Ion channels

Abstract:

Inhibition by intracellular H(+) (pH gating) and activation by phosphoinositides such as PIP(2) (PIP(2)-gating) are key regulatory mechanisms in the physiology of inwardly-rectifying potassium (Kir) channels. Our recent findings suggest that PIP(2) gating and pH gating are controlled by an intra-subunit H-bond at the helix-bundle crossing between a lysine in TM1 and a backbone carbonyl group in TM2. This interaction only occurs in the closed state and channel opening requires this H-bond to be broken, thereby influencing the kinetics of PIP(2) and pH gating in Kir channels. In this addendum, we explore the role of H-bonding in heteromeric Kir4.1/Kir5.1 channels. Kir5.1 subunits do not possess a TM1 lysine. However, homology modelling and molecular dynamics simulations demonstrate that the TM1 lysine in Kir4.1 is capable of H-bonding at the helix-bundle crossing. Consistent with this, the rates of pH and PIP2 gating in Kir4.1/Kir5.1 channels (two H-bonds) were intermediate between those of wild-type homomeric Kir4.1 (four H-bonds) and Kir4.1(K67M) channels (no H-bonds) suggesting that the number of H-bonds in the tetrameric channel complex determines the gating kinetics. Furthermore, in heteromeric Kir4.1(K67M)/Kir5.1 channels, where the two remaining H-bonds are disrupted, we found that the gating kinetics were similar to Kir4.1(K67M) homomeric channels despite the fact that these two channels differ considerably in their PIP(2) affinities. This indicates that Kir channel PIP(2) affinity has little impact on either the PIP(2) or pH gating kinetics.

Abstract:

Our understanding of the mammalian inwardly rectifying family of K+ channels (Kir family) has recently been advanced by X-ray crystal structures of two homologous prokaryotic orthologs (KirBac1.1 and KirBac3.1). However, the functional properties of these KirBac channels are still poorly understood. To address this problem, we cloned and characterized genes encoding KirBac orthologs from a wide variety of different prokaryotes and a simple unicellular eukaryote. The functional properties of these KirBacs were then examined by growth complementation in a K+ uptake-deficient strain of Escherichia coli (TK2420). Whereas some KirBac genes exhibited robust growth complementation, others either did not complement or showed temperature-dependent complementation including KirBac1.1 and KirBac3.1. In some cases, KirBac expression was also toxic to the growth of E. coli. The KirBac family exhibited a range of sensitivity to the K+ channel blockers Ba2+ and Cs+ as well as differences in their ability to grow on very low-K+ media, thus demonstrating major differences in their permeation properties. These results reveal the existence of a functionally diverse superfamily of microbial KirBac genes and present an excellent resource for the structural and functional analysis of this class of K+ channels. Furthermore, the complementation assay used in this study provides a simple and robust method for the functional characterization of a range of prokaryotic K+ channels that are difficult to study by traditional methods.

Abstract:

Recent genetic linkage studies have identified an association between missense variations in the gene encoding the Kir4.1 potassium channel (KCNJ10) and seizure susceptibility phenotypes in both humans and mice. The results of this study demonstrate that these variations (T262S and R271C) do not produce any observable changes in channel function or in predicted channel structure. It is therefore unlikely that the seizure susceptibility phenotypes associated with these missense variations are caused by changes in the intrinsic functional properties of Kir4.1.