Misfolded membrane-bound cytochrome P450 activates KAR2 induction through two distinct mechanisms

Using the mRNA differential display technique and Western blot analysis, the present study demonstrates that induction of KAR2 occurs when misfolded membrane-bound cytochrome P450, mutated in its cytosolically exposed domain, is expressed in Saccharomyces cerevisiae. Using various KAR2 promoter constructs in front of the Escherichia coli beta-galactosidase reporter gene, we found a fast and strong induction through the heat shock element (HSE), which was enhanced several fold by its adjacent GC-rich region. Additionally, a less pronounced induction was detected for the UPR element (UPRE). As expected, this response was absent in the ire1 disruptant strain. However, the HSE-mediated induction was enhanced upon disruption of IRE1 suggesting that the HSE pathway can compensate for the lack of a functional UPR pathway. Western blotting confirmed that Kar2p levels were increased to the same extent in the ire1 disruptant and in the non-disruptant strain. Removal of the P450 membrane-spanning region also abolished the UPRE-mediated induction of KAR2 transcription, but the HSE-mediated response remained. The data show for the first time that the transcription of KAR2 is significantly induced in response to a misfolded membrane-bound endoplasmic reticulum protein, and identifies the HSE and UPRE regions as KAR2 promoter elements responding to the misfolded cytosolic P450 domain and to the membrane-integrated mutant P450, respectively.