The importance of protein phosphorylation by protein kinases to regulate signalling responses is well recognized in plants; and it is becoming more and more evident that also the inverse, de-phosphorylation by protein phos-phatases, is indispensable in these processes. The key phosphorylation targets in eukaryotic proteins, including plants, are serine, threonine, and tyro-sine residues. Accordingly, protein phosphatases are organized into different groups dependent on the phosphor-amino acid residues that are targeted for dephosphorylation. Consequently, serine/threonine phosphatases, tyro-sine phosphatases and dual specificity phosphatases (targeting both tyrosine and serine/threonine) are found in plants. Eukaryotic serine/threonine protein phosphatases are classified into the superfamily PPP (phospho protein phosphatases), the PPM (protein phosphatases magnesium- or manganese-dependent) family and the FCP family, according to the amino acid sequences of their catalytic subunits (Cohen 2004; Ingebritsen and Cohen 1983) (see Fig. 1). The PPP family of plant phosphatases includes the type 1 protein phosphatases (PP1), type 2A protein phosphatases (PP2A), protein phosphatases with Kelch-repeat domain (a conserved tertiary structure forming
beta propellers) and protein phosphatases PP4, PP5, PP6, PP7. PP2B type phosphatases that are represented by calcineurin in other eukaryotes have not been detected in plants so far. The PPM superfamily consists of protein phosphatases 2C (PP2C). The new FCP family was recognized recently through CTD phosphatase-like (CPL) members with homology to the FCP1 (TFIIF-associating C-terminal domain of RNAP II phosphatase) protein serine phosphatase, which regulates transcription by dephosphorylating the carboxyl-terminal domain of the large subunit of RNA polymerase Pol II (Bang et al. 2006; Cohen 2004). Plant protein tyrosine phosphatases (PTP) are
A Fig. 1 Classification and characterization of serine/threonine protein phosphatases. First classifications of S/T protein phosphatases were based on their different capabilities to dephosphorylate specific substrates and their responses to heat-stable inhibitor proteins (Cohen 1989; Ingebritsen and Cohen 1983). Thus, PP1 preferentially dephosphorylated the P-subunit of phosphorylase kinase, whereas type 2 phosphatases dephosphorylated the a-subunit of phosphorylase kinase. PP1 was inhibited by the heat-stable inhibitor-1 (I1) and inhibitor-2 (I2), whereas type 2 phosphatases (PP2A, PP2B and PP2C) were not. PP2Cs are Mg2+/Mn2+-dependent for their activity, while PP2As are active in the absence of bivalent cations. Although this classification remains true, the information about protein primary structure has led towards more comprehensive classification according to sequence similarity of the catalytic subunit. Thereby, PP2C belongs to a distinct PPM (protein phosphatases Mg2+/Mn2+-dependent) gene family, whereas the remaining type 1 and type 2 phosphatases belong to the same (PPP) family. In comparison with the PPP gene family, the PPM members are overrepresented in plants structurally and catalytically distinct from the serine/threonine phosphatases. PTP superfamily consists of classical protein tyrosine phosphatases and dual-specificity protein phosphatases (DSPs) (see Fig. 2). The receptor-like PTPs that are abundant in animals have not been identified in plants so far.
Several reviews have been written on plant phosphatases (DeLong 2006; Farkas et al. 2007; Luan 1998; Smith and Walker 1996) and two reviews are available about Arabidopsis PP2C members (Rodriguez 1998; Schweighofer
et al. 2004). Here we will update the information about plant PTP/DSP and PP2C members with the recent data, for the most part on Arabidopsis.
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