Low conductance gramicidin A channels are head-to-head dimers of beta 6.3-helices

Abstract

Weakly conductive, atypical channels were observed to form from highly purified Val1-gramicidin A in planar lipid bilayer membranes. The structure of these low-conductance channels (minis) was investigated by a detailed study of their channel forming characteristics. The possibility that minis originate from primary structural analogs or degradation products of gramicidin was considered and ruled out. In particular, spontaneous conductance changes in single channels demonstrated that minis can derive directly and reversibly from "standard" channels having the most common conductance level. The fraction of channels which are minis does not vary with changes in membrane gramicidin concentration, indicating that mini and standard channels have the same molecularity, that is, both are dimers. The mean lifetime of mini channels is only slightly shorter than that of standard channels, indicating that the six hydrogen bonds that stabilize the head-to-head dimer are minimally affected in minis. The fraction of channels which are minis is unaffected by the ionic strength, ionic composition, or pH of the bathing solution; it is also unaffected by the lipid composition of the bilayer. These findings are consistent with the hypothesis that minis arise from minor changes in the conformation of the Val1-gramicidin A molecule near the channel entrance or exit.

Environments and conformations of tryptophan side chains of gramicidin A in phospholipid bilayers studied by Raman spectroscopy

“…Single-channel conductance studies have revealed that there are various low-conductance channel states in addition to the major high-conductance states (Busath & Szabo, 1981). Such low-conductance channels convert reversibly to the major channels and are suggested to differ from the major channels in side-chain conformations (Urry et al, 1981;Busath & Szabo, 1988). The present models for the tryptophan conformations do not exclude the possibility that other tryptophan conformations are taken in some low-conductance states.…”

Triple-barrel Organization of ENaC, a Cloned Epithelial Na+ Channel

“…That these amiloride-sensitive channels expressed in oocytes are ENaCs is supported by the fact that they were never observed in water-injected oocytes. We conclude, therefore, that the microenvironment in which ENaC resides determines in large measure its conductance and mean open and closed times (31)(32)(33). Aside from these changes, the channels displayed comparable amiloride sensitivities, ion selectivities, and gating patterns.…”

Lipid nanodomains change ion channel function

“…We also find sensitivity to batch variability in lipids, especially DOPC. Below the miscibility transition, membranes demonstrate limited stability, and the emergence of large numbers of low conductance “mini-channels” 26 precludes long recordings and makes analysis difficult. Nonetheless, the presence of two separate lifetime distributions persists at temperatures below the miscibility transition.…”