ratio (HW/BW) increased by 50%. Inducible proteasome subunits contain alternative enzymatic sites and their incorporation changes substrate specificity and turn-over. Indeed, both turn-over and substrate specificity of 20S proteasomes changed non-uniformly in the catecholamine-stimulated heart, indicating a shift in the composition of proteasome subpopulations. To determine the impact of inducible proteasome subunits on cardiac remodeling, genetically engineered mice lacking inducible subunits were subjected to the standardized b-adrenergic stimulation. Remarkably, although the knock-out mice had no obvious phenotype with normal heart development and function before treatment, cardiac remodeling deteriorated to the point of increased mortality with HW/BW ratios up by 70%. Thus, we conclude that up-regulation of proteasome subpopulations containing inducible subunits is essential for cardiac function during chronic sympathetic stimulation.