Molecular architecture of RCK domain–containing channels. (A) Schematic diagram of Kef-like channel subunits. Two of the four subunits are shown side by side to illustrate the K+ permeation pathway (arrow). The pore module (red) is tethered to an RCK domain (cyan). A second, identical RCK domain (yellow) associates tightly with the tethered domain. These paired RCK domains from the four subunits form a gating ring, as shown in B. (B) Crystal structure of the gating ring from the MthK channel (PDB accession no. 3RBZ), shown as a bird’s-eye view from above the membrane, with the transmembrane pore removed for clarity. Positions of Ca2+ ions are represented by green spheres. (C) Schematic diagram of a BK channel subunit. Each subunit contains a transmembrane voltage-sensing domain (S0–S4 helices, white), pore module (red), and two tandem RCK domains that make up the Ca2+ sensor (RCK1, magenta; RCK2, dark purple), with an architecture analogous to the one shown in A. (D) Crystal structure of the BK gating ring (PDB accession no. 3U6N) shown as a bird’s-eye view (as in B). Positions of Ca2+ ions in the Ca2+ bowl site are represented by green spheres.
Structures of prokaryotic and eukaryotic RCK domains. (A) Crystal structure of an RCK domain homodimer from the MthK channel (PDB accession no. 4L73). Positions of Ca2+ ions are represented by enlarged green spheres. (B) Cartoon illustrating the arm-in-arm architecture of the RCK dimer with subdomain anatomy as indicated. (C) Crystal structure of the tandem RCK pseudodimer from the BK channel (PDB accession no. 3U6N). The position of the Ca2+ ion in the Ca2+ bowl site is represented by a green sphere.
Ca2+-dependent conformations of MthK and BK gating rings. (A) Conformations of the MthK gating ring in (left to right) unliganded, singly liganded, and fully liganded states. N termini, which are tethered to pore-lining helices of the channel, are represented by black spheres; purple spheres represent Ba2+ ions in the singly liganded ring, and green spheres represent Ca2+ ions. RCK dimers comprising the unliganded gating ring can assume two different conformations, thus breaking fourfold symmetry. The singly liganded (Ba2+ bound) state stabilizes a single, symmetric conformation that facilitates channel opening, and opening is further facilitated by additional liganding. In the fully liganded state, distances between all N termini are uniformly greater than in the unliganded or singly liganded states, consistent with movements that could open the transmembrane pore. (B) Conformations of the BK gating ring in (left to right) the unliganded and Ca2+-bound states, in which Ca2+ (green spheres) is bound at the Ca2+ bowl. As with the MthK gating ring, binding of Ca2+ increases distances between the RCK domain N termini, which in turn underlies channel opening.
Table 1.Residues relevant to gating ring function reported in the literature
Equivalent amino acid numbers are shown for A. californica (Aplysia Slo1), mouse (mBK), and human BK (hBK). Table rows are color coded according to residue location: S6 (red), Mg2+-binding site (orange), RCK1 Ca2+-binding site (gray), and Ca2+ bowl (blue). Sequence alignment was performed using the Clustal Omega tool from EMBL-EBI and the following protein sequences: A. californica high conductance calcium-activated potassium channel (GenBank accession no. AAR27959.1), Mus musculus mbr5 mslo (GenBank accession no. AAA39746.1), and Homo sapiens calcium-activated potassium channel subunit α-1 isoform b (GenBank accession no. NP_002238.2).