The evolutionarily-conserved histidine to aspartate (His-to-Asp) phosphorelay signal transduction is common in both prokaryotes and eukaryotes. Such a phosphorelay system is generally made up of ‘a histidine (His)-kinase’, ‘a histidine-containing phosphotransmitter (HPt)’, and ‘a phospho-accepting response regulator (RR)’. In general, an HPt factor acts as an intermediate in a given multistep His-to-Asp phosphorelay. In Arabidopsis thaliana, this model higher plant has five genes (named AHP1 to AHP5), each of which seems to encode an HPt factor. Recent studies suggested that the His-to-Asp phosphorelay involving the AHP factors is at least partly implicated in signal transduction in response to cytokinin (a plant hormone). Nevertheless, the properties of AHPs have not yet been fully clarified. Here we did comparative studies of all the AHP factors, in terms of (i) expression profiles in plants, (ii) intracellular localization, (iii) ability to acquire a phosphoryl group in vitro, and (iv) ability to interact with the downstream components, ARRs (Arabidopsis response regulators). The results of this study provided us with a comprehensive view at the molecular level for understanding the functions of the AHP phosphotransmitters in the His-to-Asp phosphorelay.

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