Ratiometric GPCR signaling enables directional sensing in yeast

Abstract

Accurate detection of extracellular chemical gradients is essential for many cellular behaviors. Gradient sensing is challenging for small cells, which can experience little difference in ligand concentrations on the up-gradient and down-gradient sides of the cell. Nevertheless, the tiny cells of the yeast Saccharomyces cerevisiae reliably decode gradients of extracellular pheromones to find their mates. By imaging the behavior of polarity factors and pheromone receptors, we quantified the accuracy of initial polarization during mating encounters. We found that cells bias the orientation of initial polarity up-gradient, even though they have unevenly distributed receptors. Uneven receptor density means that the gradient of ligand-bound receptors does not accurately reflect the external pheromone gradient. Nevertheless, yeast cells appear to avoid being misled by responding to the fraction of occupied receptors rather than simply the concentration of ligand-bound receptors. Such ratiometric sensing also serves to amplify the gradient of active G protein. However, this process is quite error-prone, and initial errors are corrected during a subsequent indecisive phase in which polarity clusters exhibit erratic mobile behavior.

Molecular switch architecture drives response properties

“…receptors (X * /X T ) [13,14]. The absolute value of the total number of receptors (X T ) has no bearing on Y * .…”

“…This points to a derepression mechanism in which the decreased amount of the receptor (X T ) lowers the repression strength k 6 X T and shifts the dose response curve to the left in comparison to that of the wild-type system. A suggested example of concerted mechanism is the yeast G-signaling pathway, which exhibits both ratiometric signaling [13,14] and dose-response alignment [43].…”

“…As before, setting k 6 = 0 and k 5 = 0, respectively, result in ODEs for the activation and derepression mechanisms. An important distinction between the ODEs in (1) and the ODEs in (14) is that the receptor dynamics only has one timescale, 1/(k 1 S +k 2 ), in the former but two timescales in the latter (section S4, SI). The interplay between these two timescales allows X (t) and X * (t) to transiently respond to a stimulus at the fast timescale, followed by an eventual return towards their respective pre-stimulus levels at the slow timescale.…”

“…These two mechanisms for initiating signaling, activation and derepression, are not mutually exclusive. For example, a concerted signal initiation, whereby both activation and dererpression are used, is employed in the GTPase cycle of the yeast mating response pathway [13,14]. In this example, inactive GPCRs recruit a GAP protein and act to repress, whereas active receptors have GEF activity and act to activate.…”

“…Further analyses have examined the effect of receptor numbers [36][37][38], feedback mechanisms [39,40], removal of active receptors via endocytosis and degradation [41,42], etc. Similarly, important properties of the concerted mechanism, such as its ability to do ratiometric signaling [13,14], to align dose responses at different stages of the signaling pathway [43], as well as its robustness [29,44] are well-known. The derepression model is relatively less studied.…”