ATP-binding cassette transporters are targets for the development of antibacterial vaccines and therapies.

ATP-binding cassette (ABC) transporters are widespread among living organisms and comprise one of the largest protein families. For example, components of ABC transporters are encoded by approximately 5% of the Escherichia coli and Bacillus subtilis genomes (47, 80). These transporters are found in all species and are evolutionarily related. However, they are functionally diverse and have roles in a wide range of important cellular functions. The different ABC transporters can be assigned to classes, families and subfamilies on the basis of phylogenetic analyses (19). Structure of ABC transporters. ABC transporters are remarkably conserved in terms of the primary sequence and the organization of the domains or subunits. Characteristically, ABC transporters have a highly conserved ATPase domain (the ABC, also known as the nucleotide-binding domain) which binds and hydrolyzes ATP to provide energy for the import and export of a wide variety of substrates (or allocrites [80]). The ABC contains two highly conserved motifs, the Walker A and Walker B motifs, which together form a structure for binding ATP. However, these motifs may also be found in ATP-binding proteins not associated with transport. ABC transporters also contain a specific motif, the consensus signature sequence LSGGQ/R/KQR, which is highly conserved and is specific to the ABC superfamily. This sequence is also known as the linker peptide or C motif and is located N-terminal with respect to the Walker B motif. Further details of the functional and structural aspects of ABCs can be found in recent reviews by Jones and George (38), Kerr (41), and Schneider and Hunke (65). The ABC transporters consist of ABCs associated with hydrophobic membrane-spanning domains (MSDs), which are also known as membrane domains, transmembrane domains, or integral membrane domains. It is thought that ABC transporters typically have a common four-domain arrangement consisting of two ABCs and two MSDs that may be fused in various ways into multidomain polypeptides (32). In most ABC transporters, an MSD typically forms six putative -helical transmembrane segments (i.e., a total of 12 segments per ABC transporter) that constitute a channel through which allocrites may be transported (32). In addition, ABC transporters may also include additional proteins for specific functions (Fig. 1). Bacterial importers, also known as permeases, typically include periplasmic solute-binding proteins (SBPs) that bind to incoming allocrites and deliver them to the import complex in the inner membrane of gram-negative bacteria. Import across the outer membrane may involve outer membrane proteins (OMPs), such as porins. The importers of gram-positive bacteria, which have no periplasm or outer membrane, include equivalent allocrite-binding proteins anchored to the outside of the cell via lipid groups. This structural difference may mean that much smaller amounts of ligand-associated SBPs are needed to saturate the transporter complex than are needed for the corresponding interaction with soluble SBPs in gramnegative bacteria. SBPs are believed to confer affinity and specificity to the importers, as well as directionality, since they are not present in exporters. Instead, some gram-negative bacterial exporters include other accessory factors (AFs) that are required when the allocrite is destined for release into the extracellular medium. Other gram-negative bacterial exporters that do not require AFs include some of the exporters involved in the transport of allocrites to the periplasm or outer membrane only. In some cases, additional OMPs are required for full export of the allocrites. Some gram-positive bacterial exporters also have proteins similar to these AFs, although their role in export is not clear. Physiological roles of bacterial ABC transporters. In bacteria, ABC transporters have a diverse range of functions that may be required in response to the environments in which different bacteria find themselves. They import a variety of allocrites, including sugars and other carbohydrates (64), amino acids (33), peptides (20), polyamines (35), metal ions (16), sulfate (42), iron (44), and molybdate (66). ABC transporters are also responsible for the targeted export of other allocrites across the cytoplasmic membrane (for example, capsular polysaccharide [67] in gram-negative bacteria). Other exporters are responsible for the secretion of antibiotics in some antibiotic-producing bacteria (53) and in drug-resistant bacteria (77) or for the export of extracellular toxins. Members of another class of ABC systems have roles in cellular processes, such as translational regulation (14) and DNA repair (25). It may be presumed that bacterial species that live in diverse environments need to adapt to different conditions and may therefore require numerous ABC systems, whereas other spe* Corresponding author. Mailing address: Department of Biomedical Sciences, Defence Science and Technology Laboratory, Porton Down, Salisbury, SP4 0JQ, United Kingdom. Phone: 44-1980-614755. Fax: 44-1980-614307. E-mail: [email protected].