Mechanistically Novel Group of Membrane Transporters

ATP-binding cassette-containing (ABC) transport systems form a huge group of membrane transporters in all kingdoms of life in which they play diverse roles such as nutrient uptake, protein translocation, ion homeostasis, and drug resistance. A special feature of all previously known uptake ABC transporters in prokaryotes is the existence of extracellular substrate-binding proteins. These components act as solute receptors and as switches that stimulate ATP hydrolysis and substrate translocation through signalling substrate availability to the cytoplasmic ABC domains. In preliminary work, we have identified several hundred members of a novel group of ABC uptake systems for metals and organic solutes, the mechanism of which differs from the general concept in several respects. These transporters contain ABC modules and are driven by ATP hydrolysis. On the other hand, they display a basic activity in the absence of an ABC module and both activities are independent of extracellular solute-binding proteins. We are convinced that a detailed biochemical characterization of these systems will uncover features that are relevant in general for an understanding of the mechanism of ABC-type transport systems.