Overexpression of HAC1 gene increased levels of both intracellular and secreted human kringle fragment in Saccharomyces cerevisiae

► The unfolded protein response (UPR) was activated by overexpressing HAC1. ► The effect of HAC1 gene overexpression was investigated with the ire1Δ and hac1Δ strains. ► The effects of the overexpression of Hac1pu and Hac1pi were assessed. ► Activation of UPR is beneficial for LK8 production and induces intracellular accumulation of the protein far more than the secreted protein level.