For macroscopic currents, KcsA was reconstituted in 1:100 (mass:mass) of protein to lipid ratio, while for single channel studies we used a ratio of 1 1:10,000 (mass:mass). at the selectivity Morphothiadin filter2. Recent high resolution crystallographic analyses have provided atomic level details of K+channels trapped with the activation gate in the closed3-5or open conformation6-9and the inactivation gate in IKZF3 antibody the conductive or non-conductive conformations5,9. A series of crystal structures of KcsA caught in various degrees of gate opening and ion occupancy9have shown that access to the C-type inactivated state is associated with a sequential reduction in ion occupancy at the S2 and S3 binding sites correlated to the extent of opening at the inner bundle gate. Although these structures provide unique insights into the basic structural transitions underlying the K+channel gating cycle, a cursory look at most single-channel recordings reveal that even the simplest ion channels exhibit kinetically complex actions beyond the present set of structures. This functional heterogeneity entails conductive and non conductive says10-13as well as a variety of sub conductance levels12,14-17for which you will find no current structural correlates. We have provided evidence showing that under saturating stimulus conditions and at steady-state (when the activation gate is in its fully-open conformation), most of these gating fluctuations arise from conformational changes at the selectivity filter11,12,18,19. Not unexpectedly, the nature of the permeant ion20-24as well as a variety of mutations near the filter17,25-28have been shown to dramatically modulate the frequency and lifetimes of these gating events. In many cases, the structural effects of these perturbations are reflected in changes in the ionic occupancy at the filter, which partly explains the divergence from normal functional behavior26,29. Modal gating appears to be a characteristic feature of many K+channels, where time-dependent single-channel activity can switch abruptly between periods of high and low open probability under fixed experimental conditions30-32. In several channels, differential inactivation rates underlie some of these gating regimes31-34. The KcsA selectivity filter and adjacent regions display a considerable amount of conformational flexibility, as revealed from a comparison of existing KcsA structures in high and low K+5, partial and fully-open states9, in the presence of blockers35and in the (so-called) flipped structure observed in the E71A mutant11. It is easy to speculate that this intrinsic structural flexibility might underlie some of the heterogeneous functional behavior of the selectivity filter that leads to multiple gating modes12. Here, we have probed the functional and structural origins of modal gating in KcsA by studying a series of side-chain substitution at Glu71 position. These mutations sharply reduce entry into the C-type inactivated state while stabilizing three kinetically defined gating modes, depending on the type of side chain at position 71. These gating modes are reminiscent of those seen in wt KcsA, based on their unique intra-burst open probability (Po), and were named high-Pomode (for mutants E71A/G/C/V/S/T), low-Pomode (E71I) and Flickery mode (E71Q). High-resolution closed-state crystal structures of some of these mutations, together with molecular dynamics (MD) simulations reveal changes in the ion profiles and water occupancy in and around the selectivity filter. These observations provide an Morphothiadin initial rational to the origins of the conformational fluctuations occurring at the selectivity filter of the open-conductive channel. == RESULTS == == Variable modal kinetic behavior of wt KcsA == At steady-state, and under saturating proton concentrations, KcsA predominantly resides in the non-conductive C-type inactivated state11,12. These long silent periods are interrupted by brief sojourns into the conductive conformation, before transitioning back to the non-conductive inactivated state11(Fig. 1a). KcsA has been reported to exhibit a highly variable single channel kinetic behavior, displaying at least three unique patterns or modes of gating12(Fig. 1b). These modes are characterized by set variations in mean open and mean closed occasions: a high-Pomode with long open occasions o~100 ms, a low-Pomode with intermediate Morphothiadin open times.