A Trap-Door Mechanism for Zinc Acquisition by Streptococcus pneumoniae AdcA

Zinc is an essential element in all domains of life. Nonetheless, how prokaryotes achieve selective acquisition of zinc from the extracellular environment remains poorly understood. Here, we elucidate a novel mechanism for zinc-binding in AdcA, a solute-binding protein of Streptococcus pneumoniae. Crystal structure analyses reveal the two-domain organization of the protein and show that only the N-terminal domain (AdcA(N)) is necessary for zinc import. Zinc binding induces only minor changes in the global protein conformation of AdcA and stabilizes a highly mobile loop within the AdcA(N) domain. This loop region, which is conserved in zinc-specific solute-binding proteins, facilitates closure of the AdcA(N) binding site and is crucial for zinc acquisition. Collectively, these findings elucidate the structural and functional basis of selective zinc uptake in prokaryotes.