Effects of phospholipid surface charge on ion conduction in the K+ channel of sarcoplasmic reticulum.
Single-channel K+ currents through sarcoplasmic reticulum K+ channels were compared after reconstitution into planar bilayers formed from neutral or negatively charged phospholipids. In neutral bilayers, the channel conductance saturates with K+ concentration according to a rectangular hyperbola, with half-saturation at 40 mM K+, and maximum conductance of 220 pS. In negatively charged bilayers (70% phosphatidylserine/30% phosphatidylethanolamine), the conductance is, at a given K+ concentration, higher than in neutral bilayers. This effect of negative surface charge is increasingly pronounced at lower ionic strength. The maximum conductance at high K+ approaches 220 pS in negative bilayers, and the channel's ionic selectivity is unaffected by lipid charge. The divalent channel blocker " bisQ11 " causes discrete blocking events in both neutral and negatively charged bilayers; the apparent rate constant of blocking is sensitive to surface charge, while the unblocking rate is largely unaffected. Bilayers containing a positively charged phosphatidylcholine analogue led to K+ conductances lower than those seen in neutral bilayers. The results are consistent with a simple mechanism in which the local K+ concentration sensed by the channel's entryway is determined by both the bulk K+ concentration and the bulk lipid surface potential, as given by the Gouy-Chapman model of the electrified interface. To be described by this approach, the channel's entryway must be assumed to be located 1-2 nm away from the lipid surface, on both sides of the membrane.
D000818:Animals; D004553:Electric Conductivity; D007473:Ion Channels / Q000502:physiology*; D008051:Lipid Bilayers; D008433:Mathematics; D008954:Models, Biological; D009994:Osmolar Concentration; D010743:Phospholipids / Q000502:physiology; D011188:Potassium / Q000378:metabolism*; D012519:Sarcoplasmic Reticulum / Q000502:physiology*; D013329:Structure-Activity Relationship
D007473:Ion Channels; D008051:Lipid Bilayers; D010743:Phospholipids; D011188:Potassium