Evidence for post-translational membrane insertion of the integral membrane protein bacterioopsin expressed in the heterologous halophilic archaeon Haloferax volcanii

The gene coding for the integral membrane protein bacterioopsin (Bop), that is composed of seven transmembrane helices, was expressed in the halophilic archaeon Haloferax volcanii as a fusion protein with the halobacterial enzyme dihydrofolate reductase and with the cellulose binding domain of Clostridium thermocellum cellulosome. In each case, bacterioopsin was present both in the membrane and in the cytoplasmic fractions. Pulse-chase labeling experiments showed that the fusion protein in the cytoplasmic fraction is the precursor of the membrane-bound species. Bacterioopsin mutants that lack the seventh helix (BopΔ7) were found to accumulate only in the cytoplasmic fraction, whereas bacterioopsin mutants that lack either helices four and five (BopΔ4-5), or helices one and two (BopΔ1-2), were found in the cytoplasmic as well as in the membrane fractions. The seventh helix, when expressed alone, could target in trans the insertion of a separately expressed bacterioopsin mutant protein that has only the first six helices. These results support a model in which bacterioopsin is produced in H. volcanii as a soluble protein and in which its insertion into the membrane occurs post-translationally. According to this model, membrane insertion is directed by the seventh helix.