The N-terminal Helix Is a Post-assembly Clamp in the Bacterial Outer Membrane Protein PagP

The Escherichia coli outer membrane β-barrel enzyme PagP and its homologues are unique in that the eight-stranded barrel is tilted by about 25° with respect to the membrane normal and is preceded by a 19-residue amphipathic α-helix. To investigate the role of this helix in the folding and stability of PagP, mutants were generated in which the helix was deleted (Δ(1-19)), or in which residues predicted to be involved in helix–barrel interactions were altered (W17A or R59L). The ability of the variants to insert into detergent micelles or liposomes was studied in vitro using circular dichroism, fluorescence, Fourier transform infrared spectroscopy, electrophoretic mobility and gain of enzyme activity. The data show that PagP, initially unfolded in 5% (w/v) perfluoro-octanoic acid or 6 M guanidinium chloride, inserts spontaneously and folds quantitatively to an active conformation into detergent micelles of cyclofos-7 or into large vesicles of diC(12:0)-phosphatidylcholine (diC(12:0)PC), respectively, the latter in the presence of 7 M urea. Successful refolding of all variants into both micelles and liposomes ruled out an essential role for the helix or helix–barrel interactions in folding and membrane insertion. Measurements of thermal stability indicated that the variants R59L, W17A/R59L and Δ(1-19) were destabilised substantially compared with wild-type PagP. However, in contrast to the other variants, destabilisation of the W17A variant relative to wild-type PagP was much greater in liposomes than in micelles. Analysis of the kinetics of folding and unfolding of all variants in diC(12:0)PC liposomes suggested that this destabilisation arises predominantly from an increased dissociation of the refolded variant proteins from the lipid-inserted state. The data support the view that the helix of PagP is not required for folding and assembly, but instead acts as a clamp, stabilising membrane-inserted PagP after folding and docking with the membrane are complete.