FOLDING AND MEMBRANE INSERTION OF THE TRIMERIC BETA-BARREL PROTEIN OMPF

We have studied folding and membrane insertion of the porin OmpF and c ompared it to OmpA. Both are beta-barrel membrane proteins from the ou ter membrane of Escherichia coli, OmpF forming trimers and OmpA monome rs. Each of them can be unfolded in solubilized form in a water/urea m ixture. Refolding is initiated by dilution into a dispersion of lipid vesicles or lipid/detergent vesicles, whereupon OmpF and OmpA refold a nd insert into the membranes. Folding and insertion of the monomers pr oceed in a similar way for the two proteins, but native OmpF appears m ore slowly and with a lower yield than native OmpA because of trimeriz ation of OmpF. The dependence of the yield of refolding, membrane inse rtion, and trimerization on pH, lipid concentration, and the presence of detergent was investigated. Trimerization of OmpF is shown to take place at or in the membrane and a membrane-inserted dimer is detected as an intermediate of this process.