The transmembrane distribution of galactolipids in spinach thylakoid inside-out vesicles is opposite to that found in intact thylakoids

Abstract

The transmembrane distribution of galactolipids has been determined in spinach thylakoid inside-out vesicles obtained by fragmentation in a Yeda Press followed by partition in an aqueous dextran-polyethylene glycol two-phase system. Using the lipase from Rhizopus arrhizus to digest selectively the galactolipids in the outer monolayer of the membrane, we have found the molar outside/inside distribution to be 42±1/58±1 for monogalactosyldiacylglycerol (MGDG) and 82±0.2/18±0.2 for digalactosyldiacylglycerol (DGDG). As expected, the transmembrane distribution of galactolipids in inside-out vesicles was the opposite to that found in intact thylakoids which, under similar conditions, was found to be 62/38 for MGDG and 20/80 for DGDG. The reliability of the enzymatic approach to determine the transmembrane distribution and the relative arrangement of acyl lipids in the thylakoid membrane are discussed.