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

The observed enzymatic differences between nsp10 and nsp10Á may be caused by the latter's truncation and could, in principle, be explained by one or multiple defects, like decreased unwinding velocity and/or processivity, loss of affinity towards the substrate or uncoupling of ATPase from helicase activity. The results of the ATPase assay lead us to propose that the observed reduction of duplex unwinding may be due to unproductive ATP hydrolysis, originating from the fact that the ATPase reaction is independent of nucleic acid substrate binding. Accordingly, the input ATP in the nsp10Á assay may have been depleted before complete unwinding was achieved. Regardless of which interpretation is correct, the C-terminal 65 amino acids clearly are dispensable for the helicase activity of EAV nsp10. This result is in good agreement with the fact that the truncated protein retained all HEL1 key domains ( Figure 2A ) previously shown to be evolutionary conserved and essential in both in vitro enzyme assays and in vivo studies with virus mutants.