Self-association of unfolded outer membrane proteins.

We have investigated self-association propensities of aqueous unfolded (U(AQ)) forms of eight outer membrane proteins (OMPs), OmpA, OmpW, OmpX, PagP, OmpT, OmpLa, FadL, and Omp85. We found that high urea concentrations maintain all of these OMPs as monomers and that OmpA and OmpX remain monomeric upon dilution to 1 M urea. A pH screen showed that basic pH supports the least amount of U(AQ) OMP self-association, consistent with earlier studies showing that basic pH was optimal for better folding efficiencies. The addition of KCl increased U(AQ) OMP self-association, although the magnitudes of the responses were varied. These studies showed that urea can be used to tune the amount of U(AQ) OMP self-association and indicate that the presence of some urea may be useful in optimizing folding conditions because it diminishes aggregation.