Structural basis for the regulatory function of a complex zinc-binding domain in a replicative arterivirus helicase resembling a nonsense-mediated mRNA decay helicase

Deng, Zengqin; Lehmann, Kathleen C.; Li, Xiaorong; Feng, Chong; Wang, Guoqiang; Zhang, Qi; Qi, Xiaoxuan; Yu, Lin; Zhang, Xingliang; Feng, Wenhai; Wu, Wei; Gong, Peng; Tao, Ye; Posthuma, Clara C.; Snijder, Eric J.; Gorbalenya, Alexander E.; Chen, Zhongzhou

While attempting to solve the EAV nsp10 structure, we were confronted with the low stability of the full-length recombinant protein expressed in E. coli. We solved this problem by characterizing the C-terminally truncated nsp10Á, which lacks the 65 residues (C-terminal domain) downstream of the known HEL1 motifs. This protein was found to bind partially double-stranded DNA and display the previously reported in vitro ATPase and helicase activities. Because, compared with full-length nsp10, nsp10Á appeared to be somewhat more active as an ATPase but somewhat less active as a helicase, the C-terminal truncation may have affected the coupling of these two enzymatic activities. This suggests that the C-terminal domain may have evolved to (co)regulate nsp10 helicase-mediated functions in vivo, implying that it must be able to communicate with the nsp10 active site. This could be achieved either directly, by interacting with the nucleic acid-or ATP-binding site (the nsp10Á C-terminus is $22.5 Å apart of the active centre; Figure 2C ), or indirectly, through a protein signal transduction network. Importantly, the C-terminal domain is poorly conserved among arteri-and coronaviruses in terms of both sequence and size (Supplementary Figure S3A , and data not shown), arguing that such a putative regulatory function could be executed in a virus-and, possibly, host-specific manner.