Comparison of the structures of three circoviruses: chicken anemia virus, porcine circovirus type 2, and beak and feather disease virus

Abstract

Circoviruses are small, nonenveloped icosahedral animal viruses characterized by circular single-stranded DNA genomes. Their genomes are the smallest possessed by animal viruses. Infections with circoviruses, which can lead to economically important diseases, frequently result in virus-induced damage to lymphoid tissue and immunosuppression. Within the family Circoviridae, different genera are distinguished by differences in genomic organization. Thus, Chicken anemia virus is in the genus Gyrovirus, while porcine circoviruses and Beak and feather disease virus belong to the genus CIRCOVIRUS: Little is known about the structures of circoviruses. Accordingly, we investigated the structures of these three viruses with a view to determining whether they are related. Three-dimensional maps computed from electron micrographs showed that all three viruses have a T=1 organization with capsids formed from 60 subunits. Porcine circovirus type 2 and beak and feather disease virus show similar capsid structures with flat pentameric morphological units, whereas chicken anemia virus has stikingly different protruding pentagonal trumpet-shaped units. It thus appears that the structures of viruses in the same genus are related but that those of viruses in different genera are unrelated.

FIG. 1.

Micrographs of various circoviruses. (A) Cryomicrograph of CAV. (B) Cryomicrograph of PCV-2. (C) Micrograph of a negatively stained preparation of a mixture of CAV and BFDV. The larger, rough particles are CAV, and the smaller, smoother particles are BFDV. Scale bar, 50 nm.

FIG. 2.

Fourier shell correlations. (A) Fourier shell correlation between a map not corrected for defocus and a map combining data from 10 other micrographs corrected for defocus (solid line). The computed phase-contrast transfer function (broken line) for the appropriate level of defocus of the uncorrected map, in this case, 3,500 nm, was calculated on the assumption of 5% amplitude contrast. Note the good agreement between the zero-crossing positions. (B) Fourier shell correlation between maps computed from two half data sets, showing good agreement out to spacings of about 1.2 nm.

FIG. 3.

Determination of the absolute handedness of CAV. Average phase residuals for 45 particle images recorded at a tilt of 10° were determined by using the three-dimensional model (see Fig. 4A) and tilt-transformed orientation parameters of the corresponding untilted particle images. The contours show phase residuals for tilt transformations up to 15° along the x and y axes. At any point in the plot, the tilt angle (x²+ y²)^1/2 is the distance from the origin, and the direction of the tilt axis is arctan(y/x). The direction of the known tilt axis of the goniometer is shown as a diagonal line. The minimum phase residual (54.6°) at a tilt of about −10° around the known tilt axis indicates that the map (see Fig. 4A) is of the correct handedness. The residual (67.6°) at the point corresponding to the opposite handedness is considerably higher, indicating the degree of confidence in the determination.

FIG. 4.

Three-dimensional maps of circoviruses. (A) CAV computed from cryomicrographs. The capsid is formed from 12 pentagonal trumpet-shaped capsomeres, indicating a T=1 surface lattice containing 60 subunits. (B) PCV-2 computed from cryomicrographs. (C) BFDV computed from a micrograph of a negatively stained preparation. The structures in panels B and C are very similar, showing flat pentamer units making contacts across the twofold positions and around the threefold positions and indicating a T=1 surface lattice containing 60 protein subunits in each structure. All maps are viewed along a twofold axis.

FIG. 5.

Central sections of the three maps shown in Fig. 4. (A) CAV. (B) PCV-2. (C) BFDV. The sections are normal to a twofold axis. Protein or nucleic acid appears white. Scale bar, 5 nm.