Structure of inner capsid protein VP3.

(A) Different conformations of the inner capsid protein VP3A (cornflower blue) and VP3B (orange). Both VP3A and VP3B contain apical (red ellipse), carapace (green triangle) and dimerization domains (purple rectangle). VP3B has an extended N-terminus composed of 2 helices and connecting loops. (B) The five “helix-loop-helix-loop” N-termini of VP3B are indicated using balls and highlighted in different colors. They interact with different VP3 dimers and form a “belt” that stabilizes the inner shell. (C) The N-terminus of VP3A differs from that of VP3B. Three VP3A N-termini, highlighted by purple balls, have similar conformations, while the other two VP3A locations have different conformations (in red and cyan respectively) in the D₅ reconstruction. These two N-termini brace the polymerase (gray density) and may help to hold the polymerase to the inner surface of the capsid. The major parts of VP3A and VP3B are colored in cornflower blue and orange respectively, same as that in Fig 1B. The alignment of all VP3A show in (D) indicates that all VP3As are nearly identical, except for the different conformations of the N-terminus. (E) The alignment of the N-terminus of VP3A and VP3B clearly show four different possible N-terminal configurations for VP3. In (F), the large separations among adjacent VP3 pentamers can be seen. (G) A zoomed-in view of the boxed area in (F) shows the large separation at the VP3 dimerization domains. The black oval indicates the 2-fold axis.