The topology of phosphatidylglycerol populations is essential for sustaining photosynthetic electron flow activities in thylakoid membranes

The transmembrane distribution of phosphatidylglycerol (PG) was determined in rightside-out (RO) and inside-out vesicles (IO) obtained by fragmentatio n of spinach thylakoids in a Yeda press, followed by partition in an aqueou s dextranpolyethyleneglycol two-phase system. Using the phospholipase A(2) from porcine pancreas to digest selectively PG molecules in the outer monol ayer (exposed to the incubation medium) of the membrane, we found the molar outside/inside distribution to be 70/30 +/- 5 in RO and 40/60 +/- 3 in IO. The transmembrane distribution of PG in IO was the opposite of that in int act thylakoids (molar ratio 58/42 +/- 3). The phospholipid population which sustained most of the uncoupled photosystem II electron flow activity was localized in the inner monolayer (exposed to the thylakoid lumen) of both t hylakoid and RO membranes. In contrast, the activity in IO membranes was hi ghly dependent on the PG population located in the outer monolayer. This fi nding brings the first direct demonstration of the dependence of the photos ynthetic electron flow activity on the integrity of the inner topological p ool of PG in the thylakoid membrane. (C) 2000 Elsevier Science B.V. All rig hts reserved.