Structural insights and membrane binding properties of <scp>MGD</scp>1, the major galactolipid synthase in plants

<jats:title>Summary</jats:title><jats:p>Monogalactosyldiacylglycerol (<jats:styled-content style="fixed-case">MGDG</jats:styled-content>) and digalactosyldiacylglycerol (<jats:styled-content style="fixed-case">DGDG</jats:styled-content>) are the major lipid components of photosynthetic membranes, and hence the most abundant lipids in the biosphere. They are essential for assembly and function of the photosynthetic apparatus. In Arabidopsis, the first step of galactolipid synthesis is catalyzed by <jats:styled-content style="fixed-case">MGDG</jats:styled-content> synthase 1 (<jats:styled-content style="fixed-case">MGD</jats:styled-content>1), which transfers a galactosyl residue from <jats:styled-content style="fixed-case">UDP</jats:styled-content>‐galactose to diacylglycerol (<jats:styled-content style="fixed-case">DAG</jats:styled-content>). <jats:styled-content style="fixed-case">MGD</jats:styled-content>1 is a monotopic protein that is embedded in the inner envelope membrane of chloroplasts. Once produced, <jats:styled-content style="fixed-case">MGDG</jats:styled-content> is transferred to the outer envelope membrane, where <jats:styled-content style="fixed-case">DGDG</jats:styled-content> synthesis occurs, and to thylakoids. Here we present two crystal structures of <jats:styled-content style="fixed-case">MGD</jats:styled-content>1: one unliganded and one complexed with <jats:styled-content style="fixed-case">UDP</jats:styled-content>. <jats:styled-content style="fixed-case">MGD</jats:styled-content>1 has a long and flexible region (approximately 50 amino acids) that is required for <jats:styled-content style="fixed-case">DAG</jats:styled-content> binding. The structures reveal critical features of the <jats:styled-content style="fixed-case">MGD</jats:styled-content>1 catalytic mechanism and its membrane binding mode, tested on biomimetic Langmuir monolayers, giving insights into chloroplast membrane biogenesis. The structural plasticity of <jats:styled-content style="fixed-case">MGD</jats:styled-content>1, ensuring very rapid capture and utilization of <jats:styled-content style="fixed-case">DAG</jats:styled-content>, and its interaction with anionic lipids, possibly driving the construction of lipoproteic clusters, are consistent with the role of this enzyme, not only in expansion of the inner envelope membrane, but also in supplying <jats:styled-content style="fixed-case">MGDG</jats:styled-content> to the outer envelope and nascent thylakoid membranes.</jats:p>