Heme is a carbon monoxide receptor for large-conductance Ca2+-activated K+ channels

Carbon monoxide ( CO) is an endogenous paracrine and autocrine gaseous messenger that regulates physiological functions in a wide variety of tissues. CO induces vasodilation by activating arterial smooth muscle large conductance Ca2+-activated potassium ( BKCa) channels. However, the mechanism by which CO activates BKCa channels remains unclear. Here, we tested the hypothesis that CO activates BKCa channels by binding to channel-bound heme, a BKCa channel inhibitor, and altering the interaction between heme and the conserved heme-binding domain ( HBD) of the channel alpha subunit C terminus. Data obtained using thin-layer chromatography, spectrophotometry, mass spectrometry ( MS), and MS-MS indicate that CO modifies the binding of reduced heme to the alpha subunit HBD. In contrast, CO does not alter the interaction between the HBD and oxidized heme ( hemin), to which CO cannot bind. Consistent with these findings, electrophysiological measurements of native and cloned ( cbv) cerebral artery smooth muscle BKCa channels show that CO reverses BKCa channel inhibition by heme but not by hemin. Site-directed mutagenesis of the cbv HBD from CKACH to CKASR abolished both heme-induced channel inhibition and CO-induced activation. Furthermore, on binding CO, heme switches from being a channel inhibitor to an activator. These findings indicate that reduced heme is a functional CO receptor for BKCa channels, introduce a unique mechanism by which CO regulates the activity of a target protein, and reveal a novel process by which a gaseous messenger regulates ion channel activity.