Any object that enters or leaves a cell, whether nutrient, virus or waste product, must penetrate one or more enclosing membranes. The magnitude of this phenomenon may be estimated, for example, by the need of an actively growing Escherichia coli
cell to take up ~106
glucose molecules per second1, 2
to support the requisite metabolic demands. Cells must not only be able to import preferred substrates, but they also often have the capability to utilize a wide variety of alternate nutrients when available. With a few exceptions (such as O2
), the movement of small molecules, ions, and even some macromolecules across membranes is mediated by specialized membrane proteins known as transporters. To accommodate the diversity of molecules a cell may need to acquire from the environment, many different transporters are encoded in the genomes of organisms. In E. coli
, for example, ~10% of the genome has been classified as participating in transport processes3
and overall, more than 550 different types of transporters have been identified4
. The importance of transport activity may be appreciated from the non-trivial metabolic cost of pumping molecules across cell membranes, which can consume a significant fraction (estimated as ~10–60%, depending on conditions) of the ATP requirement of bacteria5, 6
One of the largest classes of transporters is the ABC (ATP binding cassette) transporter superfamily8–10
. These transporters use the binding and hydrolysis of ATP to power the translocation of a diverse assortment of substrates, ranging from ions to macromolecules, across membranes. ABC transporters function as either importers, bringing nutrients and other molecules into cells, or as exporters that pump toxins, drugs and lipids across membranes (Box 1
). Members of the ABC transporter family are present in organisms from all kingdoms of life; while exporters are found in both eukaryotes and prokaryotes, importers appear to be present exclusively in prokaryotic organisms. ABC transporters constitute the largest protein family in E. coli
, including ~80 distinct systems representing 5% of the genome11
, while ~50 ABC transporters are present in humans12
. Members of the seven families of human ABC transporters13
participate in cholesterol and lipid transport, multidrug resistance, antigen presentation, mitochondrial iron homeostasis, and the ATP-dependent regulation of ion channels (the cystic fibrosis transmembrane conductance regulator and the sulfonyl urea receptors); mutations of these proteins have been associated with a range of disorders including cystic fibrosis, hypercholesterolemia and diabetes.
Box 1. Ins and Outs of ABC transporters
Cellular survival requires the generation and maintenance of electrical and chemical concentration gradients across the generally impermeable cell membrane. ABC transporters are key participants in this process, and typically use the favorable chemical energy of ATP hydrolysis to translocate molecules across membranes in the thermodynamically unfavorable direction. A given ABC transporter may function as either an importer or an exporter, moving molecules in or out of cells, respectively, but no example is known of an ABC transporter that functions physiologically in both directions. ABC transporters that function as importers are found predominantly in prokaryotes, where they mediate the uptake of essential nutrients, such as amino acids, sugars, and essential metals. Substrates of ABC importers vary greatly in size and chemical nature, ranging from oligopeptides and oligosaccharides to small ions. ABC exporters are found in both prokaryotes and eukaryotes, and their substrates are typically lipophilic. In humans, ABC exporters are crucial participants in lipid, fatty acid and cholesterol export, malfunction of which underlies various diseases. Perhaps the most studied ABC transporter is the human Pgp that maintains cholesterol distribution across the leaflets of the plasma membrane73, 74
. Unfortunately, Pgp also extrudes other lipophilic compounds, including chemotherapeutic agents, resulting in multidrug resistance of tumor cells.
Whether functioning as importers or exporters, ABC transporters likely share a high degree of mechanistic similarities. For both exporters and importers, an alternating access model for transport has been suggested17, 31, 48
. The key feature of this model is the presence of a substrate binding site that can alternatively access either the extracellular side or the intracellular side of the membrane, corresponding to the “outward” and “inward” facing conformations of the transporter, respectively. ATP binding and hydrolysis drive the conformational changes that result in the alternating exposure of this binding site to the two sides of the membrane; the relative binding affinities for substrate of the two conformations will largely determine the net direction of transport. In particular, the outward facing conformation of an importer is expected to have a higher affinity for substrate than the inward facing conformation, while the opposite relationship will hold for exporters.
ABC transporters have a characteristic architecture, which consists minimally of four domains (): two transmembrane domains (TMDs) embedded in the membrane bilayer and two ABCs (also designated as the nucleotide binding domains (NBDs)) located in the cytoplasm. At the sequence level, the superfamily of ABC transporters is identified by a characteristic set of highly conserved motifs present in the ABCs; in contrast, the sequences and architectures of the TMDs are quite variable, reflecting the chemical diversity of the translocated substrates. Beyond these four domains, additional elements may be found fused to the TMDs and/or ABCs of ABC transporters that likely serve regulatory functions14
. For prokaryotic ABC transporters that function as importers, substrate translocation is also dependent on another protein component, a high affinity binding protein that specifically associates with the ligand in the periplasm for delivery to the appropriate ABC transporter15
(). As originally recognized by Heppel16
, these binding proteins are released upon osmotic shock and hence the associated transport systems, now recognized as ABC transporters, were initially identified as “shock-sensitive”.
Figure 1 Molecular architecture of ABC transporters. (a) A cartoon representation of the modular organization of ABC transporters, composed of two transmembrane domains (TMD) and two ABC domains. The binding protein component required by importers is also illustrated. (more ...)
Following the crystal structure determination of the E. coli
importer BtuCD in 200217
, the structural biology of intact ABC transporters has exploded over the past 2 years18–26
(). Recent reviews highlighting these structural advances may be found in27–33
. A brief discussion of developments in the crystallography of ABC transporters may be found in the Supplementary Information (Box S1)
. In this article, we build on this structural foundation and focus on comparative and mechanistic aspects of ABC transporters, particularly emphasizing developments involving prokaryotic members of this superfamily.
Table 1 Structurally characterized ABC transporter systems, classified by TMD fold. The nomenclature of the fold classification is from33. Note that while the outward facing conformation generally corresponds to the ATP (or suitable analogue) bound state, exceptions (more ...)