1.A.2.2.1
Prokaryotic K⁺-selective Kir channel KirBac1.1 (selectivity: K⁺ = Rb⁺ = Cs⁺ >> Li⁺, Na⁺ or NMGM) (Enkvetchakul et al., 2004), inward rectifying (Cheng et al., 2009). Closure of the Kir1.1 pH gate results from steric occlusion of the permeation path by the convergence of four leucines (or phenylalanines) at the cytoplasmic apex of the inner transmembrane helices. In the open state, K⁺ crosses the pH gate together with its hydration shell (Sackin et al. 2005). An inhibitory cholesterol binding site has been identified (Fürst et al. 2014). Conformational changes associated with an open activation gate have been identified, and these suggest an allosteric pathway that ties the selectivity filter to the activation gate through interactions between both transmembrane helices, the turret, the selectivity filter loop, and the pore helix. Specific residues involved in this conformational exchange that are highly conserved among human Kir channels have also been identified (Amani et al. 2020). Anionic lipids, especially cardiolipin, initiate a concerted rotation of the cytoplasmic domain subunits. This action buries ionic side chains away from the bulk water, while allowing water greater access to the K⁺ conduction pathway (Borcik et al. 2020). Kv1.5 channels are regulated by PKC-mediated endocytic degradation (Du et al. 2021). Pore-forming TMSs control ion selectivity and the selectivity filter conformation in the KirBac1.1 channel (Matamoros and Nichols 2021). Key functional residues involved in gating and lipid allostery of K⁺ Kir channels have been identified (Yekefallah et al. 2022).