Structure and function of the nairovirus cap-snatching endonuclease

Nairoviruses, such as Crimean-Congo hemorrhagic fever virus (CCHFV) initiate viral mRNA synthesis using an N-terminal cap-snatching endonuclease domain within their viral L protein. The endonuclease of this nairoviruses have an extended domain architecture of 350 residues with flexible insertion loops, an a7 support helix and a hybrid active site motif. Biochemical assays show that nairovirus EN activity must require manganese ions and not magnesium ions and prefer uridine rich single stranded RNA substrates.  Antibody assisted crystallization with antigen binding fragments of CCHF virus has shown a distinct two metal ion binding mode. 
Mutation analysis and viral mini replicon assays have confirmed that the dual metal ion structure is necessary for the endonuclease activity and viral transcription. Inhibitor assay shows that metal chelating compounds DPBA, L742001 and baloxavir acid inhibit the EN activity. The authors also show using co-crystal structure that baloxavir acid has higher inhibition potency and higher binding affinity.
(MCW)