PP7, a plant phosphatase representing a novel evolutionary branch of eukaryotic protein Ser/Thr phosphatases

We describe a novel protein Ser/Thr phosphatase from Arabidopsis thaliana, PP7, which is only 27-32% identical in amino acid sequence to the known phosphatases and is the most divergent member of the PPP (PP1/2A/2B) family for today. Some structural features suggest more close relationship of PP7 to the PP5/rdgC subfamily. PP7 contains all of the residues essential for the phosphatase activity and possesses three major insertions in its presumable C-terminal subdomain, which suggest its unique regulation and/or optimisation of its structure for interaction with specific substrates or regulators. A phosphatase structurally related to PP7 is expressed in rice. PP7 conservation between mono- and dicotyledonous plants may point to its essential role in the plant cell.