THE CRYSTAL-STRUCTURE OF THE DESIGNED TRIMERIC COILED-COIL COIL-V(A)L(D) - 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 VEALE HG-amide has been determined and refined to a crystallographic R-facto r of 21.4% for all data from 10-Angstrom to 2.1-Angstrom resolution. T his molecule is called coil-V(a)L(d) because it contains valine in the a heptad positions and leucine in the d heptad positions. In the trig onal crystal, three molecules, related by a crystallographic threefold axis, form a parallel. three-helix bundle. The bundles are stacked he ad-to-tail to form a continuous coiled coil along the c-direction of t he crystal. The contacts among the three helices within the coiled coi l are mainly hydrophobic: four layers of valine residues alternate wit h four layers of leucine residues to form the core of the bundle. In c ontrast, mostly hydrophilic contacts mediate the interaction between t rimers: here a total of two direct protein-protein hydrogen bonds are found. Based on the structure, we propose a scheme for designing cryst als of peptides containing continuous two-, three-, and four-stranded coiled coils.