Abstract
We studied the impact of Ba(2+) ions on the function and structure of large conductance potassium (BK) channels. Ion composition has played a crucial role in the physiological studies of BK channels due to their ability to couple ion composition and membrane voltage signaling. Unlike Ca(2+), which activates BK channels through all Regulator of K (+) Conductance (RCK) domains, Ba(2+) has been described as specifically interacting with the RCK2 domain. It has been shown that Ba(2+) also blocks potassium permeation by binding to the channel's selectivity filter. The Cryo-EM structure of the Aplysia BK channel in the presence of high concentration Ba(2+) here presented (PDBID: 7RJT) revealed that Ba(2+) occupies the K(+) S3 site in the selectivity filter. Densities attributed to K(+) ions were observed at sites S2 and S4. Ba(2+) ions were also found bound to the high-affinity Ca(2+) binding sites RCK1 and RCK2, which agrees with functional work suggesting that the Ba(2+) increases open probability through the Ca(2+) bowl site (RCK2). A comparative analysis with a second structure here presented (PDBID: 7RK6), obtained without additional Ba(2+), shows localized changes between the RCK1 and RCK2 domains, suggestive of coordinated dynamics between the RCK ion binding sites with possible relevance for the activation/blockade of the channel. The observed densities attributed to Ba(2+) at RCK1 and RCK2 sites and the selectivity filter contribute to a deeper understanding of the structural basis for Ba(2+)'s dual role in BK channel modulation, adding to the existing knowledge in this field.