The endoplasmic reticulum (ER) of eukaryotic cells serves as a checkpoint tightly monitoring protein integrity and channeling malformed proteins into different rescue and degradation routes. The degradation of several ER lumenal and membrane-localized proteins is mediated by ER-associated protein degradation (ERAD) in yeast (Saccharomyces cerevisiae) and mammalian cells. To date, evidence for the existence of ERAD-like mechanisms in plants is indirect and based on heterologous or artificial substrate proteins. Here, we show that an allelic series of single amino acid substitution mutants of the plant-specific barley (Hordeum vulgare) seven-transmembrane domain mildew resistance o (MLO) protein generates substrates for a postinsertional quality control process in plant, yeast, and human cells, suggesting conservation of the underlying mechanism across kingdoms. Specific stabilization of mutant MLO proteins in yeast strains carrying defined defects in protein quality control demonstrates that MLO degradation is mediated by HRD pathway-dependent ERAD. In plants, individual aberrant MLO proteins exhibit markedly reduced half-lives, are polyubiquitinated, and can be stabilized through inhibition of proteasome activity. This and a dependence on homologs of the AAA ATPase CDC48/p97 to eliminate the aberrant variants strongly suggest that MLO proteins are endogenous substrates of an ERAD-related plant quality control mechanism.