ASYMMETRIC GRAMICIDIN CHANNELS - HETERODIMERIC CHANNELS WITH A SINGLEF(6)VAL(1) RESIDUE

Substitution of Val(1) by 4,4,4,4',4',4'-F-6,Val in [Val(1)]]gramicidi n A ([Val(1)]gA) produces channels in which the effects of amino acid replacements on dimer stability and ion permeation are nonadditive. If only one Val(1) (in a symmetric [Val(1)]gA channel) is substituted by F(6)Val, the resulting heterodimeric channels are destabilized relati ve to both homodimeric parent channels and the single-channel conducta nce of the heterodimeric channels is reduced relative to the parent ch annels (Russell, E.W.B., L.B. Weiss, F.I. Navetta, R.E. Koeppe II, and O.S. Andersen. 1986. Single-channel studies on linear gramicidins wit h altered amino acid side chains. Effects of altering the polarity of the side chain at position #1 in gramicidin A. Biophys. J.49:673; Durk in, J.T., R.E. Koeppe II, and O.S. Andersen. 1990. Energetics of grami cidin hybrid channel formation as a test for structural equivalence. S ide-chain substitutions in the native sequence. J. Mol. Biol. 211:221- 234). To understand the basis for this destabilization, we have examin ed further the characteristics of [F(6)Val(1)]/[Xxx(1)]gA heterodimers , where Xxx = Gly, Val, and Ala. These heterodimeric channels show rap id current transitions between (at least) two current levels and displ ay asymmetric i-V characteristics. The orientation of the heterodimers relative to the applied potential was determined by asymmetric additi on of the gramicidin analogs, one to each side of a preformed bilayer. The current transitions are most clearly illustrated for [F-6,Val(1)] /[Gly(1)]gA heterodimers, which possess two finite and well defined cu rrent levels. Based on the existence of these two conductance states a nd the analysis of duration and interval distributions, we conclude th at the transitions between the two current levels correspond to confor mational transitions in ''stable'' heterodimers. In the case of [F(6)V al(1)]/[Val(1)]gA and [F(6)Val(1)]/[Ala(1)]gA heterodimers, the low-co nductance state is indistinguishable from zero. The two (or more) cond uctance states presumably correspond to different orientations of the dipolar F(6)Val(1) side chain. The distribution between the high- and the low-conductance states varies as a function of potential in [F(6)V al(1)]/[Gly(1)]gA channels. These characteristics cause the [F(6)Val(1 )]/nonpolar (Val, Ala, Gly)gA hybrid channels to serve as a ''simple'' model for understanding gating transitions in membrane-spanning chann els.