Chapter 1 - Phylogenetic and Functional Classification of ABC (ATP-binding Cassette) Systems

Abstract

ATP-binding cassette (ABC) systems constitute one of the most abundant families of proteins. ABC systems are involved not only in the import or export of a wide variety of substances, but also in many cellular processes and in their regulation. Importers constitute mainly the prokaryotic transporters dependent upon a substrate-binding protein (BPD), whose function is to provide bacteria with essential nutrients even if the latter are present in submicromolar concentrations in the environment. Exporters are found in both prokaryotes and eukaryotes and are involved in the extrusion of noxious substances, the secretion of extracellular toxins and the targeting of membrane components. The third type of ABC system is apparently not involved in transport, but rather in cellular processes such as DNA repair, translation, or regulation of gene expression. Since ATP is found principally in the cytosol, import is defined as the inwardly directed transport of a molecule into the cytosol. By contrast, export is the translocation of a molecule out of the cytosol. ABC systems of the three types can be distinguished on the basis of the design of their component parts. All the transporters are composed of four structural domains: two very hydrophobic membrane-spanning or integral membrane domains (IMs) and two hydrophilic cytoplasmic domains containing the ABC peripherally associated with IM on the cytosolic side of the membrane.