Cellular life depends on a controlled demarcation of the interior of the cell from the extracellular environment. This delineation is achieved through biological membranes. In that sense, membranes constitute for example boundaries between the living world (i.e. inside of cells) and the abiotic environment (i.e. outside of cells) or boundaries within or between cells, and the formation of biological membranes was a crucial step in the evolution of life from prebiotic precursors. Furthermore, biological membranes maintain a non-equilibrium state between the inside and outside space of single cells, subcellular compartments, and between the cells of multicellular organisms. A prerequisite for life is that membranes are not static entities but comprise constantly changing boundaries that react in response to internal and external cues.
A biological membrane is composed of a lipid bilayer and embedded membrane proteins. The lipid bilayer creates a protective barrier that allows life in a hostile environment and in the case of higher organisms, the generation of cell compartments that contain for example different redox potentials or pH values. On the other hand, this barrier is basically impermeable for all biological relevant molecules and to allow and ensure life in its present form, this function is performed by the variety of membrane proteins localized in the cell or organelle membrane. Based on the function, membrane proteins can be divided in receptors, channels and transporters. Transporters are furthermore sub-divided into primary or secondary transporters.
While secondary transporters rely on electrochemical gradients across biological membranes that have been generated by other processes, primary transporters use some sort of energetic input directly to catalyze transport across biological membranes. ABC (ATP binding cassette) transporters form the largest family among primary transporters and use the binding and hydrolysis to ultimately drive uphill transport of substrates across biological membranes.
First described in 1982, sequences projects have revealed that ABC transporters can be found in all three kingdoms of life. For example, 81 genes encode ABC transporter in E. coli, 31 in S. cerevisae and 48 in humans. Among the 48 human genes, at least 18 diseases are known that derive from mutations in the respective ABC transporter genes. The probably best-known disease is cystic fibrosis, the most common inherent and deadly disease among Caucasians. The substrate spectrum of ABC transporters is amazing and ranges from small inorganic ions such as chloride to nutrients, peptides, hydrophobic substances and even to intact proteins with a molecular weight of up to 1.5 MDa that are translocated across biological membranes at the dispense of ATP. Despite this diversity, all members share a basic blue print that is composed of two hydrophobic, membrane-spanning domains (transmembrane domain, TMD) and two water-soluble, nucleotide binding domains (NBD). While the four modules are normally encoded on separate genes in archaea and prokaryia, a fusion of these four modules is normally encountered in eukaryia. The NBD or motor domain contains all diagnostic sequence motifs within their primary structure that gave rise to this family. These sequences are the Walker A and B motifs, the C-loop or ABC signature motif and the D-loop.
Research in our laboratory aims to understand structural and functional aspects of ABC transporters on a molecular level. Therefore, we have selected a minimal set of transporters from different organisms and different function that are studied in vitro with different biochemical, biophysical, cellular and structural approaches.