Packed protein bilayers in the 0.90 angstrom resolution structure of a designed alpha helical bundle

A 12-residue peptide designed to form an ct-helix and self-associate into a n antiparallel 4-alpha-helical bundle yields a 0.9 Angstrom crystal structu re revealing unanticipated features. The structure was determined by direct phasing with the "Shake-and-Bake" program, and contains four crystallograp hically distinct 12-mer peptide molecules plus solvent for a total of 479 a toms. The crystal is formed from nearly ideal alpha-helices hydrogen bonded head-to-tail into columns, which in turn pack side-by-side into sheets spa nning the width of the crystal. Within each sheet, the cr-helices run antip arallel and are closely spaced (9-10 Angstrom center-to-center). The sheets are more loosely packed against each other (13-14 Angstrom between helix c enters). Each sheet is amphiphilic: apolar leucine side chains project from one face, charged lysine and glutamate side chains from the other face. Th e sheets are stacked with two polar faces opposing and two apolar faces opp osing. The result is a periodic biomaterial composed of packed protein bila yers, with alternating polar and apolar interfaces. All of the 30 water mol ecules in the unit cell lie in the polar interface or between the stacked t ermini of helices. A section through the sheet reveals that the helices pac ked at the apolar interface resemble the four-ct-helical bundle of the desi gn, but the helices overhang parts of the adjacent bundles, and the helix c rossing angles are less steep than intended (7-11 degrees rather than 18 de grees).