Fig. 6: VP9.

a (Left panel) N-terminal α1 helices of the VP9 trimer (in transparent blue, based on our C5 reconstruction) forming a tripod to insert into the central cavity of the VP4 trimer (in transparent beige). (Right panel) Density map view of one spike, with one VP4 subunit being color-coded by domain (similar to Fig. 4d). b Zoomed-in view of the black box from a, showing interactions of helix α1 from one VP9 subunit with the pedestal domain (gray) and the unfurling domain (red) from VP4. c Atomic model of VP9 based on local reconstruction of VP9 (Supplementary Fig. 4). The three VP9 protomers (colored lavender, dark green and pink) are shown in the ribbon diagram, highlighting the N-terminal stalk region of VP9 (made up mainly of α1–α3). The residues involved in interprotomer interactions within the stalk region are labeled and denoted by golden spheres in one VP9 molecule (in lavender). d SDS-PAGE analysis of the protein composition of purified BAV particles (in PBS, pH 7.4, Lanes 1–3) and BAV particles in acidic buffer (citrate–phosphate buffer, pH 5.0, Lanes 4–5). Lane 1, concentrated BAV particles; Lane 2, pelleted BAV particles after centrifugation at 10,000 × g, showing only a small amount of particles are precipitated; Lane 3, supernatant after centrifugation at 10,000 × g, showing most BAV particles are intact in solution; Lane 4, pelleted particles after centrifugation, showing most particles are precipitated; Lane 5, supernatant after centrifugation, showing VP9 protein has been released into solution. e Cryo-EM structure of BAV particles after acid treatment (pH 5.0), showing VP4 density as rod-shaped projections (colored in beige) on the viral core.