Novel functional insights into a modified sugar-binding protein from Synechococcus MITS9220

Paradigms of metabolic strategies employed by photoautotrophic marine picocyanobacteria have been challenged in recent years. Based on genomic annotations, picocyanobacteria are predicted to assimilate organic nutrients via ATP-binding cassette importers, a process mediated by substrate-binding proteins. We report the functional characterisation of a modified sugar-binding protein, MsBP, from a marine Synechococcus strain, MITS9220. Ligand screening of MsBP shows a specific affinity for zinc (K-D similar to 1.3 mu M) and a preference for phosphate-modified sugars, such as fructose-1,6-biphosphate, in the presence of zinc (K-D similar to 5.8 mu M). Our crystal structures of apo MsBP (no zinc or substrate-bound) and Zn-MsBP (with zinc-bound) show that the presence of zinc induces structural differences, leading to a partially-closed substrate-binding cavity. The Zn-MsBP structure also sequesters several sulphate ions from the crystallisation condition, including two in the binding cleft, appropriately placed to mimic the orientation of adducts of a biphosphate hexose. Combined with a previously unseen positively charged binding cleft in our two structures and our binding affinity data, these observations highlight novel molecular variations on the sugar-binding SBP scaffold. Our findings lend further evidence to a proposed sugar acquisition mechanism in picocyanobacteria alluding to a mixotrophic strategy within these ubiquitous photosynthetic bacteria.

Automated summary

Recent studies have challenged the metabolic strategies employed by photoautotrophic marine picocyanobacteria. The functional characterisation of a modified sugar-binding protein, MsBP, from a marine Synechococcus strain, MITS9220, revealed specific affinities for zinc and phosphate-modified sugars. Crystal structures of apo MsBP and Zn-MsBP showed structural differences induced by zinc, with the latter sequestering sulphate ions and highlighting novel molecular variations on the sugar-binding SBP scaffold. These findings lend further evidence to a proposed sugar acquisition mechanism in picocyanobacteria, suggesting a mixotrophic strategy within these ubiquitous photosynthetic bacteria.