Structural Studies of the Apo and Ca2+-Bound States of the Human BK (SLO1) Channel Gating Ring in Solution

The gating ring (GR) regulates the activity of large-conductance voltage- and Ca^(2+)-activated K^+ channels (BK) by interacting with intracellular signaling molecules. To understand the operation of this biological sensor under physiological conditions, we performed Small-Angle X-ray Scattering (SAXS) analysis, at beamline 4-2 at the Stanford Synchrotron Radiation laboratory. SAXS measurements of the purified GR were performed in the absence or in the presence of 35 μM free Ca^(2+), found to be a saturating concentration in previous work. The quality of the circularly-averaged scattering data was evaluated with Guinier analysis, while the ATSAS software suite was used to derive structural information. The radius of gyration (R_g) and maximum interparticle distance (D_(max)) of the apo GR were 48.65±1.372 A and 185 A, respectively. These values are comparable to data obtained from crystal structure of GR (3NAF), where the envelope R_g, calculated with CRYSOL, is 45.55 A, and its diameter 155.6 A. Ca^(2+)-bound GR shows a decrease in R_g to 42.77±1.058 A and D_(max) to 160 A, demonstrating the structural response of GR to Ca^(2+). Low-resolution structural models of the GR were generated from the experimental scattering pattern using DAMMIN. The Ca^(2+)-bound GR revealed notable changes in both flexible and assembly interfaces of the superstructure's constituent RCK1 (Regulator of Conductance for K^+) and RCK2 domains. Since the structural changes are resolved under physiologically-relevant conditions, we speculate that they represent the molecular transitions that initiate the Ca^(2+)-induced activation of human BK channels.