Folding and membrane insertion of the trimeric beta-barrel protein OmpF

We have studied folding and membrane insertion of the porin OmpF and compared it to OmpA. Both are beta-barrel membrane proteins from the outer membrane of Escherichia coli, OmpF forming trimers and OmpA monomers. Each of them can be unfolded in solubilized form in a water/urea mixture. Refolding is initiated by dilution into a dispersion of lipid vesicles or lipid/detergent vesicles, whereupon OmpF and OmpA refold and insert into the membranes. Folding and insertion of the monomers proceed in a similar way for the two proteins, but native OmpF appears more slowly and with a lower yield than native OmpA because of trimerization of OmpF. The dependence of the yield of refolding, membrane insertion, 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.