On the helix sense of gramicidin A single channels.
In order to resolve whether gramicidin A channels are formed by right- or left-handed beta-helices, we synthesized an optically reversed (or mirror image) analogue of gramicidin A, called gramicidin A-, to test whether it forms channels that have the same handedness as channels formed by gramicidin M- (F. Heitz et al., Biophys. J. 40:87-89, 1982). In gramicidin M- the four tryptophan residues have been replaced with phenylalanine, and the circular dichroism (CD) spectrum therefore reflects almost exclusively contributions from the polypeptide backbone. The CD spectrum of gramicidin M- in dimyristoylphosphatidylcholine vesicles is consistent with a left-handed helical backbone folding motif (F. Heitz et al., Biophys. Chem. 24:149-160, 1986), and the CD spectra of gramicidins A and A- are essentially mirror images of each other. Based on hybrid channel experiments, gramicidin A- and M- channels are structurally equivalent, while gramicidin A and A- channels are nonequivalent, being of opposite helix sense. Gramicidin A- channels are therefore left-handed, and natural gramicidin A channels in phospholipid bilayers are right-handed beta 6.3-helical dimers.
Koeppe
R E
RE
Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville 72701.
Providence
L L
LL
Greathouse
D V
DV
Heitz
F
F
Trudelle
Y
Y
Purdie
N
N
Andersen
O S
OS
D000595:Amino Acid Sequence; D000818:Animals; D002942:Circular Dichroism; D006096:Gramicidin; D007473:Ion Channels / Q000737:chemistry*; D008969:Molecular Sequence Data; D011487:Protein Conformation
D007473:Ion Channels; D006096:Gramicidin