The Asymmetric Lipid Bilayer Revealed Sidedness of the Effective Phospholipids on the Single-Channel Properties of the KcsA Potassium Channel

Abstract

Function of ion channels is modulated by the composition of lipid bilayer. Some ion channels lose their activity when they are reconstituted into alien membrane environment. Thus, membrane lipids act as a co-factor for maintaining the channel activity as in the case for other membrane proteins. For the KcsA potassium channel, an anionic phospholipid has been thought to play an essential role in its function because KcsA exhibited reduced ion conducting activity in the artificial membrane without phosphatidylglycerol (PG). Relevance of PG to the KcsA function has been further supported by crystallographic studies that revealed specific binding of PG molecules to the transmembrane region of the channel. However, little is known about the mechanism on how lipid molecules, including PG, modulate the function of KcsA channel. In this study we analyzed effects of lipids on the single-channel current properties of KcsA using artificial lipid bilayer having asymmetric lipid composition. We revealed that presence of PG in the membrane significantly increased the open probability and unitary conductance of KcsA channel. We also found that these effects were substitutable with other anionic phospholipids such as phosphatidylserine and phosphatidic acid but not by neutral or cationic phospholipids such as phosphatidylcholine or ethyl phosphatidylcholine. Furthermore, the sidedness of these anionic lipid effects was analyzed. We concluded that the binding of anionic lipids to the specific lipid-binding sites, which had been found in the KcsA crystal structure, would not be an origin of these anionic lipid effects.