Eukaryotic cells commonly employ mitogen activated protein kinases (MAPKs) to transduce extracellular signals and evoke intracellular responses 
. MAPKs are a part of an evolutionarily-conserved three-tiered signaling cascade comprised of the MAPK, a MAPK kinase (MAPKK), and a MAPKK kinase (MAPKKK). In mammalian cells MAPKs respond to diverse stimuli including hormones, stresses, and cytokines. These different stimuli will in many cases activate a common MAPK. Conversely a single stimulus will often activate multiple MAPKs. Understanding how each stimulus and each response is coordinated is often obscured by the large number of components and the functional complexity of signaling networks 
MAPK pathways are also present in the unicellular eukaryote Saccharomyces cerevisiae
(hereafter, yeast). As in higher eukaryotes, yeast use multiple MAPK pathways to respond to a variety of environmental signals 
. The two best-characterized examples are the mating pathway and the high osmolarity glycerol (HOG) response pathway (detailed in ) 
. The mating pathway operates through a cell-surface receptor that activates a canonical G protein heterotrimer. The activated G protein recruits Ste5, a scaffold protein that assembles and activates three component kinases: Ste11, Ste7 and the MAPK Fus3 
. Active Fus3 promotes events leading to cell fusion including new gene transcription, cell cycle arrest and cytoskeletal rearrangements 
. High osmotic stress activates Ste11 as well as Pbs2 and the MAPK Hog1 
. Active Hog1 promotes events leading to stress adaptation including increased glycerol production, cell cycle arrest and a pause in protein translation 
. Individually, the two pathways have well-defined components, known points of regulation, and established measures of pathway output. Together, the pathways form a signaling network that is a model for the study of signal coordination.
Hyperosmotic stress delays mating differentiation.
The mating and HOG pathways share several components, yet exhibit remarkable signal fidelity when stimulated individually 
(, shared components highlighted in green). Hyperosmotic stress does not activate Fus3 or promote mating, and mating pheromones do not activate Hog1 or the HOG pathway. Such pathway fidelity may be maintained by two mechanisms: (i) pathway insulation and (ii) pathway cross-inhibition 
. The pathway insulation model proposes that physical sequestration of components maintains specificity. For example, Ste11 exists in two scaffolded pools, one that selectively activates Fus3 and another that selectively activates Hog1 
. The pathway cross-inhibition model proposes that one pathway inhibits signaling by the competing pathway. For example, Hog1 is required to prevent the inadvertent activation of the mating response by hyperosmotic stress. When Hog1 is absent, or rendered catalytically inactive, hyperosmotic stress promotes mating. Thus it appears that Hog1 targets a component of the mating pathway to maintain fidelity 
. However, previous studies were unsuccessful in identifying the substrate(s) of Hog1 in the mating pathway.
Thus the mechanisms that prevent cross-talk remain unresolved. A related and potentially more tractable question is how cells coordinate responses when the mating and HOG pathways are activated simultaneously. To address this question investigators have treated cells simultaneously with mating pheromone and hyperosmotic stress and used pathway-specific transcription reporters to monitor signaling in individual cells 
. One group reported that the responses to these inputs are mutually exclusive 
. However a subsequent analysis identified a potential artifact, wherein cell death can produce a spurious signal in reporter assays that employ the red fluorescent protein 
. In surviving cells reporters of both pathways are activated in proportion to their respective stimuli. Thus a single cell can respond to both hyperosmotic stress and pheromone, but how these responses are prioritized or coordinated remains to be determined.
In this study we establish that the hyperosmotic stress and mating pheromone signals are coordinated. Using a broad array of activity assays, conducted over various time scales, we show that Hog1 delays and dampens the response to pheromone and does so by two distinct mechanisms: (i) negative feedback phosphorylation of a shared component (Ste50) and (ii) feed-forward phosphorylation of a negative regulator of translation (Rck2). Thus, activated Hog1 invokes pathway cross-inhibition to delay the mating differentiation response. Mating differentiation resumes once cellular osmotic balance is restored and cross-inhibition is relieved. These studies provide a model of how a cell integrates competing signals to control cell fate.