The scaffold protein Ste5 directly controls a switch-like mating decision in yeast

Abstract

Evolution has resulted in numerous innovations that allow organisms to increase their fitness by choosing particular mating partners, including secondary sexual characteristics, behavioural patterns, chemical attractants and corresponding sensory mechanisms. The haploid yeast Saccharomyces cerevisiae selects mating partners by interpreting the concentration gradient of pheromone secreted by potential mates through a network of mitogen-activated protein kinase (MAPK) signalling proteins. The mating decision in yeast is an all-or-none, or switch-like, response that allows cells to filter weak pheromone signals, thus avoiding inappropriate commitment to mating by responding only at or above critical concentrations when a mate is sufficiently close. The molecular mechanisms that govern the switch-like mating decision are poorly understood. Here we show that the switching mechanism arises from competition between the MAPK Fus3 and a phosphatase Ptc1 for control of the phosphorylation state of four sites on the scaffold protein Ste5. This competition results in a switch-like dissociation of Fus3 from Ste5 that is necessary to generate the switch-like mating response. Thus, the decision to mate is made at an early stage in the pheromone pathway and occurs rapidly, perhaps to prevent the loss of the potential mate to competitors. We argue that the architecture of the Fus3-Ste5-Ptc1 circuit generates a novel ultrasensitivity mechanism, which is robust to variations in the concentrations of these proteins. This robustness helps assure that mating can occur despite stochastic or genetic variation between individuals. The role of Ste5 as a direct modulator of a cell-fate decision expands the functional repertoire of scaffold proteins beyond providing specificity and efficiency of information processing. Similar mechanisms may govern cellular decisions in higher organisms and be disrupted in cancer.

Ultrasensitivity of the Bacillus subtilis sporulation decision

“…In many other organisms, the cell fate decision has been shown to be controlled by the ultrasensitive activation of the master regulator through positive feedback, resulting in a bistable response. Thus, two stable states of the responses correspond to two distinct cell fates (47)(48)(49)(50)(51)(52)(53). Surprisingly, despite the presence of positive feedback in the phosphorelay architecture, we have found that the phosphorelay response to KinA induction is graded and nonultrasensitive.…”

Composite low affinity interactions dictate recognition of the cyclin-dependent kinase inhibitor Sic1 by the SCFCdc4ubiquitin ligase

“…Similarly, multisite phosphorylation of the Ste5 scaffold protein cooperatively disrupts its membrane localization through electrostatic repulsion between the multiple phosphates and the negatively charged phospholipid membrane surface (6). Finally, the biochemical phenomenon of zero-order ultrasensitivity, first described for metabolic enzymes (2), can also operate in signaling complexes where the local concentration of phosphorylation sites exceeds the catalytic capacity of the kinase (8). These different types of multisite dependencies all serve to enable cellular decision making processes.…”

“…The basis for ultrasensitivity lies in the requirement for a threshold level of phosphorylation, which, in the presence of a countervailing phosphatase, confers a nonlinear dependence on kinase activity (3). Multisite phosphorylation thresholds can derive from enzyme activation mechanisms (4), decoy phosphorylation sites (5), bulk electrostatic effects (6,7), and zero-order enzyme saturation (8).…”

Spatial focalization of pheromone/MAPK signaling triggers commitment to cell–cell fusion

“…However, the MAPK module is highly versatile and can be modulated in various cell types and situations to generate distinct graded or switch-like outputs (Inder et al 2008;English et al 2015). For instance, the MAPK pheromone pathway in S. cerevisiae exhibits an essentially graded response to induce gene expression proportional to pheromone concentration (Poritz et al 2001;Paliwal et al 2007), but its role in inducing the formation of the mating projection is switch-like, a behavior dependent on the MAPK scaffold Ste5 (Hao et al 2008;Malleshaiah et al 2010). The S. pombe pheromone MAPK module, like the mammalian Raf-MEK-ERK cascade, which can functionally replace it (Hughes et al 1993), lacks a scaffold protein.…”