Regulation of sodium channel function by bilayer elasticity: the importance of hydrophobic coupling. Effects of Micelle-forming amphiphiles and cholesterol.
Membrane proteins are regulated by the lipid bilayer composition. Specific lipid-protein interactions rarely are involved, which suggests that the regulation is due to changes in some general bilayer property (or properties). The hydrophobic coupling between a membrane-spanning protein and the surrounding bilayer means that protein conformational changes may be associated with a reversible, local bilayer deformation. Lipid bilayers are elastic bodies, and the energetic cost of the bilayer deformation contributes to the total energetic cost of the protein conformational change. The energetics and kinetics of the protein conformational changes therefore will be regulated by the bilayer elasticity, which is determined by the lipid composition. This hydrophobic coupling mechanism has been studied extensively in gramicidin channels, where the channel-bilayer hydrophobic interactions link a "conformational" change (the monomer<-->dimer transition) to an elastic bilayer deformation. Gramicidin channels thus are regulated by the lipid bilayer elastic properties (thickness, monolayer equilibrium curvature, and compression and bending moduli). To investigate whether this hydrophobic coupling mechanism could be a general mechanism regulating membrane protein function, we examined whether voltage-dependent skeletal-muscle sodium channels, expressed in HEK293 cells, are regulated by bilayer elasticity, as monitored using gramicidin A (gA) channels. Nonphysiological amphiphiles (beta-octyl-glucoside, Genapol X-100, Triton X-100, and reduced Triton X-100) that make lipid bilayers less "stiff", as measured using gA channels, shift the voltage dependence of sodium channel inactivation toward more hyperpolarized potentials. At low amphiphile concentration, the magnitude of the shift is linearly correlated to the change in gA channel lifetime. Cholesterol-depletion, which also reduces bilayer stiffness, causes a similar shift in sodium channel inactivation. These results provide strong support for the notion that bilayer-protein hydrophobic coupling allows the bilayer elastic properties to regulate membrane protein function.
Lundbaek
Jens A
JA
Novo Nordisk A/S, Måløv, Denmark. lundbaek@dadlnet.dk
Birn
Pia
P
Hansen
Anker J
AJ
Søgaard
Rikke
R
Nielsen
Claus
C
Girshman
Jeffrey
J
Bruno
Michael J
MJ
Tape
Sonya E
SE
Egebjerg
Jan
J
Greathouse
Denise V
DV
Mattice
Gwendolyn L
GL
Koeppe
Roger E
RE
Andersen
Olaf S
OS
D000222:Adaptation, Physiological / Q000187:drug effects / Q000502:physiology; D002460:Cell Line; D002462:Cell Membrane / Q000187:drug effects / Q000502:physiology*; D002784:Cholesterol / Q000378:metabolism*; D004548:Elasticity; D006096:Gramicidin / Q000494:pharmacology; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D007668:Kidney / Q000187:drug effects / Q000502:physiology; D008051:Lipid Bilayers / Q000378:metabolism*; D040542:Mechanotransduction, Cellular / Q000187:drug effects / Q000502:physiology*; D008560:Membrane Fluidity / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D008823:Micelles; D015222:Sodium Channels / Q000187:drug effects / Q000502:physiology*; D013501:Surface-Active Agents / Q000378:metabolism
D008051:Lipid Bilayers; D008823:Micelles; D015222:Sodium Channels; D013501:Surface-Active Agents; D006096:Gramicidin; D002784:Cholesterol