Understanding the conformational motions of RCK gating rings

Abstract

Regulator of conduction of K⁺ (RCK) domains are ubiquitous regulators of channel and transporter activity in prokaryotes and eukaryotes. In humans, RCK domains form an integral component of large-conductance calcium-activated K channels (BK channels), key modulators of nerve, muscle, and endocrine cell function. In this review, we explore how the study of RCK domains in bacterial and human channels has contributed to our understanding of the structural basis of channel function. This knowledge will be critical in identifying mechanisms that underlie BK channelopathies that lead to epilepsy and other diseases, as well as regions of the channel that might be successfully targeted to treat such diseases.

Figure 1.

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.

Figure 2.

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.

Figure 3.

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.