Effect of salt and membrane fluidity on fluorophore motions of a gramicidin C derivative

Cannabinoid interactions with ion channels and receptors

Cannabidiol (CBD), the non-psychoactive component of Cannabis sativa, acts on a diverse selection of membrane proteins with promising therapeutic potential in epilepsy and chronic pain. One such protein is the voltage-gated sodium channel (Na_v). CBD shows a lack of specificity for sodium channels; however, the method of interaction is still unknown. In this review, we will outline the studies that report reproducible results of CBD and other cannabinoids changing membrane channel function, with particular interest on Na_v. Na_v are implicated in fatal forms of epilepsy and are also associated with chronic pain. This makes Na_v potential targets for CBD interaction since it has been reported to reduce pain and seizures. One potential method of interaction that is of interest in this review is whether CBD affects channel function by altering lipid bilayer properties, independent of any possible direct interaction with membrane channels. CBD’s ability to interact with its targets is a novel and important discovery. This discovery will not only prompt further research towards CBD’s characterization, but also promotes the application of cannabinoids as potentially therapeutic compounds for diseases like epilepsy and pain.

How lipids affect the activities of integral membrane proteins

The activities of integral membrane proteins are often affected by the structures of the lipid molecules that surround them in the membrane. One important parameter is the hydrophobic thickness of the lipid bilayer, defined by the lengths of the lipid fatty acyl chains. Membrane proteins are not rigid entities, and deform to ensure good hydrophobic matching to the surrounding lipid bilayer. The structure of the lipid headgroup region is likely to be important in defining the structures of those parts of a membrane protein that are located in the lipid headgroup region. A number of examples are given where the conformation of the headgroup-embedded region of a membrane protein changes during the reaction cycle of the protein; activities of such proteins might be expected to be particularly sensitive to lipid headgroup structure. Differences in hydrogen bonding potential and hydration between the headgroups of phosphatidycholines and phosphatidylethanolamines could be important factors in determining the effects of these lipids on protein activities, as well as any effects related to the tendency of the phosphatidylethanolamines to form a curved, hexagonal H(II) phase. Effects of lipid structure on protein aggregation and helix-helix interactions are also discussed, as well as the effects of charged lipids on ion concentrations close to the surface of the bilayer. Interpretations of lipid effects in terms of changes in protein volume, lipid free volume, and curvature frustration are also described. Finally, the role of non-annular, or 'co-factor' lipids, tightly bound to membrane proteins, is described.

Effect of increased chain packing on gramicidin-lipid interactions

To study the effect of lipid packing on the dynamics of membrane proteins, the changes in the rotational motion of gramicidin tryptophans with increased packing brought about by high hydrostatic pressure through fluorescence spectroscopy were determined. In fluid phase dimyristoylphosphatidylcholine, the rotational motion of the residues decreased slightly with increased packing, but in the gel phase a significant reversible increase was observed. The magnitude of this increase was temperature dependent and much greater at lower temperatures. Quenching studies show that the increase in rotational motion is not due to a change in the location of the peptide in the membrane under pressure. Aromatic ring stacking between residues 9 and 15 appears to be stabilized under pressure, and there is no evidence of pressure-induced changes in peptide aggregation. The increase in rotational motion could be caused by a destabilization of hydrogen bonds between the indole hydrogens and the lipid head group oxygens due to an increase in the thickness of the compressible lipid bilayer with pressure without a concomitant lengthening of the peptide. These results indicate that specific interactions between lipids and proteins may play a major role of regulating the dynamics of membrane proteins.