Transbilayer organization of the thylakoid galactolipids

Abstract

The distribution across the thylakoid membrane of its major polar lipid constituents, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), was investigated by tritium labelling of membrane vesicles of opposite sidedness. Tritium was introduced into the galactose headgroups of the lipids by oxidation with galactose oxidase, or sodium periodate, and subsequent reduction with tritiated sodium borohydride. Rightside-out and inside-out vesicles with the same overall composition were prepared from randomized thylakoids. The results indicate, that there is an asymmetric distribution of the two galactolipids across the thylakoid membrane. About 60% of both galactolipids are found in the outer and 40% in the inner half of the bilayer. Since MGDG and DGDG together comprise 75% of the matrix forming lipids, it is suggested that the remaining matrix forming lipids, in particular the anionic phosphatidylglycerol and sulphoquinovosyldiacylglycerol, are mainly distributed to the inner leaflet of the membrane.