Fig. 3.8.

Subunit packing and intersubunit interactions in the all-pentamer T  =  7 structure of polyomavirus. (a) T  =  7 icosahedral lattice as viewed along the icosahedral twofold axis. The five-, three-, and twofold symmetry elements are denoted by pentagon, triangle, and oval symbols. (b) Crystallographic structure of polyomavirus, viewed along the twofold axis as in (a), showing the close packing of pentamers on a T  =  7 icosahedral lattice (obtained from VIPER, Reddy et al. 2001). Pentamers sit on the five- and six-coordinated positions of the lattice. Two of these locations are denoted. (c) The icosahedral asymmetric unit consisting of a pentamer at the six-coordinated position and a subunit (shown in pink) from the pentamer at the fivefold axis. The subunits in the pentamer were colored differently (obtained from VIPER, Reddy et al. 2001). Application of the five-, three-, and twofold symmetry generates the structures shown in (b). The CTAs emerging from each subunit are clearly seen. (d) A schematic representation of the interchange of CTA arms between the pentamers that hold the capsid together (Liddington et al. 1991). The pentameric subunits are depicted as triangles inside a pentagon. The subunits in the pentamers at the sixfold node are colored in different colors as in (c). The pentamer at the fivefold axis is shown in white. The CTAs emerging from each subunit is shown in the same color as the subunit. The CTA from the pentavalent pentamer is shown in gray. Three distinct types of inter-pentameric interactions involving interchange of CTAs are observed in the structure. First type, between the pentavalent pentamer and two hexavalent pentamers related by the local threefold axis (gray, red, and green CTAs), between the two hexavalent pentamers related by the local twofold axis (yellow and cyan), and the third type, between the hexavalent pentamers related by the icosahedral twofold axis (blue–blue)