Supplementary Figure 6: Structural comparison of actin-binding proteins from pathogens and hypothetical models for structural activation of YopO.

(a) Superposition of toxofilin (PDB:2Q97) on the structure of YopO:actin, following the colour scheme in Fig. 1a, with toxofilin in yellow. The N- and C- terminal residues of the toxofilin construct are shown as yellow and magenta spheres, respectively. The C-terminus of toxofilin wraps around actin subdomain 4 in a manner similar to that of YopO, although the topologies and the exact interaction interfaces differ.

(b) Superposition of Iota toxin component Ia (PDB:3BUZ) on the structure of YopO:actin, following the colour scheme in Fig. 1a, with Iota toxin Ia in magenta.

(c) Hypothetical model of YopO activation. The GDI domain is shown in red and the kinase domain is shown in blue, with actin shown as a surface representation in marine. In this hypothetical model of the inactive structure, the backbone helix of the GDI domain would be straight and the kinase active site would likely be inaccessible. In the transition from the inactive YopO to YopO in the YopO:actin complex, two potential “intermediate” complexes are possible. Actin could first bind to either the kinase domain or the GDI domain, though the latter is more plausible due to a greater solvation free energy gain for the formation of this interface as compared to binding of actin to the kinase domain. The series of conformational changes that lead to sandwiching of actin between the two domains would lead to the bending of the backbone helix of the GDI domain and the kinase substrate binding groove being exposed as in the YopO:actin structure.