Ubiquitin-dependent signal amplification in lipid saturation sensing

Cellular membranes are dynamic platforms whose composition and biophysical properties are surveyed by sensor proteins to maintain homeostasis. How these sensors convert weak physical cues into robust biochemical outputs remains unclear. Failure to preserve membrane homeostasis, however, results in cellular stress and organelle dysfunction. Here, we investigate the prototypical yeast lipid saturation sensor Mga2 to reveal how lipid-controlled rotational movements in the transmembrane region are decoded into robust transcriptional responses. Using a fully reconstituted in vitro ubiquitylation system with quantitative fluorescence-based readouts, we uncover how Mga2 amplifies subtle, fluctuating membrane signals into pronounced differences in ubiquitylation. Kinetic modeling demonstrates that this amplification does not require deubiquitylating enzymes. Instead, negative feedback arises from a diversion of ubiquitin flux toward inhibitory autoubiquitylation of the cognate E3 ligase Rsp5. Lipid saturation shifts this ubiquitin flux, thereby enabling several-fold signal amplification. Our findings provide a mechanistic framework for how membrane property sensors convert weak, fluctuating physical inputs into robust biochemical outcomes, and establish ubiquitin flux partitioning as a general principle of signal amplification in cellular surveillance mechanisms.