Helix sense of gramicidin channels as a "nonlocal" function of the primary sequence
Gramicidin A (gA) channels are dimers formed by right-handed beta 6.3-helical monomers. The stereochemical basis for the preference of a right-handed conformation remains obscure, but it has earlier been demonstrated that the handedness can be shifted by changing the chirality of each residue in the LD-sequence and therefore is determined by the peptide itself and not by channel-membrane interactions. We now examine the contributions of Trp15, the central Val residues 6-8, and residues 1-5. None of these alone are sufficient to specify the helix sense. To examine the D-Val6-L-Val7-D-Val8 sequence, the register of the 3 valines was shifted by one to L-Val5-D-Val6-L-Val7. The resulting analogue, [Val5,D-Ala8]gA, forms channels with a conductance and duration that are both somewhat less than those of gA channels. The reduced channel duration can be attributed to a steric conflict between the side chains of Val1 in one monomer and Val5 in the other monomer. The helix handedness is not altered by this modification, as shown by circular dichroism and two-dimensional nuclear magnetic resonance spectroscopy and by hybrid channel experiments. [Val5,D-Ala8]gA forms hybrid channels with gA (which forms right-handed channels), but not with des-Val1-gA- (which forms left-handed channels). Similar hybrid channel analysis shows that des-Trp15-gA and [L-Ala1,D-Ala2,L-Ala3,D-Ala4]gA also form right-handed channels. We conclude that the helix handedness most probably is a complex function of the arrangement of both the D-Val-L-Val-D-Val and the L-Trp-(D-Leu-L-Trp)3 segments.