The influence of lipids on MGD1 membrane binding highlights novel mechanisms for galactolipid biosynthesis regulation in chloroplasts.

Mono- and digalactosyldiacylglycerol (MGDG and DGDG) are the most abundant lipids of photosynthetic membranes (thylakoids). In Arabidopsis green tissues, MGD1 is the main enzyme synthesizing MGDG. This monotopic enzyme is embedded in the inner envelope membrane of chloroplasts. DGDG synthesis occurs in the outer envelope membrane. Although the suborganellar localization of MGD1 has been determined, it is still not known how the lipid/glycolipid composition influences its binding to the membrane. The existence of a topological relationship between MGD1 and "embryonic" thylakoids is also unknown. To investigate MGD1 membrane binding, we used a Langmuir membrane model allowing the tuning of both lipid composition and packing. Surprisingly, MGD1 presents a high affinity to MGDG, its product, which maintains the enzyme bound to the membrane. This positive feedback is consistent with the low level of diacylglycerol, the substrate of MGD1, in chloroplast membranes. By contrast, MGD1 is excluded from membranes highly enriched in, or made of, pure DGDG. DGDG therefore exerts a retrocontrol, which is effective on the overall synthesis of galactolipids. Previously identified activators, phosphatidic acid and phosphatidylglycerol, also play a role on MGD1 membrane binding via electrostatic interactions, compensating the exclusion triggered by DGDG. The opposite effects of MGDG and DGDG suggest a role of these lipids on the localization of MGD1 in specific domains. Consistently, MGDG induces the self-organization of MGD1 into elongated and reticulated nanostructures scaffolding the chloroplast membrane.-Sarkis, J., Rocha, J., Maniti, O., Jouhet, J., Vié, V., Block, M. A., Breton, C., Maréchal, E., Girard-Egrot, A. The influence of lipids on MGD1 membrane binding highlights novel mechanisms for galactolipid biosynthesis regulation in chloroplasts.