ER proteostasis and temperature differentially impact the mutational tolerance of influenza hemagglutinin

Here, we present a systematic and quantitative test of the hypothesis that the composition and activities of the endoplasmic reticulum (ER) proteostasis network impact mutational tolerance of secretory pathway client proteins. We focus on influenza hemagluttinin (HA), a viral coat protein that folds in the host's ER via a complex but well-characterized pathway. By integrating chemical methods to modulate the unfolded protein response with deep mutational scanning to assess mutational tolerance, we discover that upregulation of ER chaperones broadly enhances HA mutational tolerance across numerous sites and secondary/tertiary structure elements, including sites targeted by host antibodies. Remarkably, this host chaperone-enhanced mutational tolerance is observed at the same HA sites where mutational tolerance is most reduced by propagation at a fever-like temperature. Thus, host ER proteostasis mechanisms and temperature modulate HA mutational tolerance in opposite directions. This finding has important implications for influenza evolution, because influenza immune escape is contingent on HA possessing sufficient mutational tolerance to acquire antibody resistance while still maintaining the capacity to fold and function. More broadly, this work provides the first experimental evidence that the composition and activities of the ER proteostasis network critically define the mutational tolerance and, therefore, the evolution of secretory pathway client proteins. Overall design: RNA-seq characterizing a clonal HEK293T-Rex cell line, expressing DHFR ATF6f, Tet XBP1s, and the tetracycline repressor. These cell lines were treated with small molecules for 24 hours (in triplicate) to modulate the proteostasis environment in a stress-independent manner, at either 37C or 39C. XBP1s was activated by treatment with 0.1 ug/mL Doxycycline; ATF6f/XBP1s were activated by treatment with 0.1 ug/mL Doxycycline and 1 uM TMP; basal cells were vehicle-treated (0.01% DMSO). These cells were previously characterized in Shoulders et al. Cell Reports, 2013.

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Phillips AM, Doud MB, Gonzalez LO, Butty VL, Lin YS, Bloom JD, Shoulders MD