Induction of Sis1 promotes fitness but not feedback in the heat shock response

ABSTRACT Previously, we combined modeling and experiments to demonstrate that the heat shock response (HSR) functions as a negative feedback loop in which undefined chaperone clients activate the HSR by sequestering Hsp70, and subsequent induction of Hsp70 deactivates the response (Zheng et al., 2016; Krakowiak et al., 2018). Here, we formally define newly synthesized proteins (NSPs) as a major class of HSR activators and determine the role of Sis1, a co-chaperone of Hsp70, in HSR regulation. We develop and experimentally validate a new mathematical model that incorporates NSPs and Sis1. Unexpectedly, genetic decoupling and pulse-labeling experiments reveal that Sis1 induction promotes fitness during prolonged stress rather than providing negative feedback to the HSR. These results support an overall model in which NSPs signal the HSR by sequestering Sis1 and Hsp70, while induction of Hsp70 – but not Sis1 – attenuates the response.