Calcium-binding site in AA10 LPMO from suggests modulating effects during environmental survival and infection.

Despite major efforts toward its eradication, cholera remains a major health threat and economic burden in many low- and middle-income countries. Between outbreaks, the bacterium responsible for the disease, , survives in aquatic environmental reservoirs, where it commonly forms biofilms, for example, on zooplankton. -acetyl glucosamine-binding protein A (GbpA) is an adhesin that binds to the chitinaceous surface of zooplankton and breaks its dense crystalline packing thanks to its lytic polysaccharide monooxygenase (LPMO) activity, which provides with nutrients. In addition, GbpA is an important colonization factor associated with bacterial pathogenicity, allowing the binding to mucins in the host intestine. Here, we report the discovery of a cation-binding site in proximity of the GbpA active site, which allows Ca, Mg, or K binding close to its carbohydrate-binding surface. In addition to the X-ray crystal structures of cation-LPMO complexes (to 1.5 Å resolution), we explored how the presence of ions affects the stability and activity of the protein. Calcium and magnesium ions were found to bind to GbpA specifically, with calcium ions - abundant in natural sources of chitin - having the strongest effect on protein stability. When the cation-binding site was rendered non-functional, a decrease in activity was observed, highlighting the importance of the structural elements stabilized by calcium. Our findings suggest a cation-binding site specific to GbpA and related LPMOs that may fine-tune binding and activity for its substrates during environmental survival and host infection.