Structural bases for blockade and activation of BK channels by Ba ions.

We studied the impact of Ba 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, which activates BK channels through all (RCK) domains, Ba has been described as specifically interacting with the RCK2 domain. It has been shown that Ba also blocks potassium permeation by binding to the channel's selectivity filter. The Cryo-EM structure of the BK channel in the presence of high concentration Ba here presented (PDBID: 7RJT) revealed that Ba occupies the K S3 site in the selectivity filter. Densities attributed to K ions were observed at sites S2 and S4. Ba ions were also found bound to the high-affinity Ca binding sites RCK1 and RCK2, which agrees with functional work suggesting that the Ba increases open probability through the Ca bowl site (RCK2). A comparative analysis with a second structure here presented (PDBID: 7RK6), obtained without additional Ba, 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 at RCK1 and RCK2 sites and the selectivity filter contribute to a deeper understanding of the structural basis for Ba's dual role in BK channel modulation, adding to the existing knowledge in this field.