This special issue focuses on two important types of ionmotive ATPases: P-type ATPases that are characterised by the formation of a phosphoprotein intermediate and V-type ATPases found in the diverse compartments belonging to the eukaryotic vacuolar system, which includes not only components of the secretory and endocytotic membrane system but also the plasma membrane.The 1940s and the early 1950s were of pivotal importance for the newly emerging field of electrophysiology. In the course of this decennium, physiologists witnessed the gradual development of the concept of active transport of inorganic ions in cells. It became at that time, partly by the use of radioisotopes, gradually but undeniably clear that biological membranes do not act as barriers impermeable to ions like Na + or K + but that instead there exists a continuous transmembrane exchange of ions, and hence, that in order to ensure a steady state, cells had to actively move Na + and K + against concentration gradients. It was soon realised that the thus generated transmembrane electrochemical ion gradient can be used to drive other ions or non-ionic compounds uphill and that it allows the generation of electrical activity in the form of action potentials. However, it was also clear that preserving this transmembrane gradient, in face of the ongoing drain imposed by the various ion movements, requires the continuous input of energy. It is not surprising that evolution has pioneered many different types of active transport systems to counter these leaks. But the question remained how can chemical energy be harnessed to catalyse a vectorial transmembrane transport of ions?It was Jens C. Skou in Denmark who, about 50 years ago in 1957, first pointed to an adenosine triphosphatase as the motor of this active transport system [1], a discovery that only 40 years later earned him the Nobel prize for Chemistry 1997: "for the first discovery of an ion-transporting enzyme, Na + , K + -ATPase" (see Nobel lecture at http://nobelprize.org/ nobel_prizes/chemistry/laureates/1997/skou-lecture.html and [2]).The 1960s represent the decennium of the Ca 2+ -transporting ATPases. First, the Ca 2+ transport ATPase from skeletal muscle sarcoplasmic reticulum (SERCA) and shortly thereafter that of erythrocyte plasma membrane (PMCA) joined the scene. The challenge became now to unravel the individual steps of the transport process. The phosphorylation of an aspartate residue in the ATPase was rapidly recognised as a necessary step in the catalytic cycle. The formation of this intermediate represents the hallmark of these ATPases, hence the name of this group of transporters, the P-type ATPases.But not all transport ATPases make use of a phosphoprotein intermediate. The idea of the V-type ATPases took much more time to mature (see [3] for a review). The Vtype ATPases are now known to form an independent group of ion-transport ATPases using a molecular motor reminiscent of that found in the bacterial F-type ATPases. It took indeed another 30 years after the discovery of the ...
