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The synthesis of complex sugars is a key aspect of microbial biology. Cyclic β-1,2-glucan (CβG) is a circular polysaccharide critical for host interactions of many bacteria, including major pathogens of humans ( Brucella ) and plants ( Agrobacterium ). CβG is produced by the cyclic glucan synthase (Cgs), a massive multi-domain membrane protein. So far, its structure as well as the mechanisms underlining the synthesis have not been clarified. Here we use cryo-electron microscopy (cryo-EM) and functional approaches to study Cgs from A. tumefaciens . We were able to determine the structure of this complex protein machinery and clarify key aspects of CβG synthesis. Our research opens new possibilities for combating pathogens that rely on polysaccharide virulence factors and can lead to new synthetic biology approaches for producing complex cyclic sugars.