Divergent roles of Hsp70 chaperones in orthoflavivirus protein secretion and virion formation.

Orthoflaviviruses, such as tick-borne encephalitis virus (TBEV) and West Nile virus (WNV), can cause severe neurological disease and remain without specific antiviral treatments. We found that orthoflavivirus envelope (E) and non-structural protein 1 (NS1) interact with heat shock protein 70 (Hsp70) chaperones, key regulators of protein homeostasis and existing cancer drug targets. We examined how Hsp70 and endoplasmic reticulum-resident BiP contribute to viral protein secretion and infectivity of tick and mosquito-borne orthoflaviviruses. Targeting the Hsp70 nucleotide-binding domain with small-molecule inhibitor YM-1 significantly reduced infectivity of multiple orthoflaviviruses, while substrate-binding domain inhibitor PES-Cl specifically impaired NS1 secretion of tick-borne orthoflaviviruses. Protein degradation inhibitors restored NS1 expression in BiP-deficient cells but failed to rescue NS1 secretion. These data indicate that while BiP is essential for secretion of tick-borne orthoflavivirus NS1, it is not required for infectivity. The antiviral effect of YM-1 likely reflects inhibition of other chaperones or additional cellular targets.