Fm. Pringle et al., Analysis of the capsid processing strategy of Thosea asigna virus using baculovirus expression of virus-like particles, J GEN VIROL, 82, 2001, pp. 259-266
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.