Abstract

Bordetella pertussis, the causative agent of whooping cough, is a reemerging public health threat. While the Tripartite Tricarboxylate Transporter (TTT) system BctCBA was previously implicated solely in citrate uptake, we demonstrate that the solute-binding protein BctC specifically binds citrate chelated with Zn2+ and Ni2+. To elucidate the molecular mechanism of this interaction, we determined the crystal structures of BctC in three states: apo, open (citrate-bound), and closed (citrate-zinc-bound), defining the structural determinants for metal-citrate recognition. Furthermore, transcriptional analyses show that bctCBA operon expression is upregulated under divalent cation limitation. Comparative analyses suggest that citrate-mediated divalent cation binding is a widespread feature among bacterial TTT homologs. Finally, in silico modeling of the full BctCBA complex predicts an elevator-type transport mechanism. Together, these findings redefine the functional scope of BctCBA, revealing a sophisticated strategy by which B. pertussis exploits organic chelators to acquire essential trace metals during infection.