Putative interaction site of VP4 in TrV. Open-book view of the surface electrostatic potential of VP4 and the rest of a penton. The charge distribution appears to be complementary in both cases, except at the centre, on the fivefold axis. The residues that confer opposite charges to this region in CrPV are Glu4009 and Lys3008, which form a salt bridge that cannot occur in TrV. This may be one of the reasons why VP4 is not icosahedrally ordered in the TrV capsid.