FIGURE 11-9 The four classes of ATP-powered transport proteins. The locations of specific examples are indicated below each class. P-class pumps are composed of two catalytic α subunits, which become phosphorylated as part of the transport cycle. Two β subunits, present in some of these pumps, may regulate transport. Only one α and one β subunit are depicted here. V-class and F-class pumps do not form phosphoprotein intermediates, and almost all transport only protons. Their structures are similar and contain similar proteins, but none of their subunits are related to those of P-class pumps. V-class pumps couple ATP hydrolysis to transport of protons against a concentration gradient, whereas F-class pumps normally operate in the reverse direction and use the energy in a proton concentration or voltage gradient to synthesize ATP. All members of the large ABC superfamily of proteins contain two transmembrane (T) domains and two cytosolic ATP-binding (A) domains, which couple ATP hydrolysis to solute movement. These core domains are present as separate subunits in some ABC proteins (as depicted here) but are fused into a single polypeptide in other ABC proteins. See T. Nishi and M. Forgac, 2002, Nat. Rev. Mol. Cell Biol. 3:94; C. Toyoshima et al., 2000, Nature 405:647; D. McIntosh, 2000, Nat. Struct. Biol. 7:532; and T. Elston, H. Wang, and G. Oster, 1998, Nature 391:510.