Heme oxygenase‐1 exists as dimers in endoplasmic reticulum

Abstract

Heme oxygenase‐1 (HO‐1) catalyzes the oxidative degradation of heme to biliverdin. Previous studies suggest that HO‐1 exists as a monomer in ER. In the present study, we performed chemical cross‐linking and fluorescence resonance energy transfer (FRET) experiments to show that HO‐1 forms dimers in ER. However, the dimerization was not observed with a truncated HO‐1 which lacks the C‐terminal transmembrane segment (TMS) (aa 266‐285) and exhibits cytosolic and nuclear localization, indicating that the TMS is essential for the self‐assembly of HO‐1 in native membrane. To identify the interface of TMS‐TMS interaction, W270 predicted by molecular modeling with stronger hydrophobic force in the interface was mutated and the effect on protein subcellular localization and dimerization was assessed. The results showed that the W270A mutant was present exclusively in ER and formed dimers as the wild type HO‐1. Interestingly, W270N mutant was localized not only in ER but also in cytosol and nucleus as well, suggesting that it is susceptible to proteolytic cleavage. Moreover, W270N mutation appeared to interfere with dimeric state as revealed by the lower FRET efficiency. Collectively, these data suggest that the dimerization driven by the hydrophobic interaction in the TMS is implicated in the stabilization of HO‐1 in ER. This work was supported by a grant from National Science Council of Taiwan (NSC‐95‐2320‐B‐001‐008).