Protein level variability determines phenotypic heterogeneity in proteotoxic stress response

Cell-to-cell variability in stress response is a bottleneck for the construction of accurate and predictive models that could guide clinical diagnosis and treatment of diseases as for instance cancers. Indeed such phenotypic heterogeneity can lead to fractional killing and persistence of a subpopulation of cells resistant to a given treatment. The heat shock response network plays a major role in protecting the proteome against several types of injuries. We combine high-throughput measurements and mathematical modeling to unveil the molecular origin of the phenotypic variability in the heat shock response network. Although the mean response coincides with known biochemical measurements, we found a surprisingly broad diversity in single cell dynamics with a continuum of response amplitudes and temporal shapes for several stimuli strengths. We theoretically predict that the broad phenotypic heterogeneity is due to network ultrasensitivity together with variations in the expression level of chaperons controlled by heat shock factor 1. We experimentally confirm this prediction by mapping the response amplitude to concentrations chaperons and heat shock factor 1 expression level.