THE CONDUCTING FORM OF GRAMICIDIN-A IS A RIGHT-HANDED DOUBLE-STRANDEDDOUBLE HELIX

The linear pentadecapeptide antibiotic, gramicidin D, is a naturally o ccurring product of Bacillus brevis known to form ion channels in synt hetic and natural membranes. The x-ray crystal structures of the right -handed double-stranded double-helical dimers (DSDHR) reported here ag ree with N-15-NMR and CD data on the functional gramicidin D channel i n lipid bilayers. These structures demonstrate single-file ion transfe r through the channels. The results also indicate that previous crysta l structure reports of a left-handed double-stranded double-helical di mer in complex with Cs+ and K+ salts may be in error and that our evid ence points to the DSDHR as the major conformer responsible for ion tr ansport in membranes.