Figure 6. Substrate release from the interior wall of BamA allows β-barrel closure, triggering release from the Bam complex.

(A) N274 resides within a large hydrophobic patch that encompasses at least six β-strands at the N-terminal region of the LptD β-barrel. The left panel shows the structure of LptD in cartoon form. The color scheme is the same as in Figure 4, with N274 indicated in red, and six hydrophobic residues that crosslink strongly to BamA in blue. The right panel shows an electrostatic surface plot generated using APBS, presented in the same orientation as the cartoon (left). Colors in the electrostatic surface plot represent potential rather than crosslinking residues. Red represents negative potential, white represents neutral potential, and blue represents positive potential. (B) The region around N274 directly interacts with BamA. Crosslinking was tested as described in Figure 4. (C) The N274I mutation suppresses the folding defects associated with LptD4213, allowing release from BamA, as judged by a reduction in crosslinking efficiency. (D) Model for substrate β-barrel closure and release from the Bam complex. N274I suppresses the folding defect associated with LptD4213 by facilitating release of the N-terminal strands of LptD from the interior of BamA.