Fig. 5.

Structure of TcB-TcC-Cdc42 alone and in complex with TcA. a Crystal structure of the TcB-TcC-Cdc42 complex. The C-terminal region of Cdc42 (N167–P179, green) forms a helix inside the cocoon. b Top view into TcB-TcC-Cdc42, with the TcC model removed for illustrative purposes. The C-terminal Cdc42 α-helix (green) can be seen attached to one side of the cocoon, whereas remaining Cdc42 density is too disordered to resolve. c Surface representation of the binding pocket of the hydrophobic C-terminal Cdc42 helix in the TcB section of the cocoon. The molecular surface of TcB is colored according to its hydrophobicity⁵², with hydrophobic regions shown in ochre and polar regions in white. d Model of the C-terminal Cdc42 α-helix in the binding pocket of TcB. Two orientations are shown. Side chains that face the Cdc42 α-helix are indicated. e Rigid-body fit of the crystal structure of TcB-TcC-Cdc42 into the cryo-EM density map of ABC-Cdc42. Density corresponding to the α-helix of Cdc42 is also present in the cryo-EM structure of the holotoxin at the same site. f Comparison of cryo-EM density reconstructions of ABC(WT) (PDB 6H6F) and ABC-Cdc42 (this work). The surfaces of the TcA, TcB, and TcC subunits are transparent, and the density corresponding to the ADP-ribosyltransferase TccC3HVR (WT, left) and Cdc42 (right) is dark gray. The latter is shown at a lower threshold and filtered to 15 Å resolution