13.9: ABC Transporters: Importer
ATP-binding cassette or ABC transporters are a class of ATP-driven pumps that hydrolyze ATP to move solutes across the membrane. They can be grouped into importers and exporters. While exporters are present in all domains of life, importers exist only in bacteria and some plants.
In bacteria, based on the number of transmembrane helices and the chemical nature of their substrates, the ABC importers can be divided into three types:
- Type I imports mostly metabolites such as sugars and amino acids (maltose importer in Escherichia coli)
- Type II importers import trace non-metabolites such as metal chelates and vitamins (cobalamin importers in Escherichia coli), and
- Type III also known as ECF importers present in Lactobacillus brevis.
Additionally, for the substrate to bind and be translocated, the bacterial importers have substrate-binding proteins (SBP). In gram-negative bacteria, a high-affinity periplasmic SBP typically delivers the solute to the importer present in the inner membrane that pumps it into the cytosol. However, gram-positive bacteria lack the periplasm; hence their binding protein is often a lipid-anchored external SBP, while some also have the binding proteins fused to the transmembrane domain of the transporter itself.
ABC importers are absent in mammals which increases the promise of these importers as therapeutic targets. For example, Mycobacterium tuberculosis, which causes tuberculosis, remains a significant health concern owing to its high mortality rates worldwide. It imports substrates to meet the requirement of nutrient uptake, maintenance of cell integrity, communication, and pathogenicity. Therefore, targeting its importers with antimicrobialsthat mimic transporter substrates, could be a potential drug treatment option.
