CHAPTER 2 INTEGRATION OF UNEQUAL LENGTH b SUBUNITS INTO F1Fo ATP SYNTHASE Introduction F1Fo ATP synthases provide the bulk of cellular energy production in both eukaryotes and prokaryotes (3, 5, 6). Enzymes in this family utilize the electrochemical gradient of protons across membranes in order to synthesize ATP from ADP and inorganic phosphate in a coupled reaction (16). In Escherichia coli, F,Fo ATP synthase is a complex enzyme composed of approximately twenty-two polypeptides with the stoichiometry of u3 3y~sab2C10 (6, 7). The F, portion is composed of the subunits a3 3yis and is responsible for enzymatic catalysis. The Fo portion of the enzyme consists of the ab2C10 Subunits and is responsible for the translocation of protons through the membrane. Electron microscopy has shown that the Fl and Fo sectors are linked by two slender stalk structures (11). During ATP synthesis proton translocation drives the rotation of the central stalk, which consists of subunits ys, within the u3 3 hexamer held stationary by the peripheral stalk. This rotation propagates the conformational changes in the active sites located at the up interfaces driving catalytic activity (3, 15, 62, 65, 281, 282). The 6 subunit of Fl and a dimer of two identical b subunits from Fo comprise the peripheral stalk acting as the stator. The 6 subunit has been visualized seated at the top of the Fl a3 3 hexamer (187). However, recent evidence has suggested that the 6 subunit may be positioned slightly to the side of F1 in association with a single a subunit (100, 141-143).