Polyhistidine triad proteins of pathogenic streptococci

The polyhistidine triad (Pht) proteins are an intriguing family of proteins found on the surface of members of the genus Streptococcus. Their defining feature is the presence of multiple copies of the eponymous His triad motif HxxHxH. This review focuses on the Pht proteins of Streptococcus pneumoniae, which contribute to virulence and are leading candidates for inclusion in protein-based pneumococcal vaccines. They appear to have multiple functions, including metal ion homeostasis, evasion of complement deposition and adherence of bacteria to host cells. Across the streptococci, there are many Pht homologs, which can be grouped according to structural features. Critically, there is considerable potential to use members of the Pht protein family as components of vaccines targeted at other streptococci.