Refolding and oriented insertion of a membrane protein into a lipid bilayer.

We have studied the refolding and membrane insertion of the outer membrane protein OmpA of Escherichia coli. The protein was extracted from its native membrane by sonication in the presence of urea and dissolved in the urea/water mixture in unfolded form. In this form it was purified. Upon addition of preformed lipid vesicles, the protein spontaneously refolded and inserted into the vesicle membranes. The vesicles had to be small and the lipids had to be in the fluid state. The insertion occurred in an oriented manner.