The Heme-Bound Human BK Channel Gating Ring is a Co Sensor

Carbon monoxide (CO) is a recognized gasotransmitter that regulates BK channel activity, causing vasodilatory effects. It is thought that CO interacts with the intracellular portion of BK channels, which consists of the assembly of structural domains RCK1 and RCK2 into the ligand-sensing Gating Ring superstructure. Several mechanisms have been proposed to account for the BK/CO interaction (Heinemann et al., Chem Commun 2014). We have tested the hypothesis that CO interacts with BK channels via heme, which associates with the RCK1-RCK2 linker region, previously described as the BK Cytochrome-c-like domain (Yusifov et al., Biophysical J, 2709-Pos 2014). We investigated the molecular interaction of CO with the purified human BK channel Gating Ring using absorption spectroscopy: we found that the spectrum of the Gating Ring complexed with the reduced form of heme [Fe(II) protoporphyrin-IX] was significantly altered in the presence of CO and produced a Soret peak at 422 nm and β/α bands at 538 and 574 nm, typical spectral characteristic of hemoproteins complexed with CO. Furthermore, CO-dependent conformational changes were assessed using ANS (8-Anilino-1-naphthalenesulfonic acid) fluorescence spectra of the Gating Ring/reduced heme complex in the presence of increasing [CO]. The intensity of GR/ANS emission diminished in a [CO]-dependent manner (K0.5=54±3.8nM), indicating less ANS association, and therefore the occurrence of CO-dependent GR structural rearrangements. On the contrary, when heme was substituted by protoporphyrin-IX, a heme analog that lacks iron, CO addition did not alter ANS fluorescence. These results suggest that, in BK channels, the Heme/Cytochrome-c-like domain complex acts as a CO sensor, binding CO via heme and inducing a conformational change to the BK channel Gating Ring at submicromolar [CO]. This structural transition likely represents one of the mechanisms underlying the CO-dependent regulation of the human BK channel.