The structures of three different human rhinovirus 14 (HRV14)-Fab complexes have been explored with X-ray crystallography and cryoelectron microscopy procedures. All three antibodies bind to the NIm-IA site of HRV14, which is the β-B–β-C loop of the viral capsid protein VP1. Two antibodies, Fab17-IA (Fab17) and Fab12-IA (Fab12), bind bivalently to the virion surface and strongly neutralize viral infectivity whereas Fab1-IA (Fab1) strongly aggregates and weakly neutralizes virions. The structures of the two classes of virion-Fab complexes clearly differ and correlate with observed binding neutralization differences. Fab17 and Fab12 bind in essentially identical, tangential orientations to the viral surface, which favors bidentate binding over icosahedral twofold axes. Fab1 binds in a more radial orientation that makes bidentate binding unlikely. Although the binding orientations of these two antibody groups differ, nearly identical charge interactions occur at all paratope-epitope interfaces. Nucleotide sequence comparisons suggest that Fab17 and Fab12 are from the same progenitor cell and that some of the differing residues contact the south wall of the receptor binding canyon that encircles each of the icosahedral fivefold vertices. All of the antibodies contact a significant proportion of the canyon region and directly overlap much of the receptor (intercellular adhesion molecule 1 [ICAM-1]) binding site. Fab1, however, does not contact the same residues on the upper south wall (the side facing away from fivefold axes) at the receptor binding region as do Fab12 and Fab17. All three antibodies cause some stabilization of HRV14 against pH-induced inactivation; thus, stabilization may be mediated by invariant contacts with the canyon.

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HK97 Prohead II is an early intermediate in the maturation of HK97, a T = 7 dsDNA-tailed bacteriophage related to bacteriophage lambda. Previously, selected capsid-protein genes of HK97 were expressed in Escherichia coli and spontaneously assembled to form an icosahedral capsid that followed a maturation pathway closely similar to the authentic virion. The crystal structure of the mature HK97 capsid (Head II) made in this way was reported at 3.5 A resolution. Additional high-resolution structures of intermediates are needed to understand the maturation mechanism. The crystal structure of expressed Prohead II will elucidate the early steps of HK97 assembly. Crystals of the Prohead II mutant W336F were grown in 0.1 M HEPES pH 7.5, 0.2 M CaCl(2) and 2-3% PEG 4000 at a Prohead II concentration of 16.5 mg ml(-1). It was not possible to grow high-quality crystals of wild-type Prohead II. Diffraction was observed to 5 A resolution from these crystals on beamline 14BM-C at the Advanced Photon Source and data were collected to 5.5 A with a completeness of 77%. The space group was P2(1)3, with unit-cell parameter a = 707.0 A and four particles in the unit cell. The particles are on the body diagonals of the cubic cell, with icosahedral threefold axes coincident with crystallographic threefold axes. Self-rotation function and locked-rotation function analysis determined the particle orientation and a one-dimensional R-factor search along the body diagonal indicated that the particle centers were close to (1/4, 1/4, 1/4) and symmetry-related positions. Molecular-replacement averaging and phase extension are under way.

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