However, capacitance measurements also allow for extracting information on the conformational switch associated with binding of ions and/or substrate to transporters: it has, for instance, been possible to infer the conformational transition associated with phosphorylation of the Na +/K +-pump from the recorded change in capacitance ( Lu et al., 1995). Voltage-gated channels were the first membrane proteins on which capacitance measurements were conducted: the total C M of a cell expressing these proteins was shown to comprise a voltage-independent component, which was ascribed to the membrane, and a voltage-dependent component, which was attributed to mobile charges within the voltage sensors of the channel ( Bean and Rios, 1989). These add to the total charge required to recharge the membrane upon prior voltage change. Several studies – most of them conducted in cells that overexpressed a membrane protein – demonstrated the contribution of the same to the C M: the effect can be accounted for by mobile charges within the protein. However, a change in membrane area is not the only factor that can affect capacitance. Using this approach, it has been possible to detect changes in membrane area that result from the fusion of a single vesicle with the plasma membrane ( Neher and Marty, 1982 Fernandez et al., 1984 von Gersdorff and Matthews, 1994). In addition, voltage-clamp recordings also allow for determining the cell membrane capacitance (C M). The two hallmarks of electrophysiological recordings are high sensitivity and high temporal resolution: it is technically feasible to monitor currents through single molecules and to resolve conformational transitions, which occur on the microsecond scale.

gating currents, Armstrong and Bezanilla, 1973).

ion flux through ion channels, Hodgkin and Huxley, 1952) or which arise from synchronized movement of charged residues or ions within the membrane electric field (i.e. Voltage-clamp recordings have been used for the past 70 years to assess currents through membranes, which are either resistive (i.e.

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