KINETICS OF FOLDING AND MEMBRANE INSERTION OF A BETA-BARREL MEMBRANE-PROTEIN

We have studied the kinetics of folding and membrane insertion of the outer membrane protein OmpA of Escherichia coli. In the native structu re, its membrane-inserted domain forms a beta-barrel. The protein was unfolded in solubilized form in water/urea, and refolding was induced by dilution of urea and simultaneous addition of lipid vesicles, Three transitions along the folding pathway could be distinguished. Their c haracteristic times lie below a second, in the range of minutes, and i n the range of an hour. The fast process corresponds to the transition from the unfolded state in water/urea to a misfolded state in water, the moderately slow process to a transition from the misfolded state t o a partially folded state in the membrane, and the slow process to th e transition from the partially folded to the native state. The partia lly folded state in the membrane is interpreted as the analogue of the molten globule state of soluble proteins.