Effect of light-harvesting complex II on ion transport across model lipid membranes

The effect of the incorporation of the major Light-harvesting complex of ph otosystem II (LHCII) to planar bilayer lipid membranes (BLMs) formed from s oybean asolectin and unilamellar small liposomes formed from egg-yolk phosp hatidylcholine on ion transport across the lipid bilayer has been studied. The specific conductivity of the BLM rises from 5.2+/-0.8x10(-9) up to 510x 10(-9) O(-1)cm(-2) upon the incorporation of LHCII. The conductivity of the membrane with LHCII depends upon the ionic strength of the bathing solutio n and is higher by a factor of five when the KCI concentration increases fr om 0.02 to 0.22 M. Such a strong effect has not been observed in the same s ystem without LHCII. The liposome model is also applied to analyse the effe ct of LHCII on the bilayer permeability to protons. Unilamellar liposomes w ith a diameter less than 50 nm have been prepared, containing (trapped insi de) Neutral Red, a pigment sensitive to proton concentration. A gradient of protons on the membrane is generated by the acidification of the liposome suspension and spectral changes of Neutral Red are recorded in time, reflec ting the penetration of protons into the internal space of liposomes. Two c omponents of proton permeation across Liposome membranes are observed: a fa st one (proceeding within seconds) and a slow one (operating on the time sc ale of minutes). The rate of both components of proton transport across LHC II-containing membranes is higher than for liposomes alone. The enhancement effect of LHCII on the ion transport across the lipid membrane is discusse d in terms of aggregation of the pigment-protein complexes. The possible ph ysiological importance of such an effect in controlling ion permeability ac ross the thylakoid membrane is discussed. (C) 2000 Elsevier Science S.A. Al l rights reserved.