As noted earlier, all members of the very large and diverse ABC superfamily of transport proteins contain two transmembrane (T) domains and two cytosolic ATP-binding (A) domains (see Figure 7-6). The T domains, each built of six membrane-spanning a helices, form the pathway through which the transported substance (substrate) crosses the membrane and determine the substrate specificity of each ABC protein. The sequences of the A domains are «30-40 percent homologous in all members of this superfamily, indicating a common evolutionary origin. Some ABC proteins also contain an additional exoplasmic substrate-binding subunit or regulatory subunit.
The plasma membrane of many bacteria contains numerous permeases that belong to the ABC superfamily. These proteins use the energy released by hydrolysis of ATP to transport specific amino acids, sugars, vitamins, or even pep-tides into the cell. Since bacteria frequently grow in soil or pond water where the concentration of nutrients is low, these
ABC transport proteins enable the cells to import nutrients against substantial concentration gradients. Bacterial per-meases generally are inducible; that is, the quantity of a transport protein in the cell membrane is regulated by both the concentration of the nutrient in the medium and the metabolic needs of the cell.
In E. coli histidine permease, a typical bacterial ABC protein, the two transmembrane domains and two cytosolic ATP-binding domains are formed by four separate subunits. In gram-negative bacteria such as E. coli, the outer membrane contains porins that render them highly permeable to most small molecules (see Figure 5-14). A soluble histidine-binding protein is located in the periplasmic space between the outer membrane and plasma membrane. This soluble protein binds histidine tightly and directs it to the T subunits of the permease, through which histidine crosses the plasma membrane powered by ATP hydrolysis. Mutant E. coli cells that are defective in any of the histidine permease subunits or the soluble binding protein are unable to transport histidine into the cell, but are able to transport other amino acids whose uptake is facilitated by other transport proteins. Such genetic analyses provide strong evidence that histidine per-mease and similar ABC proteins function to transport various solutes into bacterial cells.
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