Despite the lack of sequence conservation on the surface of these structures, an analysis of the surface electrostatics reveals that the face that is curved in YopN–TyeA and open in MxiC possesses a conserved negatively charged patch (Fig. 3c). This patch spreads across one face of the C-terminal half of the molecule (displayed in red on the right-hand side of the surface shown in Fig. 3c). The helices that line this face undergo minor repacking and can accommodate the large movements of the surrounding domains. The conserved patch on this face suggests a role for this region in interactions with partner proteins. If this is a binding face for another component of the T3SS, it is interesting to note that this interface is conserved despite it being intramolecular in one homologue and intermolecular in another.