Fig. 1.

Structure of the CCMV capsid and its novel geometrical features. (a) Stereoview of the protein shell as a Cα tracing. Color coding is defined in (b). The yellow cage represents the edges of a truncated icosahedron. (b) A truncated icosahedron model [38] shown in the same orientation as in (a). Positions of icosahedral rotation axes are marked by yellow symbols (pentagons: five-fold rotation axes; triangles: three-fold rotation axes; ovals: two-fold rotation axes). The central triangle with one five-fold (top) and two three-fold axes (lower left and right) at its vertices and containing polygons labeled A, B and C defines the icosahedral asymmetric unit. The polygons represent chemically identical protein subunits and within this area they occupy slightly different geometrical (chemical) environments and this is indicated by differences in their coloring. Polygons with subscripts are related to A, B and C by icosahedral symmetry (i.e. A to A₅ by five-fold rotation). The apparent three-fold rotation axis at the center of the asymmetric unit Ivertices of cage in (a)] is not exact (quasi three-fold axis: white triangle) as it relates icosahedral threefold axes (quasi six-fold axes) to a five-fold axis outside of its local environment. Similarly, polygons labeled A and B₅ form a quasi two-fold axis (white oval). Putative calcium-binding locations in one asymmetric unit of CCMV are marked by brown circles. Interactions between B₂-C and between C-B₅ polygons are defined by 180° dihedral angles (side view at top right) whereas bends similar in magnitude occur at the B-C and C-A polygon interfaces (138° and 142°, respectively) (side view at bottom right). (c) A rhombic triacontahedron model [38] shown in the same orientation and labeled identically to the truncated icosahedron model. The A, B and C polygons are co-planar within each asymmetric unit. Two such asymmetric units are co-planar by icosahedral two-fold symmetry giving the rhombic solid its prominent diamond shaped facets. The interaction between B₂ and C remains planar (a 180° dihedral angle between two-fold related asymmetric units, top right) whereas the interaction of C and B₅ adopts a 144° dihedral angle (bottom right). Hexamers are therefore best described as trimers of dimers.