ERAD ubiquitin ligases: multifunctional tools for protein quality control and waste disposal in the endoplasmic reticulum

In eukaryotic cells terminally misfolded proteins of the secretory pathway are retarded in the endoplasmic reticulum (ER) and subsequently degraded in a ubiquitin-proteasome-dependent manner. This highly conserved process termed ER-associated protein degradation (ERAD) ensures homeostasis in the secretory pathway by disposing faulty polypeptides and preventing their deleterious accumulation and eventual aggregation in the cell. The focus of this paper is the functional description of membrane-bound ubiquitin ligases, which are involved in all critical steps of ERAD. In the end we want to speculate on how the modular architecture of these entities ensures the specificity of substrate selection and possibly accomplishes the transport of misfolded polypeptides from the ER into the cytoplasm.