Regulation of membrane fluidity by RNF145‐triggered degradation of the lipid hydrolase ADIPOR2

The regulation of membrane lipid composition is critical for cellular homeostasis. Cells are particularly sensitive to phospholipid saturation, with increased saturation causing membrane rigidification and lipotoxicity. How mammalian cells sense membrane lipid composition and reverse fatty acid (FA)‐induced membrane rigidification is poorly understood. Here we systematically identify proteins that differ between mammalian cells fed saturated versus unsaturated FAs. The most differentially expressed proteins were two ER‐resident polytopic membrane proteins: the E3 ubiquitin ligase RNF145 and the lipid hydrolase ADIPOR2. In unsaturated lipid membranes, RNF145 is stable, promoting its lipid‐sensitive interaction, ubiquitination and degradation of ADIPOR2. When membranes become enriched in saturated FAs, RNF145 is rapidly auto‐ubiquitinated and degraded, stabilising ADIPOR2, whose hydrolase activity restores lipid homeostasis and prevents lipotoxicity. We therefore identify RNF145 as a FA‐responsive ubiquitin ligase which, together with ADIPOR2, defines an autoregulatory pathway that controls cellular membrane lipid homeostasis and prevents acute lipotoxic stress. Synopsis How mammalian cells sense and control membrane lipid composition remains unclear. Here, the E3 ligase RNF145 is found as rheostat to tune levels tuning the levels of the ER‐resident lipid hydrolase ADIPOR2 in response to changing fatty acid levels, thus preventing lipotoxicity. RNF145 is a uniquely lipid‐sensitive, endoplasmic reticulum (ER)‐resident ubiquitin E3 ligase. The lipid hydrolase ADIPOR2 is also localised to the ER and maintains cell viability in response to fluctuations in dietary fatty acids. RNF145 responds to changes in fatty acid saturation, causing either ADIPOR2 ubiquitination, ER membrane extraction and proteasome‐dependent degradation, or RNF145 auto‐degradation with unrestricted ADIPOR2 activity. The fatty acid regulation of RNF145 plays an essential role in relieving lipotoxicity by highly saturated phospholipids through ADIPOR2 activity. Graphical Abstract A lipid‐sensitive ubiquitin E3 ligase serves as essential rheostat to prevent lipotoxicity by highly saturated phospholipids.