On the conductance heterogeneity in membrane channels formed by gramicidin A. A cooperative study

The relative frequency of low-conductance variants of gramicidin A channels in lipid bilayers was determined in parallel experiments in two different laboratories. A common gramicidin stock solution was tested in both labs and, initially, gave rise to significantly different proportions (9% v. 23%) of "mini" channels in the two labs. The lipid and gramicidin solutions were exchanged to identify the source of the difference: When using solutions prepared in lab A (Andersen), lab B (Busath) observed 9% minis, consistent with the original findings in lab A; when using the gramicidin solution prepared in lab B, lab A observed 18% minis, consistent with the original findings in lab B. The experimental apparatus and analysis techniques are therefore not the source of the discrepancy; rather, the difference appears to stem from some factor(s) related to the gramicidin, lipid, and electrolyte solutions. It appears that the mini frequency cannot reflect intrinsic characteristics of the channel-forming peptide, but rather must, at least in part, reflect environmental modulations of channel properties. This has implications for the interpretation of multi-channel experiments on gramicidin A.