Increased Activity of Large Conductance Ca<SUP>2&#8290;</SUP>-Activated K<SUP>&#8290;</SUP> Channels in Negatively-Charged Lipid Membranes

Increased Activity of Large Conductance Ca<SUP>2&#8290;</SUP>-Activated K<SUP>&#8290;</SUP> Channels in Negatively-Charged Lipid Membranes

<P> The effects of membrane surface charge originated from lipid head groups on ion channels were tested by analyzing the activity of single large conductance Ca<SUP>2&#8290;</SUP>-activated K<SUP>&#8290;</SUP> (maxi K) channel from rat skeletal muscle. The conductances and open-state probability (P<SUB>o</SUB>) of single maxi K<SUB> </SUB>channels were compared in three types of planar lipid bilayers formed from a neutral phosphatidylethanolamine (PE) or two negatively-charged phospholipids, phosphatidylserine (PS) and phosphatidylinositol (PI). Under symmetrical KCl concentrations (3∼1,000 mM), single channel conductances of maxi K channels in charged membranes were 1.1∼1.7 times larger than those in PE membranes, and the differences were more pronounced at the lower ionic strength. The average slope conductances at 100 mM KCl were 251&#8273;9.9, 360&#8273;8.7 and 356&#8273;12.4 (mean&#8273;SEM) pS in PE, PS and PI membranes respectively. The potentials at which P<SUB>o</SUB> was 1/2, appeared to have shifted left by 40 mV along voltage axis in the membranes formed with PS or PI. Such shift was consistently seen at pCa 5, 4.5, 4 and 3.5. Estimation of the effect of surface charge from these data indicated that maxi K channels sensed the surface potentials at a distance of 8∼9 Å from the membrane surface. In addition, similar insulation distance (7∼9 Å) of channel mouth from the bilayer surface charge was predicted by a 3-barrier-2-site model of energy profile for the permeation of K<SUP>&#8290;</SUP> ions. In conclusion, despite the differences in structure and fluidity of phospholipids in bilayers, the activities of maxi K channels in two charged membranes composed of PS or PI were strikingly similar and larger than those in bilayers of PE. These results suggest that the enhancement of conductance and P<SUB>o</SUB> of maxi channels is mostly due to negative charges in the phospholipid head groups.