The hydrophobic core of the GCN4 leucine-zipper dimerization domain is formed by a parallel helical association between nonpolar side chains at the a and d positions of the heptad repeat. Here we report a self-assembling coiled-coil array formed by the GCN4-pAe peptide that differs from the wild-type GCN4 leucine zipper by alanine substitutions at three charged e positions. GCN4-pAe is incompletely folded in normal solution conditions yet self-assembles into an antiparallel tetraplex in crystals by formation of unanticipated hydrophobic seams linking the last two heptads of two parallel double-stranded coiled coils. The GCN4-pAe tetramers in the lattice associate laterally through the identical interactions to those in the intramolecular dimer–dimer interface. The van der Waals packing interaction in the solid state controls extended supramolecular assembly of the protein, providing an unusual atomic scale view of a mesostructure.