Some plasma membrane sensor proteins that detect nutrients look a lot like the transporters that move those small molecules across the membrane. On page 327, Wu et al. present a model suggesting that the sensors work like them, too—minus the transport step.
Previous work showed that a single mutation in the Ssy1p amino acid sensor increased its basal signaling level and made it hyperresponsive to extracellular ligand. Starting from those data, the team developed a model of how the sensor might control transcriptional activation of amino acid transporters.
According to the model, the sensor could sit in the membrane with its ligand-binding site facing the intracellular or extracellular space. In its unbound state, the sensor freely flips between inside- and outside-facing conformations. But ligand freezes the sensor in one conformation or the other so it cannot readily shift between the two sides of the...
