Abstract

MAP Kinase cascades are ubiquitous enzymatic modules that serve as signal transduction machinery to allow eukaryotic cells to detect and respond to external stimuli. The yeast Saccharomyces cerevisiae uses two MAP Kinase pathways to sense separately osmolarity and mating pheromone. Previous observations suggest that these two pathways faithfully respond to their respective signals despite the pathways sharing common components. By measuring the response of single cells to simultaneous stimulation of both pathways, we show not only that the MAP Kinase pathways are not faithful to their inputs under these conditions but also that these cascades behave like a toggle switch, allowing only a single output even when presented with both inputs. These results imply that crosstalk between MAP kinase pathways is an integral part of signal transduction and processing in eukaryotic cells.