A MICROSECOND KINETIC STUDY OF THE PHOTOGENERATED MEMBRANE POTENTIAL OF BACTERIORHODOPSIN WITH A FAST RESPONDING DYE

Static and dynamic studies of the potential-sensitive membrane probe RH421 in dimyristoylphosphatidylcholine vesicles

The dynamics of the potential-sensitive styryl dye RH421 in dimyristoylphosphatidylcholine vesicles have been investigated above and below the main phase transition temperature using iodine-laser temperature-jump relaxation spectrophotometry and time-resolved fluorescence lifetime measurements. Equilibrium fluorescence titrations have shown that the affinity of the dye for the membrane is much higher in the liquid-crystalline state than in the gel state. The interaction can be described by either a partition or a binding model and a theory is presented providing a relation between these two approaches. In the liquid-crystalline state bound dye exhibits steady-state fluorescence relaxation processes in the submicrosecond and millisecond time range following a temperature jump. Time-resolved fluorescence measurements show a variation in the fluorescence lifetime across the emission spectrum, suggesting an excited-state process occurring on the subnanosecond time scale. These processes are most likely related to dye and/or lipid reorientation following the temperature jump or excitation pulse. Temperature-dependent changes in the fluorescence excitation spectrum of bound dye suggest that the dye exists in at least two different sites within the membrane.

Spectroscopic investigations of the potential-sensitive membrane probe RH421

The absorbance spectra, fluorescence emission and excitation spectra, and fluorescence anisotropy of the potential-sensitive styryl dye RH421 have been investigated in aqueous solution and bound to the lipid membrane. The potential-sensitive response of the dye has been studied using a preparation of membrane fragments containing a high density of Na+, K(+)-ATPase molecules. In aqueous solution the dye is sensitive both to changes in pH and ionic strength. Evidence has been found that the dye readily aggregates in aqueous solution. Aggregation is enhanced by an increase in ionic strength. The aggregates formed display a low fluorescence intensity. At high pH values (above approx. 8) changes in the dye's fluorescence spectra are observed, which may be due to a reaction of the dye with hydroxide ions. When bound to the membrane the dye also exhibits concentration-dependent fluorescence changes. The potential-sensitive response of the dye in Na(+),K(+)-ATPase membrane fragments after addition of MgATP in the presence of Na+ ions cannot be explained by a purely electrochromic mechanism. The results are consistent with either a potential-dependent equilibrium between membrane-bound dye monomers and membrane-bound dimers, similar to that previously proposed for the dye merocyanine 540, or with a field-induced structural change of the membrane.

The behavior of a fast-responding barbituric acid potential-sensitive molecular probe in bovine heart submitochondrial particles

The barbituric acid probe diBa-C2-(5) responds to the formation of a membrane potential (delta psi) in bovine heart submitochondrial particles (SMP) by a CCCP-reversible, 5-7 nm red shift of the probe absorption spectrum. This shift can be enhanced by the addition of nigericin, an observation that indicates that the probe is specifically sensitive to delta psi. Probe-SMP binding analyses indicate that, relative to the resting state, the ratio of the dye dissociation constant to the maximum number of binding sites decreases by a factor of 30 when delta psi is generated. This observation suggests that the origin of the potential-dependent shift of the probe absorption spectrum is increased occupancy of the SMP membrane by diBa-C2-(5). The time course of the ATP-induced diBa-C2-(5) spectral shift in SMP was complete in nominally 0.2 s and could be described by a single-exponential rate equation. There was no evidence for a slower-phase signal when the data collection time period was increased to 250 s. The apparent first-order rate constants obtained from the single exponential analyses of the barbituric acid ATP-generated signal, however, were a linear function of probe concentration at fixed SMP membrane concentration. The resulting second-order rate constant obtained by linear regression was nominally 1 x 10(7) M (dye)-1 s-1; this value is two to three orders of magnitude higher than that of a number of other well-established probes of delta psi in mitochondrial preparations. Based on the invariance of the kinetics of the oxidation of cytochromes c and c1 by ATP-driven reversed electron transport in the presence and absence of the probe, diBa-C2-(5) does not appear to permeate the SMP membrane on a time scale of milliseconds to several minutes.

