Structural characterization of the Streptococcus pneumoniae carbohydrate substrate-binding protein SP0092

Streptococcus pneumoniae is an opportunistic respiratory pathogen that remains a major cause of morbidity and mortality globally, with infants and the elderly at the highest risk. S. pneumoniae relies entirely on carbohydrates as a source of carbon and dedicates a third of all uptake systems to carbohydrate import. The structure of the carbohydrate-free substrate-binding protein SP0092 at 1.61 Å resolution reveals it to belong to the newly proposed subclass G of substrate-binding proteins, with a ligand-binding pocket that is large enough to accommodate complex oligosaccharides. SP0092 is a dimer in solution and the crystal structure reveals a domain-swapped dimer with the monomer subunits in a closed conformation but in the absence of carbohydrate ligand. This closed conformation may be induced by dimer formation and could be used as a mechanism to regulate carbohydrate uptake.