ASSEMBLY OF F-0 SECTOR OF ESCHERICHIA-COLI H- INTERDEPENDENCE OF SUBUNIT INSERTION INTO THE MEMBRANE( ATP SYNTHASE )

The F-0 sector of the Escherichia coli H+ transporting ATP synthase is composed of a complex of three subunits, each of which traverses the inner membrane. We have studied the interdependence of subunit inserti on into the membrane in a series of chromosomal mutants in which the p rimary mutation prevented insertion of one of the F-0 subunits. Subuni t insertion was assessed using Western blots of mutant membrane prepar ations. Subunit b and subunit c were found to insert into the membrane independently of the other two F-0 subunits. On the other hand, subun it a was not inserted into membranes that lacked either subunit b or s ubunit c. The conclusion that subunit a insertion is dependent upon th e co-insertion of subunits b and c differs from the conclusion of seve ral studies, where subunits were expressed from multicopy plasmids.