SigmaR1 is an auxiliary translocon factor with lipid-binding activity that regulates protein and lipid droplet homeostasis

Sigma Non-Opioid Intracellular Receptor 1 (SigmaR1) is a member of the sigma family of receptors that interacts with a variety of psychotomimetic drugs and is involved in a wide range of cellular and physiological functions. Despite its increasing importance in human physiology and disease, the subcellular localization of SigmaR1 and its molecular function remain poorly defined. Using endogenous tagging and cell fractionation, we show that SigmaR1 is a type II integral ER membrane protein that is specifically enriched at ER sheets. A short region at the N-terminus of SigmaR1 promotes its ER-sheet localization. Importantly, our biochemical studies demonstrate that SigmaR1 directly interacts with components of the translocon complex including TRAPα and Nicalin. In addition, we found that a β-barrel at the C-terminal of SigmaR1 binds phosphatidylcholine (PC), and the binding of PC strengthens the association of SigmaR1 with the translocon complex. SigmaR1 knockout systematically impaired cellular protein and lipid homeostasis, resulting in accumulation of lipid droplets in hepatocytes. Collectively, we propose that SigmaR1 is an auxiliary translocon factor that binds lipids to regulate protein and lipid droplet homeostasis, which may underlie the broad and vital roles of SigmaR1 in physiology and disease.