Structural Insights and Membrane Binding Properties of MGD1, the Major Galactolipid Synthase in Plants

As major lipid components of photosynthetic membranes, Monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the most abundant lipids in the biosphere. They are essential for the assembly and function of the photosynthetic apparatus.
In Arabidopsis, the first step of galactolipid synthesis is catalyzed by MGDG synthase 1 (MGD1), which transfers a galactosyl residue from UDP-galactose to diacylglycerol (DAG). MGD1 is a monotopic protein embedded in the inner envelope membrane of chloroplasts. Once produced, MGDG is transferred to the outer envelope membrane, where DGDG synthesis occurs, and to thylakoids.

The crystal structures of the apo-form and the UDP complex were solved to 2.5 and 2.25 Ang. resolution, respectively. Key among other structural features are the long and flexible region of MGD1, a requirement for diacyglycerol binding and the pocket formed at the interface of two domains that form the donor sugar binding site. Furthermore, the binding mode of the enzyme to the membrane was unambiguously revealed following tests on biomimetic Langmuir monolayers, which gave insights into chloroplast membrane biogenesis.

The structural plasticity of MGD1, ensuring a very rapid capture and utilization of DAG, and its interaction with anionic lipids possibly driving the construction of lipoproteic clusters, are consistent with the role of this enzyme, not only in the expansion of the inner envelope membrane, but also in supplying MGDG to the outer envelope and nascent thylakoid membranes.