FIG. 1.

The poliovirus 3CD structure. (a) There are two molecules of 3CD in the asymmetric unit. In this stereo view, one molecule is red and one is blue. The 3C and 3D domains are tethered together by a polypeptide linker region (upper black arrow). Within each molecule of 3CD, there is no direct contact between the 3C and 3D domains, except through the linker. The length of the polypeptide linker and the position of the protease site within the 3C domain (lower purple arrow) preclude intramolecular cleavage of 3CD. Regions of greatest variability between 3CD and the 3C^pro and 3D^pol structures are shown in green. (b) Representative electron density from an omit-phased 2F_o − F_c map contoured at 1.4σ is shown in stereo. This view shows density surrounding a proposed Zn²⁺ site at an interface between the 3D domains of two symmetry-related copies of 3CD, in gray and light blue. The Zn²⁺ ion appears to be tetrahedrally coordinated by Asp446, His453, His455, and Cys464. Density for the uppermost 3D domain comes from an α helix, while the density in the lower 3D domain includes β strands.