Permeability properties of large unilamellar vesicles of thylakoid lipids

The permeability properties of large unilamellar vesicles of chloroplast thylakoid lipid DGDG to 86Rb+, 36Cl- and 3H-D- glucose have been determined. In addition, the permeabilities of binary, ternary, and quaternary mixtures of thylakoid lipids to 86Rb+ have been measured. Vesicles of DGDG were found to be 60-130 fold more permeable to Rb+ and 46-76 fold less permeable to Cl than PC vesicles. Vesicles of DGDG and PC were similar in glucose permeability. Electron spin resonance measurement of DGDG bilayer fluidity with the spin probes 5-doxyl-stearate and 12-doxy-stearate indicated that DGDG bilayers were not significantly more fluid than PC bilayers, indicating that fluidity differences could not account for the observed differences in ion permeability. The addition of 50 mol% of MGDG to DGDG vesicles had no effect on Rb+ permeability, suggesting that the hexagonal-II phase preference of MGDG does not increase bilayer permeability. The addition of SQDG lead to a large increase in Rb+ permeability. The calculated permeability coefficient to Rb+ for a DGDG/MGDG/SQDG/PG (1/2/0.5/0.5) mixture similar to that of thylakoid membranes was 2.0-10-9 cm·s-1. This value is three orders of magnitude higher than that for phospholipid systems, and ten-fold higher than that for vesicles of DGDG. It is proposed that the high permeability of thylakoid lipids to cations is the main cause of low observed thylakoid membrane electrical potentials and large proton gradients in chloroplasts