Structural characterisation of the HT3 motif of the polyhistidine triad protein D from Streptococcus pneumoniae.

The bacterium Streptococcus pneumoniae (the pneumococcus) is a major human pathogen that requires Zn for its survival and virulence in the host environment. Polyhistidine triad protein D (PhtD) has a known role in pneumococcal Zn homeostasis. However, the mechanistic basis of PhtD function remains unclear, partly due to a lack of structural information. Here, we determined the crystal structure of the fragment PhtD , containing the third Zn -binding histidine triad (HT) motif of the protein. Analysis of the structure suggests that Zn binding occurs at the surface of the protein and that all five HT motifs in the protein bind Zn and share similar structures. These new structural insights aid in our understanding of how the Pht proteins facilitate pneumococcal Zn acquisition.