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 which could guide clinical diagnosis and treatment of certain diseases, for example, cancer. Indeed, such phenotypic heterogeneity can lead to fractional killing and persistence of a subpopulation of cells which are resistant to a given treatment. The heat shock response network plays a major role in protecting the proteome against several types of injuries. Here, 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 stimulus strengths. We theoretically predict that the broad phenotypic heterogeneity is due to network ultrasensitivity together with variations in the expression level of chaperones controlled by the transcription factor heat shock factor 1. Furthermore, we experimentally confirm this prediction by mapping the response amplitude to chaperone and heat shock factor 1 expression levels. The heat shock response network protects the proteome against injuries: Damages are detected by the heat shock factor 1 and repaired by molecular chaperones. In this paper, high‐throughput measurements highlight the phenotypic variability of the response at the single‐cell level and, guided by mathematical modeling, reveal its molecular origin: ultrasensitivity of the network and variations in the expression level of molecular chaperones.