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Abstract:
Membrane-spanning gramicidin channels remain unique because of their small size, well-characterized function and well-defined structure. In organic solvents, the gramicidins are conformationally polymorphic; but a large body of work (summarized in Refs 1,2) shows conclusively that the predominant channel form is a head-to-head dimer of two single-stranded (SS) β-helices — confirming an early prediction. We therefore take issue with the suggestion in a recent editorial in Nature Structural Biology that the major conformer responsible for ion movement across membranes is a double-stranded (DS) dimer. The DS gramicidin crystal structures determined by Duax and collaborators, which were presented at the conference described in the editorial, constitute an important advance, but they do not relate to the active channel structure.
The identity of the channel structure was established by 1980 based on experiments that probed the general organization of gramicidin monomers in membrane-spanning channels. The experiment's aim was to distinguish between two different folding motifs (Fig. 1b): DS dimers, which were known to exist in organic solvents; and SS dimers, which further could be distinguished by their orientation. Single-channel experiments with gramicidin analogs modified at their N-termini showed that the N-termini are close together in the membrane-spanning channel. The introduction of a charged residue at the N-terminus abolishes channel activity; but such a replacement is tolerated at the C-terminus. By 1978, the evidence was strongly in support of the head-to-head SS dimer. Single-channel experiments using more subtle N-terminal modifications provided additional support for this structure.
