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Large-scale reorganization of protein interactions characterizes many biological processes, yet few systems are accessible to biophysical studies that display this property. The capsid protein of Nudaurelia capensis omega Virus (NomegaV) has previously been characterized in two dramatically different T = 4 quasi-equivalent assembly states when expressed as virus-like particles (VLPs) in a baculovirus system. The procapsid (pH 7), is round, porous, and approximately 450 A in diameter. It converts, in vitro, to the capsid form at pH 5 and the capsid is sealed shut, shaped like an icosahedron, has a maximum diameter of 410 A and undergoes an autocatalytic cleavage at residue 570. Residues 571-644, the gamma peptide, remain associated with the particle and are partially ordered. The interconversion of these states has been previously studied by solution X-ray scattering, electron cryo microscopy (CryoEM), and site-directed mutagenesis. The particle structures appear equivalent in authentic virions and the low pH form of the expressed and assembled protein. Previously, and before the discovery of the multiple morphological forms of the VLPs, we reported the X-ray structure of authentic NomegaV at 2.8 A resolution. These coordinates defined the fold of the protein but were not refined at the time because of technical issues associated with the approximately 2.5 million reflection data set. We now report the refined, authentic virus structure that has added 29 residues to the original model and allows the description of the chemistry of molecular switching for T = 4 capsid formation and the multiple morphological forms. The amino and carboxy termini are internal, predominantly helical, and disordered to different degrees in the four structurally independent subunits; however, the refined structure shows significantly more ordered residues in this region, particularly at the amino end of the B subunit that is now seen to invade space occupied by the A subunits. These additional residues revealed a previously unnoticed strong interaction between the pentameric, gamma peptide helices of the A and B subunits that are largely proximal to the quasi-6-fold axes. One C-terminal helix is ordered in the C and D subunits and stabilizes a flat interaction in two interfaces between the protein monomers while the other, quasi-equivalent, interactions are bent. As this helix is arginine rich, the comparable, disordered region in the A and B subunits probably interacts with RNA. One of the subunit-subunit interfaces has an unusual arrangement of carboxylate side chains. Based on this observation, we propose a mechanism for the control of the pH-dependent transitions of the virus particle.

The structures of the double-shelled rice dwarf virus and of its single-shell core have been determined by cryoelectron microscopy and image reconstruction. The core carries a prominent density located at each of the icosahedral faces of its T = 1 lattice. These protrusions are formed by outer shell trimers, tightly inserted at the threefold positions of the core. Such configuration of the core may guide the assembly of the outer shell, aided by lateral interactions between its subunits, into a T = 13 lattice. The organization of the phytoreovirus capsid elucidates for the first time a general model for assembling two unique T numbers of quasi-equivalence.

The three-dimensional structure of a self-assembled, recombinant hepatitis E virus particle has been solved to 22-Å resolution by cryo-electron microscopy and three-dimensional image reconstruction. The single subunit of 50 kDa is derived from a truncated version of the open reading frame-2 gene of the virus expressed in a baculovirus system. This is the first structure of a T = 1 particle with protruding dimers at the icosahedral two-fold axes solved by cryo-electron microscopy. The protein shell of these hollow particles extends from a radius of 50 Å outward to a radius of 135 Å. In the reconstruction, the capsid is dominated by dimers that define the 30 morphological units. The outer domain of the homodimer forms a protrusion, which corresponds to the spike-like density seen in the cryo-electron micrograph. This particle retains native virus epitopes, suggesting its potential value as a vaccine.

T4 encodes two dispensable proteins that bind to the outer surface of the mature capsid. Soc (9 kDa) stabilizes the capsid against extremes of alkaline pH and temperature, but Hoc (40 kDa) has no perceptible effect. Both proteins have been developed as display platforms. Their positions on the hexagonal surface lattice of gp23*, the major capsid protein, were previously defined by two-dimensional image averaging of negatively stained electron micrographs of elongated variant capsids. We have extended these observations by reconstructing cryo-electron micrographs of isometric capsids produced by a point mutant in gene 23, for both Hoc+.Soc+ and Hoc+.Soc- phages. The expected T = 13 lattice was observed, with a single Hoc molecule at the center of each gp23* hexamer. The vertices are occupied by pentamers of gp24*: despite limited sequence similarity with gp23*, the respective monomers are similar in size and shape, suggesting they may have the same fold. However, gp24* binds neither Hoc nor Soc; in situ, Soc is visualized as trimers at the trigonal points of the gp23* lattice and as monomers at the sites closest to the vertices. In solution, Soc is a folded protein ( approximately 10% alpha-helix and 50-60% beta sheet) that is monomeric as determined by analytic ultracentrifugation. Thus its trimerization on the capsid surface is imposed by a template of three symmetry-related binding sites. The observed mode of Soc binding suggests that it stabilizes the capsid by a clamping mechanism and offers a possible explanation for the phenotype of osmotic shock resistance.