Anionic Phospholipids Shift the Conformational Equilibrium of the Selectivity Filter in the KcsA Channel to the Conductive Conformation: Predicted Consequences on Inactivation

pH-induced conformational changes in the selectivity filter of a potassium channel lead to alterations in its selectivity and permeation properties

The Selectivity Filter (SF) in tetrameric K+ channels, has a highly conserved sequence, TVGYG, at the extracellular entry to the channel pore region. There, the backbone carbonyl oxygens from the SF residues, create a stack of K+ binding sites where dehydrated K+ binds to induce a conductive conformation of the SF. This increases intersubunit interactions and confers a higher stability to the channel against thermal denaturation. Indeed, the fit of dehydrated K+ to its binding sites is fundamental to define K+ selectivity, an important feature of these channels. Nonetheless, the SF conformation can be modified by different effector molecules. Such conformational plasticity opposes selectivity, as the SF departs from the "induced-fit" conformation required for K+ recognition. Here we studied the KirBac1.1 channel, a prokaryotic analog of inwardly rectifying K+ channels, confronted to permeant (K+) and non-permeant (Na+) cations. This channel is pH-dependent and transits from the open state at neutral pH to the closed state at acidic pH. KirBac1.1 has the orthodox TVGYG sequence at the SF and thus, its behavior should resemble that of K+-selective channels. However, we found that when at neutral pH, KirBac1.1 is only partly K+ selective and permeates this ion causing the characteristic "induced-fit" phenomenon in the SF conformation. However, it also conducts Na+ with a mechanism of ion passage reminiscent of Na+ channels, i.e., through a wide-open pore, without increasing intersubunit interactions within the tetrameric channel. Conversely, when at acidic pH, the channel completely loses selectivity and conducts both K+ and Na+ similarly, increasing intersubunit interactions through an apparent "induced-fit"-like mechanism for the two ions. These observations underline that KirBac1.1 SF is able to adopt different conformations leading to changes in selectivity and in the mechanism of ion passage.

Steady-state and time-resolved fluorescent methodologies to characterize the conformational landscape of the selectivity filter of K+ channels

A variety of equilibrium and non-equilibrium methods have been used in a multidisciplinary approach to study the conformational landscape associated with the binding of different cations to the pore of potassium channels. These binding processes, and the conformational changes resulting therefrom, modulate the functional properties of such integral membrane properties, revealing these permeant and blocking cations as true effectors of such integral membrane proteins. KcsA, a prototypic K+ channel from Streptomyces lividans, has been extensively characterized in this regard. Here, we revise several fluorescence-based approaches to monitor cation binding under different experimental conditions in diluted samples, analyzing the advantages and disadvantages of each approach. These studies have contributed to explain the selectivity, conduction, and inactivation properties of K+ channels at the molecular level, together with the allosteric communication between the two gates that control the ion channel flux, and how they are modulated by lipids.