Endoplasmic reticulum (ER)-resident ubiquitin ligases are essential to cellular homeostasis and diverse signaling pathways, functioning in protein quality control, lipid metabolism, innate immunity, and interorganelle communication. While best known for their roles in ER-associated degradation (ERAD) of misfolded proteins, accumulating evidence shows that they also mediate the regulated turnover of functional ER proteins and contribute to ER-phagy, thereby expanding their roles in ER homeostasis. This review summarizes recent advances in understanding substrate recognition mechanisms employed by ER ubiquitin ligases and how these enzymes coordinate ERAD and ER-phagy, with a primary focus on mammalian systems. We further discuss their roles in ER homeostasis and immune responses, and how their dysregulation contributes to diseases such as neurodegeneration and immune disorders.
