Crystal Structure of the C-Terminal Cytoplasmic Domain of Non-Structural Protein 4 from Mouse Hepatitis Virus A59

Xu, Xiaoling; Lou, Zhiyong; Ma, Yanlin; Chen, Xuehui; Yang, Zhangsheng; Tong, Xiaohang; Zhao, Qi; Xu, Yuanyuan; Deng, Hongyu; Bartlam, Mark; Rao, Zihe

Ser425 from each monomer in the mutant dimer is oriented on the outside of the dimer, far away from intersecting C-termini of the dimer (Fig. 1) . Thus, mutating Cys425 to Ser does not directly influence the interaction between the C-termini of these two monomers. The mutant C425S dimer could be obtained by crystallographic symmetry of the WT nsp4C structure (Fig 3D) . In WT nsp4C, a disulfide bond at Cys425 causes the C-termini of the two monomers to extend in different orientations, and thus resulting in flexibility and undetectable electron density differences at the C-termini of the monomers. In the case of the C425S mutant, the disulfide bond is dissociated and the dimer adopts a more stable conformation by crystal packing: the flexible Ctermini of two monomers are stabilized by hydrogen bond interactions between the amino acids in the Ala488-Thr492 (b4) and Ser450-Met452 (b3) loops, which form stable anti-parallel bsheets at the bottom of the dimer, resulting in clear electron density from residues Ser489-Leu495.