The histidine-containing phosphotransfer (HPt) domain is a novel protein module with an active histidine residue that mediates phosphotransfer reactions in the two-component signaling systems. A multistep phosphorelay involving the HPt domain has been suggested for these signaling pathways. The crystal structure of the HPt domain of the anaerobic sensor kinase ArcB has been determined [1]. The domain consists of six alpha helices containing a four-helix bundle-folding. The pattern of sequence similarity of the HPt domains of ArcB and components in other signaling systems can be interpreted in light of the three-dimensional structure and supports the conclusion that the HPt domains have a common structural motif both in prokaryotes and eukaryotes. In S. cerevisiae ypd1p this domain has been shown to contain a binding surface for Ssk1p (response regulator receiver domain containing protein Pfam:PF00072) [2].

1: Insights into multistep phosphorelay from the crystal structure of the C-terminal HPt domain of ArcB. Kato M, Mizuno T, Shimizu T, Hakoshima T; Cell 1997;88:717-723. PMID:9054511

2: Ssk1p response regulator binding surface on histidine- containing phosphotransfer protein ypd1p. Porter SW, Xu Q, West AH; Eukaryot Cell 2003;2:27-33. PMID:12582120