Figure 1.8. Two parallel models of the leucine zipper structure (based upon Fig. 4 of Oas et al. (1990)).
Helices are represented by cylinders with arrows showing the orientation. The leucine heptad is shown as dark spheres and the alternate heptad repeat of hydrophobic residues is repesented by the light spheres. (A) Parallel version of the interdigitation model of Landschulz et al. (1988). Parallel association of helices combined with interdigitation of leucyl sidechains in this model results in an assymetric molecule. (B) The coiled-coil model in which both the leucyl sidechains and those sidechains from residues four along in the sequence pack together to form a hydrophobic dimer interface (see top view). This results in a rotationally symmetric molecule (see small arrow), which is consistent with the symmetry seen in 1H NMR spectra of the GCN4 leucine zipper (Oas et al., 1990; Saudek et al., 1990, 1991b). The superhelical twist of the coiled-coil is omitted from (B) for simplicity.