FIG. 9.

IBDV assembly model. Mature VP2 is originally synthesized as a precursor, pVP2. The removable C-terminal extension (71 residues) contains the molecular switch, an amphipathic α-helix (segment 443-452), and is represented as a cylinder followed by a random coil. (A) When pVP2 (or shorter pVP2 variants) is expressed in a recombinant baculovirus system in the absence of other viral components, irregular helical tubes (all-hexamer structures) are formed (left); nonetheless, if the initial product is mature VP2, T=1 all-pentamer capsids form spontaneously (right). (B) The processing rate of the pVP2 C-terminal extension determines whether the C-terminal region (shown as a loop) of VP3 (green) can interact with pVP2. If processing is rapid, amphipathic α-helices associate to form a bundle, and a pentamer is formed with bent contacts between VP2 trimers (for simplicity, only a monomer is shown). In contrast, slow processing allows a longer, stable interaction between VP2 and VP3 C termini. Wavy lines indicate the dynamism of the VP3 interaction with VP2; six VP3 chains temporally stabilize hexamer formation between VP2 trimers with flat contacts, but the interaction ratio between pVP2 intermediates and VP3 might be higher.