Analysis of the capsid processing strategy of Thosea asigna virus using baculovirus expression of virus-like particles

Thosea asigna virus (TaV), a putative member of the genus Betatetravirus of the family Tetraviridae, is predicted to have a novel capsid expression strategy compared with other characterized tetraviruses. The capsid precursor protein is cleaved twice to generate three proteins. Two of the proteins, L (58·3 kDa) and S (6·8 kDa), are incorporated into the TaV virion. The third, non-structural protein, produced from the N terminus of the precursor protein, is up to 17 kDa in size and is of unknown function. The TaV capsid precursor protein sequence without the 17 kDa N-terminal region was modelled against the solved structure from Nudaurelia ω virus (NωV) using SwissModel. The TaV model was very similar to the solved structure determined for subunit A of NωV and had features that are conserved between tetraviruses and nodaviruses, including the positioning of the cleavage site between the L and S capsid proteins. The production of virus-like particles (VLPs) using the baculovirus expression system was used to analyse the capsid processing strategy employed by TaV. VLPs were formed in both the presence and absence of the 17 kDa N-terminal region of the capsid precursor. VLPs were not formed when the L and S regions were expressed from separate promoters, indicating that cleavage between the L and S capsid proteins was an essential part of TaV capsid assembly. Expression of the TaV 17 kDa protein in bacteria did not produce intracellular tubules similar to those formed by bacterial expression of the p17 protein from Helicoverpa armigera stunt virus.