Effects of overexpression of an unfolded protein response (UPR) gene were investigated in recombinant Saccharomyces cerevisiae strains harboring 16 copies of the gene for human kringle protein, LK8. In S. cerevisiae, Had p plays a crucial role as a transcription activator and Ire1 p as endonuclease acting on the HAC1 transcript to remove an intron and induce the UPR. The disruption of the two genes was detrimental to LK8 expression, and interestingly, the hac1 Delta strain was not able to utilize galactose as a carbon source and concomitantly delayed cell growth compared with the wild type and the ire1 Delta, strains. In a complementation test, the growth defect was partially recovered by the overexpression of the HAC1 gene controlled by the GAL1 promoter. Additional activation of UPR was mediated by the GAL1 promoter driven coexpression of the HAC1 gene and enhanced the intracellular and secreted levels of LK8 by 4- and 1.6-fold, respectively. The UPR was essentially required for the heterologous production of LK8. Furthermore, Hac1p is the factor promoting LK8 protein production as well as cell growth in recombinant S. cerevisiae strains. This result indicates that the additional activation of UPR might be a good option for overproduction of heterologous proteins in S. cerevisiae. (C) 2012 Elsevier Ltd. All rights reserved.