Atomic structure of the translation regulatory protein NS1 of bluetongue virus

Bluetongue virus (BTV) non-structural protein 1 (NS1) regulates viral protein synthesis and exists as tubular and non-tubular forms in infected cells but how tubules assemble and how protein synthesis is regulated are unknown. Here, we report near-atomic resolution structures of two NS1 tubular forms determined by cryo-electron microscopy. The two tubular forms are different helical assemblies of the same NS1 monomer, consisting of an N-terminal foot-, a head- and body domains connected to an extended C-terminal arm, which wraps atop the head domain of another NS1 subunit through hydrophobic interactions. Deletion of the C-terminus prevents tubule formation but not viral replication, suggesting an active non-tubular form. Two zinc finger-like motifs are present in each NS1 monomer and tubules are disrupted by divalent cation chelation and restored by cation addition, including Zn(2+), suggesting a regulatory role of divalent cations in tubule formation. In vitro luciferase assays show that the NS1 non-tubular form upregulates BTV mRNA translation, while zinc-finger disruption decreases viral mRNA translation, tubule formation and virus replication, confirming a functional role. Thus, the non-tubular form of NS1 is sufficient for viral protein synthesis and infectious virus replication and the regulatory mechanism involved operates through divalent cation-dependent conversion between the non-tubular and tubular forms.