Structural and Functional Studies on the Interaction of GspC and GspD in the Type II Secretion System
(A) An ‘open book’ view of the GspCHR–GspDN0-N1 binary complex in surface representation. Residues in the interface are colored according to the degree of burial upon complex formation: yellow, up to 40% reduction in accessible surface area (ASA); orange, 40–70% reduction in ASA; and brown, more than 70% reduction in ASA. Atoms participating in intermolecular hydrogen bond formation are colored in cyan. (B) Same view as in (A) with the interaction surfaces colored according to the solvent accessible electrostatic potential. The interaction surface is contoured by black lines. (C) Anti-parallel β1HR–β1N0 interactions in the GspCHR–GspDN0-N1 complex. The upper strand is β1N0. Interacting residues are shown as sticks and labeled. Hydrogen bonds are shown as dashed lines. A σA-weighted 2FO–FC electron density map contoured at 1.2 σ is shown as a dark blue mesh. (D) Interface surface of a homology model of the V. cholerae GspC–GspD complex . Residues in the interface are colored according to the color of the interacting partner: GspD in cyan and GspC in green. The residues that were subjected to mutational analysis are colored in magenta and labeled. (E) Amino acid sequence alignments of the HR domains of GspC and the N0 domains of GspD from ETEC and V. cholerae. The corresponding secondary structure elements are shown above the sequences. Residues that make intermolecular Van der Waals contacts and H-bonds in the ETEC GspCHR–GspDN0-N1 complex are labeled by triangles and stars, respectively. The residues that were subjected to mutational analysis in V. cholerae GspC and GspD are indicated by magenta triangles underneath the alignments.