ABC Transporters: Importer

ABC importers use ATP hydrolysis to move solutes from outside the cell into the cytosol. In Gram-negative bacteria, a substrate-binding protein delivers the solute to the importer in the inner membrane. ATP binding causes the ATPase domains to dimerize and change the transporter's conformation, releasing the solute into the cytosol. This process represents a form of primary active transport.

Substrate-binding proteins selectively recognize and bind solutes, then deliver them to the ABC importer. In Gram-negative bacteria, the periplasmic SBP brings the solute to the transporter in the inner membrane. In Gram-positive bacteria, the lipid-anchored SBP carries the solute through the thick peptidoglycan layer. This interaction brings the ATPase domains closer together, enabling ATP binding and the transport mechanism.

Gram-negative bacteria have a periplasmic space where a high-affinity SBP delivers solutes to the inner membrane importer. Gram-positive bacteria lack a periplasm, so their SBP is typically a lipoprotein anchored to the cell membrane or fused to the transporter itself. Despite these structural differences, both use the same ATP hydrolysis mechanism to transport solutes into the cytosol.

Type I importers transport metabolites like sugars and amino acids, exemplified by maltose importers in E. coli. Type II importers transport trace non-metabolites such as metal chelates and vitamins, including cobalamin importers. Type III, also called ECF importers, are found in organisms like Lactobacillus brevis. Each type is defined by the chemical nature of its substrate and transmembrane helix configuration.

ABC importers are absent in mammals, making them selective targets for antimicrobial therapy. Pathogenic bacteria like Mycobacterium tuberculosis rely on importers for nutrient uptake, cell integrity, and pathogenicity. Drugs that mimic transporter substrates could block these essential functions without harming human cells, offering a promising treatment strategy for tuberculosis and other bacterial infections.

ATP hydrolysis triggers a conformational change in the transporter that releases the solute from the binding site into the cytosol. The ATPase domains first bind two ATP molecules after the substrate-binding protein delivers the solute. The energy from ATP hydrolysis powers the structural rearrangement necessary to move the solute across the membrane and complete the transport cycle.

Solutes first enter the periplasmic space through porins in the outer membrane of Gram-negative bacteria. Once in the periplasm, a substrate-binding protein recognizes and binds the solute, then carries it to the ABC importer located in the inner membrane. This two-stage entry system ensures selective and efficient transport of specific solutes into the cell.