Structure and function of multiple Ca2+-binding sites in a K+ channel regulator of K+ conductance (RCK) domain

Regulator of K(+) conductance (RCK) domains control the activity of a variety of K(+) transporters and channels, including the human large conductance Ca(2+)-activated K(+) channel that is important for blood pressure regulation and control of neuronal firing, and MthK, a prokaryotic Ca(2+)-gated K(+) channel that has yielded structural insight toward mechanisms of RCK domain-controlled channel gating. In MthK, a gating ring of eight RCK domains regulates channel activation by Ca(2+). Here, using electrophysiology and X-ray crystallography, we show that each RCK domain contributes to three different regulatory Ca(2+)-binding sites, two of which are located at the interfaces between adjacent RCK domains. The additional Ca(2+)-binding sites, resulting in a stoichiometry of 24 Ca(2+) ions per channel, is consistent with the steep relation between [Ca(2+)] and MthK channel activity. Comparison of Ca(2+)-bound and unliganded RCK domains suggests a physical mechanism for Ca(2+)-dependent conformational changes that underlie gating in this class of channels.