Fig. 4.

The structure of the CoV nsp7-8 complex. A) The hollow hexadecameric ring of the SARS-CoV nsp7-8 complex has a positively charged channel (blue surface shading) that is likely important for RNA binding. The outside of the hexadecamer is predominantly negatively charged (red surface shading). B) The SARS-CoV nsp8 crystal structure (pdb 2AHM) resembles a ‘golf club' with a long stick at the N-terminus (N) and a head-like shape at the C-terminus (C). The nsp8 structure can adopt two conformations, here shaded green (nsp8-I) and orange (nsp8-II). C) In the hexadecamer each of the two nsp8 structures is present four times and complemented by eight nsp7 subunits that act as mortar. Two orientations of the nsp7 subunit are indicated (orange and green). D) The FCOV heterotrimeric nsp7-8 complex (pdb 3ub0) consists of one nsp8 (shaded green) subunit and two nsp7 subunits (orange and blue). The nsp8 subunit adopts an nsp8-I conformation.