Maximal Activity of KcsA, Kirbac1.1 and Kir2.1 Channels are Differentially Regulated by Membrane Thickness

Abstract

Ion channels may be regulated by numerous factors, including the physiochemical properties of the membrane in which they are imbedded. Hydrophobic matching between the hydrophobic thickness of the bilayer and the channel's hydrophobic length is thought to minimize the energetic penalty that would be needed to solvate hydrophobic residues or exposed lipid tails. Here we examine the role of hydrophobic matching in regulating the activity of 3 potassium channels, KcsA, KirBac1.1 and human Kir2.1. Purified channels were reconstituted into membranes containing 25 mol% POPG and 75 mol% diCn:1PC (14 < n < 22) (+ 1 mol% PI(4,5)P2 for Kir2.1). 86Rb+ influx assays indicate that KcsA channels are maximally active in thick membranes (diC20:1PC), while both KirBac1.1 and Kir2.1 channels were maximally active in thinner membranes (eg. < diC18:1PC). Single channel recordings in lipid bilayers of the same compositions indicate that membrane thickness affects the open probability of KcsA, but not unitary conductance. SAXS was used to quantify the hydrophobic thickness of each of our experimental conditions in order to quantify the energy associated with hydrophobic matching for these 3 proteins. Our initial calculations suggest the energies associated with membrane stretching or curvature are too great to account for hydrophobic matching in these channels, and suggest tilting of the α-helices of the proteins are required for these channels to be maximally active. Molecular dynamic simulations provide further insight into the molecular details associated with hydrophobic matching for each of these channels.