The DDF catalytic triad. The colour code is the same as that in Figs. 3 and 5. This figure highlights the structural conservation of the DDF triad in VP1 and in VP3, in both cases exactly in the same location with respect to the jelly roll and the small β-sheets shown in green. The first aspartic acid, which points into the interior of the particle, is not conserved in CrPV VP3, in which it is proline. The conservation of the second aspartic acid (indicated by the arrows) is likely to be a consequence of structural constraints (see text). The conservation of the DDF motif in the same location in VP1 and in VP3 in several dicistroviruses is intriguing, given that the two proteins have only 12% sequence identity and have diverged substantially otherwise.