Biochemical Characterization of the Histidine Triad Protein PhtD as a Cell Surface Zinc-Binding Protein of Pneumococcus

Zinc homeostasis is critical for pathogen host colonization. Indeed, during invasion, Streptococcus pneumoniae has to finely regulate zinc transport to cope with a wide range of Zn2+ concentrations within the various host niches. AdcAII was identified as a pneumococcal Zn2+-binding protein; its gene is present in an operon together with the phtD gene. PhtD belongs to the histidine triad protein family, but to date, its function has not been clarified. Using several complementary biochemical methods, we provide evidence that like AdcAII, PhtD is a metal-binding protein specific for zinc. When Zn2+ binds (K-d = 131 +/- 10 nM), the protein displays substantial thermal stabilization. We also present the first direct evidence of a joint function of AdcAII and PhtD by demonstrating that their expression is corepressed by Zn2+, that they interact directly in vitro, and that they are colocalized at the bacterial surface. These results suggest the common involvement of the AdcAII-PhtD system in pneumococcal zinc homeostasis.