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Structure Of Eukaryotic Type III Glutamine Synthetase Lacking Ring-Ring Quaternary Contacts

There are three major classes of the enzyme Glutamine Synthetase (GS), denoted as GSI, GSII and GSIII in both prokaryotes and eukaryotes, that share widely conserved active-site residues but vary in average protein length, oligomeric assembly state and cofactor requirements. Key structures have been determined for all prokaryotic and eukaryotic GS classes except the GSIII class of eukaryotic origin, where even basic questions about its functional oligomeric state are unresolved. Here, we report the 3.2Å cryo-EM structure of eukaryotic GSIII from the ancient organism of green algal lineage Ostreococcus tauri. Using a combined structural and biochemical approach, we demonstrate that O. tauri GSIII self-assembles and functions exclusively as a single hexameric ring. This is unlike most GSs in other classes, where they form double-stacked ring assemblies instead. Major structural differences in the ring-ring stacking across the classes suggest evolutionary pressure may have favored higher-order interactions that might be beneficial but are not necessarily required for the catalytic GS performance.