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

The dimer conformation of WT nsp4C. In the WT nsp4C structure, two monomers in one asymmetric unit are linked by a 2.0 Ã… Cys425-Cys425 disulfide bond to form a dimer (Fig. 1 ). The C-terminal of nsp4 is exposed to the cytosol, suggesting that the nsp4C should bind to the ER membrane via its N-terminal. Notably, the N-termini of the two molecules are not aligned in the WT nsp4C dimer. Instead, they are projected in different orientations, with one molecule rotated by about 150u around the y-axis relative to the other molecule (Fig. 1) . The N-terminal amino acids Cys425, Lys421, Glu422, Lys426, and Asn429 are located in the contact surface between the two monomers. The hydrophobic cavity of WT nsp4C is exposed on the dimer surface, and the C-termini of the two monomers extend in different orientations. We propose that the surface properties of the WT nsp4C dimer are representative of an ''open'' conformation given that its positive, negative and hydrophobic areas are all exposed (Fig. 1 ). This conformation is in conflict with the hypothesis that nsp4C binds to the membrane as a dimer via their N-termini; it is rational only if nsp4C binds the membrane as a monomer.