The crystal structure of the designed trimeric coiled coil coil-VaLd: implications for engineering crystals and supramolecular assemblies.

The three-dimensional structure of the 29-residue designed coiled coil having the amino acid sequence acetyl-E VEALEKK VAALESK VQALEKK VEALEHG-amide has been determined and refined to a crystallographic R-factor of 21.4% for all data from 10-A to 2.1-A resolution. This molecule is called coil-VaLd because it contains valine in the a heptad positions and leucine in the d heptad positions. In the trigonal crystal, three molecules, related by a crystallographic threefold axis, form a parallel three-helix bundle. The bundles are stacked head-to-tail to form a continuous coiled coil along the c-direction of the crystal. The contacts among the three helices within the coiled coil are mainly hydrophobic: four layers of valine residues alternate with four layers of leucine residues to form the core of the bundle. In contrast, mostly hydrophilic contacts mediate the interaction between trimers: here a total of two direct protein--protein hydrogen bonds are found. Based on the structure, we propose a scheme for designing crystals of peptides containing continuous two-, three-, and four-stranded coiled coils.