An intrahelical salt bridge within the trigger site stabilizes the GCN4 leucine zipper

We previously reported that a helical trigger segment within the GCN4 leuci ne zipper monomer is indispensable for the formation of its parallel two-st randed coiled coil. Here, we demonstrate that the intrinsic secondary struc ture of the trigger site is largely stabilized by an intrahelical salt brid ge. Removal of this surface salt bridge by a single amino acid mutation ind uced only minor changes in the backbone structure of the GCN4 leucine zippe r dimer as verified by nuclear magnetic resonance. The mutation, however, s ubstantially destabilized the dimeric structure. These findings support the proposed hierarchic folding mechanism of the GCN4 coiled coil in which loc al helix formation within the trigger segment precedes dimerization.