Ubiquitin-dependent signal amplification in lipid saturation sensing

Abstract Cellular membranes are dynamic platforms whose composition and biophysical properties are surveyed by sensor proteins to maintain homeostasis. Failure to preserve membrane homeostasis, however, results in cellular stress and organelle dysfunction. Using the prototypical lipid saturation sensor Mga2, we explored how weak physical cues that modulate rotational movements in the transmembrane region are converted into decisive biochemical outputs that ultimately control the production of unsaturated fatty acids. Using reconstituted systems and purified components, we quantitatively follow the ubiquitylation of Mga2 via the E3 ligase Rsp5. Kinetic modeling reveals that several rounds of ubiquitylation contribute to signal amplification and uncovers the first transfer of ubiquitin onto Mga2 as a kinetic gatekeeper. The minimal system composed of the sensor protein and a canonical E1-E2-E3 ubiquitylation cascade is sufficient to discriminate saturated and unsaturated membrane lipid environments even in the absence of deubiquitylating enzymes such as Ubp2. Our findings provide a mechanistic framework for how membrane property sensors convert weak, fluctuating physical cues into robust biochemical outcomes.