All members of this family are proteins from Streptococcal species. The proteins are characterised by having a HxxHxH motif that usually occurs multiple times throughout the protein [1]. The histidine triad is predicted to bind metal cations, in particular Zn2+. The zinc is transferred, on the surface of the streptococcus from the Strep_his_triad protein, a zinc scavenger, to apo-ADCAII, a cell-surface lipoprotein transporter [3] that leads to Zn2+ uptake [2] into the bacterium.

1: Recombinant PhpA protein, a unique histidine motif-containing protein from Streptococcus pneumoniae, protects mice against intranasal pneumococcal challenge. Zhang Y, Masi AW, Barniak V, Mountzouros K, Hostetter MK, Green BA; Infect Immun 2001;69:3827-3836. PMID:11349048

2: New insights into histidine triad proteins: solution structure of a Streptococcus pneumoniae PhtD domain and zinc transfer to AdcAII. Bersch B, Bougault C, Roux L, Favier A, Vernet T, Durmort C; PLoS One. 2013;8:e81168. PMID:24312273

3: AdcAII, a new pneumococcal Zn-binding protein homologous with ABC transporters: biochemical and structural analysis. Loisel E, Jacquamet L, Serre L, Bauvois C, Ferrer JL, Vernet T, Di Guilmi AM, Durmort C; J Mol Biol. 2008;381:594-606. PMID:18632116