CRYSTAL-STRUCTURE OF A BOVINE NEUROPHYSIN-II DIPEPTIDE COMPLEX AT 2.8-A DETERMINED FROM THE SINGLE-WAVELENGTH ANOMALOUS SCATTERING SIGNAL OF AN INCORPORATED IODINE ATOM

The crystal structure of a dipeptide complex of bovine neurophysin II has been solved at 2.8 angstrom resolution solely by using single-wavelength anomalous scattering data from a single iodinated derivative. The asymmetric unit is an elongated tetramer of dimensions 110 x 40 x 30 angstrom, composed of two dimers related by pseudo twofold symmetry. Each monomer consists of two homologous layers, each with four antiparallel beta-strands. The two regions are connected by a helix followed by a long loop. Monomer-monomer contacts involve antiparallel beta-sheet interactions, which form a dimer with two layers of eight beta-strands. One peptide per monomer occupies the principal hormone-binding pocket formed by part of the amino-terminal region and parts of the connecting helix and loop, with binding to protein consistent with conclusions drawn from solution studies. Dimer-dimer contacts involve the Tyr49 region adjacent to this site. A fifth dipeptide, of unknown biological significance, helps to stabilize one of the monomer-monomer interfaces and the tetramer network in the crystal.