Targeted removal of damaged/misfolded proteins is primarily done by the ubiquitin (Ub)-proteasome system (UPS). Generally, the UPS degrades a protein molecule via two main steps: ubiquitination of the protein and subsequent degradation of the ubiquitinated protein by the proteasome. The two steps are not always directly coupled. The proteasome can directly uptake nearby polyubiquitinated proteins via its Ub receptor subunits but extra-proteasomal Ub receptors may be required for shuttling the remote substrates to the proteasome for degradation. We have previously demonstrated that UPS function is impaired during acute myocardial ischemia/reperfusion (I/R) and this impairment plays a major role in I/R injury due to failure to timely remove damaged/misfolded proteins. The molecular underpinnings of UPS impairment in I/R injury remains poorly understood. This study shows an important role of Ubiquilin1, a bona fide extra-proteasomal Ub receptor, in cardiac ubiquitination-proteasome coupling during I/R. Mice with cardiomyocyte-restricted knockout of the Ubiquilin1 gene (CR-Ubqln1KO) or with Ubiquilin1 overexpression (Ubqln1OE) were subject to myocardial I/R created via left anterior descending artery ligation (for 30min) and subsequent release (for 24hr). The I/R induced decline of left ventricular maximum dP/dt, elevation of minimum dP/dt, and infarct size were significantly greater in CR-Ubqln1KO mice, and conversely, were markedly attenuated in Ubqln1OE mice, compared with their littermates with a respective control genotype. The accumulation of ubiquitinated proteins in I/R myocardium was increased by CR-Ubqln1KO and reduced by Ubqln1OE. Furthermore, in cultured cardiomyocytes, Ubqln1OE was able to enhance the degradation of a surrogate UPS substrate as well as a bona fide misfolded protein linked to human proteinopathy. These exciting new findings demonstrate for the first time that inadequate coupling between ubiquitination and proteasomal degradation hinders the degradation of damaged proteins in I/R myocardium and represents a major pathogenic factor to acute I/R injury. It is also suggested that facilitating the coupling is potentially a novel therapeutic strategy for reducing I/R injury.