Lipid nanodomains change ion channel function

Abstract

Signaling proteins and neurotransmitter receptors often associate with saturated chain and cholesterol-rich domains of cell membranes, also known as lipid rafts. The saturated chains and high cholesterol environment in lipid rafts can modulate protein function, but evidence for such modulation of ion channel function in lipid rafts is lacking. Here, using raft-forming model membrane systems containing cholesterol, we show that lipid lateral phase separation at the nanoscale level directly affects the dissociation kinetics of the gramicidin dimer, a model ion channel.

Figure 1

a) gramicidin channels in 5/2 dopc/spm/chl at 30 °C, 1 m kcl, 100 mv. horizontal bar is 2 sec in length. vertical bar is 3pa. b) logarithmically binned histogram of gramicidin lifetimes in 5/2/ dopc/chl at 30 °C and single exponential fit to log probability. c) gramicidin channels in 1/1/1 dopc/spm/chl at 32 °C (7 °C above miscibility transition for this mixture)), 1 m kcl, 100 mv. horizontal bar is 2 sec in length. vertical bar is 3pa. d) logarithmically binned histogram of gramicidin lifetimes in 1/1/1 dopc/spm/chl at 32 °C with single exponential (green trace) and double exponential fits (fuchsia trace).

Figure 2

a) lifetimes of gramicidin channels plotted against inverse temperature. black circles represent lifetimes for slow channels in dopc/spm/chl 1/1/1, red squares represent fast channels in dopc/spm/chl 1/1/1. blue triangles represent lifetimes of gramicidin in dopc/chl 5:2. error bars represent standard deviations of the means. cross hatched area represents the approximate location of the miscibility transition for the 1/1/1 mixture. b) conductance for fast gramicidin channels in the 1/1/1 mixture (black squares) and for channels in dopc/chl 5/2. black dashed line represents a log-linear fit to the fast channel lifetimes (r sq = .96). data from reference (34) for gramicidin in diphytanolyphosphatidylcholine membranes is replotted for comparison (orange dotted line). see si for lifetime and conductance plots against temperature in centigrade.

Figure 3

differential scanning calorimetry of lipid mixtures. scans are records from cooling scans staring at 60 °C and ending at 5° C, at a rate of 1 °C/minute. green line represents dopc/spm/chl 2/2/1 multilamellar vesicle 25 mg/ml. the black line represents dopc/spm/chl multilamellar vesicle 25 mg/ml with 100× the amount of gramicidin and twice the amount of ethanol used in electrophysiology experiments. the red line represents dopc/spm/chl 1/1/1 multilamellar vesicles 30mg/ml. the blue line represents dppc 5 mg/ml at 1/20 the scale for reference.