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 ri ght-handed conformation remains obscure, but it has earlier been demon strated that the handedness can be shifted by changing the chirality o f each residue in the LD-sequence and therefore is determined by the p eptide itself and not by channel-membrane interactions, We now examine the contributions of Trp(15), the central Val residues 6-8, and resid ues 1-5. None of these alone are sufficient to specify the helix sense . To examine the D-Va(6)-L-Val(7)-D-Val(8) sequence, the register of t he 3 valines was shifted by one to L-Val(5)-D-Val(6)-Val(7). The resul ting analogue, [Val(5),D-AlA(8)]gA, forms channels with a conductance and duration that are both somewhat less than those of gA channels. Th e reduced channel duration can be attributed to a steric conflict betw een the side chains of Val(1) in one monomer and Val(5) in the other m onomer. The helix handedness is not altered by this modification, as s hown by circular dichroism and two-dimensional nuclear magnetic resona nce spectroscopy and by hybrid channel experiments. [Val(5),D-Ala(8)]g A forms hybrid channels with gA (which forms right-handed channels), b ut not with des-Val(1)-gA(-) (which forms left-handed channels). Simil ar hybrid channel analysis shows that des-Trp(15)-gA and [L-Ala(1),D-A la(2),L-Ala(3),D-Ala(4)]gA also form right-handed channels. We conclud e 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.