Fig. 2.

Oligomeric assembly of Bsu Anti-TRAP (AT). The 53-residue AT protomer (AT₁) features a short N-terminal helix leading into a “zinc ribbon” motif and ending in a C-terminal helix. Assembly of AT₃ occurs via a helical bundle that buries ∼ 4,200 Å². In solution AT₃ is in an equilibrium with dodecamers (AT₁₂) featuring a tetrahedral arrangement of AT₃ stabilized by a network of ion pairs between the amino terminus of Met1 and the sidechain carboxylate of Asp7 of adjacent trimers. A transient AT₆ intermediate is hypothesized to arise from dimerization of AT₃ via formation of two sets of Met1-Asp7 ion pairs and burial of ∼ 1,100 Å²; dimerization of two AT₆ assemblies to form AT₁₂ is favored by formation of ten additional sets of ion pairs and burial of another ∼ 4,600 Å².