The question of whether a protein whose natural sequence is inverted adopts
a stable fold is still under debate. We have determined the 2.1-Angstrom c
rystal structure of the retro-GCN4 leucine zipper. In contrast to the two-s
tranded helical coiled-coil GCN4 leucine zipper, the retro-leucine zipper f
ormed a very stable, parallel four-helix bundle, which now lends itself to
further structural and functional studies.