BamA with a transmembrane β-barrel and soluble POTRA motifs mediates OMP biogenesis NMR relaxation studies show POTRA1–2 is flexibly linked to POTRA3–5 in BamA Residual dipolar couplings indicate POTRA1–2 and POTRA4–5 behave as rigid species Disulfides that restrict POTRA2–3 flexibility impair in vivo function of BamA