Abstract
Histidine phosphorylation of proteins is increasingly recognised as an important regulatory posttranslational modification in eukaryotes as well as prokaryotes. The HP (Histidine Phosphatase) superfamily, named for a key catalytic His residue, harbors two known groups of protein phosphohistidine phosphatases (PPHPs). The bacterial SixA protein acts as a regulator of His-Asp phosphorelays with two substrates characterized in vitro and/or in vivo. The recently characterized eukaryotic PHPP PGAM5 only has one currently known substrate, NDPK-B, through which it helps regulate T-cell signaling. SixA and PGAM5 appear to share no particular sequence or structural features relating to their PPHP activity suggesting that PHPP activity has arisen independently in different lineages of the HP superfamily. Further members of the HP superfamily may thus harbor (additional) unsuspected PHPP activity.
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