Charge translocation by the Na,K-pump: I. Kinetics of local field changes studied by time-resolved fluorescence measurements

Membrane fragments containing a high density of Na,K-ATPase can be noncovalently labeled with amphiphilic styryl dyes (e.g., RH 421). Phosphorylation of the Na,K-ATPase by ATP in the presence of Na+ and in the absence of K+ leads to a large increase of the fluorescence of RH 421 (up to 100%). In this paper evidence is presented that the styryl dye mainly responds to changes of the electric field strength in the membrane, resulting from charge movements during the pumping cycle: (i) The spectral characteristic of the ATP-induced dye response essentially agrees with the predictions for an electrochromic shift of the absorption peak. (ii) Adsorption of lipophilic anions to Na,K-ATPase membranes leads to an increase, adsorption of lipophilic cations to the decrease of dye fluorescence. These ions are known to bind to the hydrophobic interior of the membrane and to change the electric field strength in the boundary layer close to the interface. (iii) The fluorescence change that is normally observed upon phosphorylation by ATP is abolished at high concentrations of lipophilic ions. Lipophilic ions are thought to redistribute between the adsorption sites and water and to neutralize in this way the change of field strength caused by ion translocation in the pump protein. (iv) Changes of the fluorescence of RH 421 correlate with known electrogenic transitions in the pumping cycle, whereas transitions that are known to be electrically silent do not lead to fluorescence changes. The information obtained from experiments with amphiphilic styryl dyes is complementary to the results of electrophysiological investigations in which pump currents are measured as a function of transmembrane voltage. In particular, electrochromic dyes can be used for studying electrogenic processes in microsomal membrane preparations which are not amenable to electrophysiological techniques.

NMR, calorimetric, spin-label, and optical studies on a trifluoromethyl-substituted styryl molecular probe in dimyristoylphosphatidylcholine vesicles and multilamellar suspensions: a model for location of optical probes

NMR, calorimetric, and optical spectroscopic studies have been performed on a trifluoromethyl-substituted styryl molecular probe bound to vesicles and multilamellar suspensions formed from dimyristoylphosphatidylcholine (DMPC). In the fluorine NMR spectrum at 35 degrees C there are two partially resolved resonances, but these collapse to an apparently single resonance at temperatures above 60 degrees C. However, a line-shape analysis is not consistent with exchange between two sites on an NMR time scale, and the two resonances are assumed to be due to probe sites in the inner and outer leaflets of the vesicles. Two fluorescence lifetimes, each associated with one of these sites, characterize the decay curves for the molecular probe bound to DMPC vesicles. The shift reagent Eu(FOD)3 and several nitroxide spin labels covalently bound to lipophilic structures strongly attenuate the lower frequency component of the fluorine NMR spectrum and also shift the other resonance to higher frequencies. The effect of two spin labels on the probe fluorine T2 relaxation time has been used to estimate the distance between the spin label unpaired electron and the trifluoromethyl group. The location of the spin label site in the membrane was determined from the effect of the unpaired electron on the lipid 13C linewidths. A model for the location of the probe in the bilayer was developed from the above information and refined using molecular mechanics calculations on a probe-DMPC lipid complex. The long axis of the probe parallels the bilayer normal; the styryl-group portion of the optical chromophore is located slightly below the glycerol backbone, and the remainder of the chromophore extends well into the hydrophobic region of the bilayer. Therefore, the optical properties of the probe should not be significantly influenced by alterations of the membrane surface charge density. Parameters derived from DSC studies in the gel-to-lipid crystal phase transition of DMPC are extremely sensitive to the probe. Even at 0.0001 mol fraction of probe, the transition is substantially broadened, and the delta H for the transition has increased, just as one predicts for the formation of a tight complex described above.

Bacterial rhodopsins monitored with fluorescent dyes in vesicles and in vivo

Three retinal-containing pigments have been detected in Halobacterium halobium membranes: bacteriorhodopsin (bR), halorhodopsin (hR), and slow-cycling rhodopsin (sR). The first two hyperpolarize the cell membrane by electrogenic transport of H+ and Cl-, respectively. The third pigment, sR, may be a photosensory receptor since mutants lacking bR and hR retain their retinal-dependent phototaxis responses. We monitored light-induced changes in fluorescence of several voltage-sensitive dyes in cells and membrane vesicles. Red light-induced potential changes generated by bR and hR were similar to signals described previously. Signals generated by hR could be identified using four criteria: wavelength dependence, Cl- dependence, shunting by valinomycin and K+, and the absence of these signals in hR-deficient mutants. The absence (detection limit approximately 0.5 mV) of hyperpolarization signals in bR-hR-sR+ vesicles and cells shows that sR photochemical reactions are nonelectrogenic. Two signals independent of bR and hR were measured: blue light caused a decrease and red light an increase in dye fluorescence. Both signals appear to derive from sR on the basis of their retinal-dependence and action spectra. In a retinal-deficient mutant strain (Flx3R), both sR signals appeared after addition of all-trans retinal. In this strain retinal also restores phototaxis sensitivity within the same time scale. The retinal concentration dependence for all four parameters monitored--the attractant (red) and repellent (blue) phototaxis, and the red light and blue light-induced fluorescence signals--is the same. This correlation is consistent with the hypothesis that both attractant and repellent responses are mediated by sR, as suggested by Bogomolni and Spudich (Proc. Natl. Acad. Sci. USA. 79:6250-6254 (1982)).