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 st rategy 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 thi rd, non-structural protein, produced from the N terminus of the precursor p rotein, 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 modelle d against the solved structure from Nudaurelia omega virus (N omegaV) using SwissModel. The TaV model was very similar to the solved structure determi ned for subunit A of N omegaV and had features that are conserved between t etraviruses and nodaviruses, including the positioning of the cleavage site between the L and S capsid proteins. The production of virus-like particle s (VLPs) using the baculovirus expression system was used to analyse the ca psid processing strategy employed by TaV, VLPs were formed in both the pres ence and absence of the 17 kDa N-terminal region of the capsid precursor. V LPs were not formed when the L and S regions were expressed from separate p romoters, indicating that cleavage between the L and S capsid proteins was an essential part of TaV capsid assembly. Expression of the TaV 17 kDa prot ein in bacteria did not produce intracellular tubules similar to those form ed by bacterial expression of the p17 protein from Helicoverpa armigera stu nt virus.