Gramicidin-induced enhancement of transbilayer reorientation of lipids in the erythrocyte membrane

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Abstract
Incorporation of the channel-forming antibiotic gramicidin into the membrane of human erythrocytes highly (up to 30-fold) enhances rates of reorientation (flip) of lysophosphatidylcholine and palmitoylcarnitine to the inner membrane layer after their primary incorporation into the outer layer. Despite the high increase of flip rates by gramicidin, the asymmetric orientation of the inner membrane layer phospholipids phosphatidylethanolamine and phosphatidylserine is stable as demonstrated by the lack of accessibility of these lipids toward cleavage by exogenous phospholipase A2. On the other hand, gramicidin enhances the rate of cleavage of outer membrane layer phosphatidylcholine by phospholipase A2, which indicates changes in the packing of phosphatidylcholine following gramicidin binding. The increase of flip becomes detectable when about 105 copies of gramicidin per cell have been bound (gramicidin to membrane phospholipid ratio of 1:2000). This is a 1000-fold higher concentration than that required for an increase of K+ permeability mediated by the gramicidin channel. Acceleration of flip is thus not simply correlated with channel formation. The enhancement of flip is markedly dependent on structural details of gramicidin. Formylation of its four tryptophan residues abolishes the effect. Even at high concentrations of formylated gramicidin at which the extents of binding of native and of formylated gramidicin to the membrane are comparable, no flip acceleration is produced. Enhancement of flip by gramicidin occurs after a temperature-dependent lag phase. At 37.degree. C, flip rates begin to increase within a few minutes and at 25.degree. C, only after 3 h. This lag phase is most likely not due to limitations by the rate of binding of gramicidin to the membrane. At gramicidin concentrations at which flip is enhanced, gramicidin, but not Trp-N-formylated gramicidin, produces aqueous leaks in the erythrocyte membrane permeable to large ions (choline, oxalate) and nonelectrolytes (erythritol, sucrose). In the following paper, flip acceleration and production of aqueous leaks by gramicidin are shown to be accompanied by the formation of hexagonal HII lipid structures, and the relationship between the phenomena is discussed [Tournois, H., Leunissen-Bijvelt, J., Haest, C. W. M., De Gier, J., and De Kruijff, B. (1987) Biochemistry (following paper in this issue)].