Membrane Localization of Sc Proteins Promotes Graded S in the Yeast MAP Kinase Cas

all-or-none or ‘‘switch-like’ ’ manner (Figure 1A), in which indi-vidual cells in the population are either ‘‘on’ ’ or ‘‘off,’ ’ with no intermediate response [2]. By converting the analog input into a binary output, this signaling behavior can facilitate all-or-none cellular decisions that control irreversible changes in biological states such as cell-cycle progression [2, 3], as well as responses to stress [4]. Ste5 or any MAPK scaffold plays a major role in promoting a graded mode of signaling. Furthermore, it has been hypoth-esized that scaffold proteins could be equally likely to make signaling more switch-like, rather than less so [18]. Here, we dissect the determinants of graded signaling in the yeast pheromone response pathway with single-cell assays and a variety of tools for modulating signaling input at different pathway steps. We find that graded behavior is robust in that it resists genetic perturbation. However, the core MAP kinas