19] Electrical approach to study rhodopsin activation in single cells with early receptor current assay

Abstract

The ERC is a conformation-dependent charge motion similar to the gating currents of ionic channels. Both the waveforms and bandwidth of ERCs and ionic channel gating currents are similar, providing support to the initial suggestion that the ERP was a kind of gating current. In ionic channels the electrostatic field promotes motion of alpha-helical elements that stimulate large-scale molecular events that promote opening of the ionic pore. In ionic channels gating currents of expressed channel mutants has contributed significantly to understanding the mechanism of activation. Given the known role of electrical processes to rhodopsin activation, the ERC approach applied to mutant and wild-type visual pigments is likely to lead to a fuller understanding of the mechanism of conformational activation. This method is currently well suited to investigate the later phases of rhodopsin activation that are thought to be electrostatic in nature. We anticipate that ERC studies will make significant contributions to understanding how the breakdown of the electrostatic interaction between the PSB and its counterion is initiated and propagated to induce the proton uptake on the cytoplasmic surface of the pigment and the shaping of the transducin docking domain. We encourage collaboration to apply the ERC methodology to interesting mutant pigments and retinal analogs. We expect that the ERC methodology can soon be applied to understand rapid charge displacements associated with photochemistry (i.e., R1), the effects of transduction proteins on R2, and the measurement of electrical processes during cone visual pigment activation.