The structure of cucumber mosaic virus (CMV; strain Fny) has been determined to 3.2Å resolution using X-ray crystallography. Despite having only 19% capsid protein sequence identity (34% similarity) to cowpea chlorotic mottle virus (CCMV), the core structures of these two members of the Bromoviridae family are highly homologous. As suggested by the previous low resolution structural studies, the 305Å diameter (maximum) of CMV is ~12Å larger than CCMV. In CCMV, the structures of the A, B, and C subunits are nearly identical except for the N-terminus. In contrast, the structures of two loops in subunit A of CMV differ from those in B and C. These loops are 6 and 7 residues longer than the analogous regions in CCMV. Unlike CCMV, the capsid of CMV does not undergo swelling at pH 7.0 and is stable at pH 9.0. This may be partly due to the fact that the N-termini of the B and C subunits form a unique bundle of 6 amphipathic helices orientated down into the virion core at the 3-fold axes. In addition, while CCMV has a cluster of aspartic acid residues at the quasi three-fold axis that are proposed to bind metal in a pH dependent manner, this cluster is replaced by complementing acids and bases in CMV. Finally, this structure clearly demonstrates that the residues important for aphid transmission lie at the outer-most portion of the ßH-ßI loop and yields details of the portions of the virus that are hypothesized to mediate binding to the aphid mouthparts.

Left Image: Schematic representation of the T=3, truncated icosahedron. The A,B,C denote the three identical subunits and the black dots represent the N-terminal alpha helices. The quasi 3 and 6-fold axes are also noted.
Right Image: Surface representation of the CMV caspid colored according to radial distance. In the schematic, the labeled A, B, and C subunits are those that are in the general orientation used for the following diagrams. The subunits used to represent the icosahedral asymmetric unit were chosen to demonstrate the quasi six-fold axis and are not related by a quasi three-fold axis. In both figures, the icosahedral three-fold (quasi six-fold), five-fold, two-fold, and the quasi three-fold axes are labeled. In the schematic, six white circles are positioned around one of the quasi six-fold axes to approximate the location of the hexameric bundle of N-terminal helices described in the following figures.

Left Image: Comparison between C subunits of CMV and CCMV. The C-a backbone of CMV is shown in red and CCMV in blue. The program MolView was used for this alignment and 91 residues yielded a root-mean-square deviation of 1.3Å. Some of the key areas of differences are labeled.
Right Image: Comparison of the CMV A and C subunit structures. The C-a backbone of the A and C subunits are shown in blue and red, respectively. The RNA interior is towards the bottom of the diagram. The approximate location of the three-fold axis (for the C subunit) and the five-fold axis (for the A subunit) are represented by the black line.

On the left is a C-alpha backbone of one of the three subunits. The coloring scheme is where the color ramps from red to blue as it extends from the N to C termini. The residues denoted by the mauve balls are those which, when mutated, affect the ability of aphids to transmit the viruses as shown in the table on the right. As shown below, this same loop appears to be binding a metal ion.

Upper left: Surface rendering of a portion of the CMV surface. Interestingly, the only acidic patch on the surface is this aphid transmission loop. Lower Left: Details of the acidic aphid transmission loop in comparison to the calcium binding region of calmodulin. As shown in the lower right panel, there appears to be a metal ion bound to this loop structure. Furthermore, many of the mutations that abrogate aphid transmission represent a loss or reversal of these acidic residues that interact with this isolated density.

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