Cytochrome C-like Domain within the Human BK Channel

Large-conductance, voltage- and Ca2+-activated K+ (BK) channels, potent regulators of cellular excitability, are modulated by numerous signaling molecules, including heme. Heme binds reversibly to the BK channel at site 612CKACH616, a conserved heme regulatory motif (HRM) in the cytochrome c protein family, and it is located within the ~120-residue linker connecting RCK1 and RCK2 domains. This linker also possesses terminal α-helices, reminiscent of cytochrome c family proteins. Most of this region has thus far evaded structural definition. To gain structural insight into this functionally significant region, we performed a sequence alignment of the BK linker region with mitochondrial cytochrome c and cytochrome c domains from different hemoproteins. We found that, in addition to the HRM motif, key structural and functional elements of cytochrome c proteins are conserved in the BK RCK1- RCK2 linker: firstly, the portion of the BK region resolved in the available atomic structures shares secondary structure elements with cytochrome c domains proteins; secondly, cytochrome c domains Met80, the second axial ligand to the heme iron, aligns with BK channel M691. In addition to its role in electron shuttling, cytochrome c domains exhibit various catalytic properties such as peroxidase activity, i.e. the oxidation of suitable substrates using peroxides. We found that the linker region endows human BK channels with peroxidase activity, with an apparent H2O2 affinity ~40-fold higher than mitochondrial cytochrome c under basal conditions, and this activity was diminished by 612CKACH616, and M691 substitutions. These results indicate that the BK channel possesses a new module, similar to cytochrome c domains of the hemoproteins, which may confer novel physiological functions to these ubiquitous ion channels.