Different Classes of Pumps Exhibit Characteristic Structural and Functional Properties

The general structures of the four classes of ATP-powered pumps are depicted in Figure 7-6, with specific examples in each class listed below. Note that the members of three classes (P, F, and V) transport ions only, whereas members of the ABC superfamily primarily transport small molecules.

All P-class ion pumps possess two identical catalytic a subunits that contain an ATP-binding site. Most also have two smaller p subunits that usually have regulatory functions. During the transport process, at least one of the a sub-units is phosphorylated (hence the name "P" class), and the transported ions are thought to move through the phospho-rylated subunit. The sequence around the phosphorylated residue is homologous in different pumps. This class includes the Na+/K+ ATPase in the plasma membrane, which main-

direction to utilize energy in a proton concentration or electrochemical 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 (depicted here), but are fused into a single polypeptide in other ABC proteins. [See T Nishi and M. Forgac, 2002, Nature Rev. Mol. Cell Biol. 3:94; G. Chang and C. Roth, 2001, Science 293:1793; C. Toyoshima et al., 2000, Nature 405:647; D. Mcintosh, 2000, Nature Struc. Biol. 7:532; and T Elston, H. Wang, and G. Oster, 1998, Nature 391:510.]

Exoplasmic face

Exoplasmic face

Cytosolic face

P-class pumps

Plasma membrane of plants, fungi, bacteria (H+ pump)

Plasma membrane of higher eukaryotes (Na+/K+ pump)

Apical plasma membrane of mammalian stomach (H+/K+ pump)

Plasma membrane of all eukaryotic cells (Ca2+ pump)

Sarcoplasmic reticulum membrane in muscle cells (Ca2+ pump)

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