Membrane phosphate headgroups' modulation of permeation of alkaline cations in gramicidin channels.

The function of membrane proteins is modulated by lipid bilayers. The permeation of ions in gramicidin A channels (gA) is markedly distinct in monoglyceride and phospholipid membranes. It was previously demonstrated that membrane phosphate headgroups accelerate the rate of proton transfer in gA. However, the permeation of alkalines in gA channels is considerably slower in phospholipid than in monoglyceride membranes. In this study, gA channels were reconstituted in various membranes of ceramides, monoglycerides, phospholipids, or sphingolipids. It is demonstrated that single channel conductances to alkalines are similar among bilayers consisting of phospholipids and sphingolipids, and ceramides and monoglycerides. The presence of phosphate headgroups in membranes (and not the double acyl chains in lipids) attenuates alkaline permeation and enhances the proton transfer permeation in gA channels. In ceramide membranes in low ionic strength (<250 mM) solutions, gA channels become dysfunctional. The experimental results are discussed in regard to membrane/solution and membrane/protein interfaces.

Enhancement of proton transfer in ion channels by membrane phosphate headgroups.

The transfer of protons (H+) in gramicidin (gA) channels is markedly distinct in monoglyceride and phospholipid membranes. In this study, the molecular groups that account for those differences were investigated using a new methodology. The rates of H+ transfer were measured in single gA channels reconstituted in membranes made of plain ceramides or sphingomyelins and compared to those in monoglyceride and phospholipid bilayers. Single-channel conductances to protons (gH) were significantly larger in sphingomyelin than in ceramide membranes. A novel and unsuspected finding was that H+ transfer was heavily attenuated or completely blocked in ceramide (but not in sphingomyelin) membranes in low-ionic-strength solutions. It is reasoned that H-bond dynamics at low ionic strengths between membrane ceramides and gA makes channels dysfunctional. The rate of H+ transfer in gA channels in ceramide membranes is significantly higher than that in monoglyceride bilayers. This suggests that solvation of the hydrophobic surface of gA channels by two acyl chains in ceramides stabilizes the gA channels and the water wire inside the pore, leading to an enhancement of H+ transfer in relation to that occurring in monoglyceride membranes. gH values in gA channels are similar in ceramide and monoglyceride bilayers and in sphingomyelin and phospholipid membranes. It is concluded that phospho headgroups in membranes have significant effects on the rate of H+ transfer at the membrane gA channel/solution interfaces, enhancing the entry and exit rates of protons in channels.