Ion transport across model lipid membranes containing light-harvesting complex II: an effect of light

The effect of light on proton transport across lipid membranes of small unilamellar liposomes containing incorporated major light-harvesting pigment-protein complex of Photosystem II (LHCII) has been studied with the application of pH-sensitive dyes entrapped inside vesicles. Proton permeability coefficient for LHCII-modified membranes was found to be about twice as high as in the case of the control pure lipid vesicles. Illumination of the samples with light absorbed by the LHCII-bound photosynthetic pigments considerably affects the kinetics of proton transport: it increases the rate and decreases the steady-state level of proton gradient across the membranes. The effect was interpreted in terms of heat-induced conformational changes of LHCII molecular structures that affect proton buffering capacity of this protein. Both the control and the LHCII-modified lipid membranes have been found to be practically impermeable to Ca(++) ions, as demonstrated by fluorescence of liposome-entrapped calcium-sensitive probe calcium crimson. The slight differences in the proton transport across the LHCII-containing membranes under the presence of Ca(++) suggest calcium binding to this antenna protein.