Effect of phosphatidylglycerol depletion on the surface electric properties and the fluorescence emission of thylakoid membranes

To explore the possible effect of phosphatidylglycerol (PG) on the surface electric properties and chlorophyll fluorescence characteristics we used electric light scattering technique and 77 K chlorophyll fluorescence of thylakoid membranes from a cyanobacterium, Synechocystis PCC6803 (wild type) and its pgsA mutant defective in PG synthesis. We found a strong decrease in the permanent and induced electric dipole moments of the mutant thylakoids, following long-term PG depletion parallel with a decrease of the emission peak from PSI and an increase of the emission peak from PSII. Partial recovery of the electric state of thylakoid membranes was observed at re-addition of PG to the mutant cells depleted of PG for 21days. This change in the electric dipole moments is probably due to a decrease in PG content and progressive structural alterations in the macroorganization of the photosynthetic complexes induced by PG deprivation.Our results suggest that the depletion of a lipid, which carries a negative charge, despite its small contribution to the overall lipid content, significantly perturbs the surface charge of the membranes. These changes are related with the chlorophyll fluorescence emission ratios of two photosystems and may partly explain our earlier results concerning the PG requirement for the function and assembly of photosystems I and II reaction centers.