ABC transporter, lipid A-core flippase, MsbA (ABC_transpr_lipidA)
Type: Family

ABC transporters belong to the ATP-Binding Cassette (ABC) superfamily, which uses the hydrolysis of ATP to energise diverse biological systems. ABC transporters minimally consist of two conserved regions: a highly conserved ATP binding cassette (ABC) and a less conserved transmembrane domain (TMD). These can be found on the same protein or on two different ones. Most ABC transporters function as a dimer and therefore are constituted of four domains, two ABC modules and two TMDs.

ABC transporters are involved in the export or import of a wide variety of substrates ranging from small ions to macromolecules. The major function of ABC import systems is to provide essential nutrients to bacteria. They are found only in prokaryotes and their four constitutive domains are usually encoded by independent polypeptides (two ABC proteins and two TMD proteins). Prokaryotic importers require additional extracytoplasmic binding proteins (one or more per systems) for function. In contrast, export systems are involved in the extrusion of noxious substances, the export of extracellular toxins and the targeting of membrane components. They are found in all living organisms and in general the TMD is fused to the ABC module in a variety of combinations. Some eukaryotic exporters encode the four domains on the same polypeptide chain .

The ABC module (approximately two hundred amino acid residues) is known to bind and hydrolyse ATP, thereby coupling transport to ATP hydrolysis in a large number of biological processes. The cassette is duplicated in several subfamilies. Its primary sequence is highly conserved, displaying a typical phosphate-binding loop: Walker A, and a magnesium binding site: Walker B. Besides these two regions, three other conserved motifs are present in the ABC cassette: the switch region which contains a histidine loop, postulated to polarise the attaching water molecule for hydrolysis, the signature conserved motif (LSGGQ) specific to the ABC transporter, and the Q-motif (between Walker A and the signature), which interacts with the gamma phosphate through a water bond. The Walker A, Walker B, Q-loop and switch region form the nucleotide binding site .

The 3D structure of a monomeric ABC module adopts a stubby L-shape with two distinct arms. ArmI (mainly β-strand) contains Walker A and Walker B. The important residues for ATP hydrolysis and/or binding are located in the P-loop. The ATP-binding pocket is located at the extremity of armI. The perpendicular armII contains mostly the α helical subdomain with the signature motif. It only seems to be required for structural integrity of the ABC module. ArmII is in direct contact with the TMD. The hinge between armI and armII contains both the histidine loop and the Q-loop, making contact with the gamma phosphate of the ATP molecule. ATP hydrolysis leads to a conformational change that could facilitate ADP release. In the dimer the two ABC cassettes contact each other through hydrophobic interactions at the antiparallel β-sheet of armI by a two-fold axis .

The ATP-Binding Cassette (ABC) superfamily forms one of the largest of all protein families with a diversity of physiological functions . Several studies have shown that there is a correlation between the functional characterisation and the phylogenetic classification of the ABC cassette . More than 50 subfamilies have been described based on a phylogenetic and functional classification .

This family consists of a single polypeptide chain transporter in the ATP-binding cassette (ABC) transporter family, MsbA, which exports lipid A . It may also act in multidrug resistance being linked to the efflux of amphipathic drugs . Lipid A, a part of lipopolysaccharide, is found in the outer leaflet of the outer membrane of most Gram-negative bacteria. Members of this family are restricted to the Proteobacteria (although lipid A is more broadly distributed) and often are clustered with lipid A biosynthesis genes .