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

It was recently reported that the C-terminal fragment from K398 to T492 of nsp4 is dispensable for viral replication [33] . Generally speaking, nsp3 and nsp4 are reported to be inserted into the ER membrane via their hydrophobic sequences [24, 25] , while other nsps lacking hydrophobic sequences are believed to localize in the cytoplasm [2] . The trans-membrane domain of nsp3 is sufficient to mediate ER membrane association of the cytosolic protein EGFP [24] . The poliovirus (PV) membrane proteins 2BC and 2C are also dispensable for viral RNA replication, but are exclusively associated with the formation of PV replication vesicles; 2BC recruits the COPII complex to the ER membranes to trigger vesicle formation [37, 38] . Furthermore, topology study revealed that the N-terminal may serve as signal peptide, and that the hydrophilic region between the first and second transmembrane domain is exposed to the ER lumen and is glycosylated in the ER [25] . Other hydrophilic regions of nsp4 are too short to interact with other proteins, and nsp4C was detected on the double membrane vesicles from SARS-CoV infected cells [34] , thus the cytosol exposed C-terminal of nsp4 should be involved in interactions with other viral or host cell proteins in replication.