Ser/Thr phosphatase

We know of Ser/Thr phosphate specific protein phosphatases (referred to in the following as Ser/Thr phosphatases) and Tyr phosphate specific protein phosphatases. The latter are dealt with in Chapter 8. 

The Ser/Thr phosphatases exist mostly as heterodimers composed of a catalytic subunit and another subunit to which a specific function for localization of protein phosphatases is often attributed. In the case of protein phosphatase I, this subunit is known as the targeting subunit, its function is described in more detail in 7.7. 

Regulation of the Ser/Thr phosphatases takes place predominantly by three mechanisms. 

Specific inhibitor proteins for Ser/Thr phosphatases exist which can control the activity of the protein phosphatases. These inhibitors are generally subject to regulation themselves, e.g., by phosphorylation (Fig. 7.17). 

The molecular basis of CsA s biological activity is the formation of a complex with the immunophilin cyclophilin A (CyPA) in such a way that the CsA-CyPA complex can associate with calcineurin (CN), which is a Ca2+/calmodulin-dependent Ser/Thr phosphatase. As a result, dephosphorylation of a nuclear factor of activated T-cells (NFAT) is inhibited. 

Table 7.1 The properties of phosphoprotein (Ser/Thr) phosphatases (reproduced from ref. 4 with permission of Dr D. Barford and Trends in Biochemical Sciences)...

Fig. 7.2 Tlie crystal structure of mammalian Ser/Thr protein phosphatase-1, complexed with the toxin mycrocystin was determined at 2.1 A resolution. PPl has a single domain with a fold, distinct from that of the protein tyrosine phosphatases. The Ser/Thr protein phosphatase-1, is a metalloenzyme with two metal ions positioned at the active site with the help of a p-a-p-o-p scaffold. A dinuclear ion centre consisting of Mn2+ And Fe2+ g situated at the catalytic site that binds the phosphate moiety of the substrate. Ser/Thr phosphatases, PPl and PP2A, are inhibited by the membrane-permeable ocadaic acid and by cyclic hexapeptides, known as microcystins. The toxin molecule is depicted as a ball-and-stick structure. On the left and on the ri t, two different views of the same molecule are shown. Microcystin binds to three distinct regions of the phosphatase to the metaLbinding site, to a hydrophobic groove, and to the edge of a C-terminal groove in the vicinity of the active site. At the surface are binding sites for substrates and inhibitors. These ribbon models are reproduced vnth permission of the authors and Nature from ref. 9.

Comparing the structures in Fig. 7.2 and Plate 13 reveals how highly conserved the catalytic domains of Ser/Thr phosphatases are. Therefore their functional diversity must be due to different regulatory and targeting subunits that direct the phosphatase to discrete subcellular locations (Fig. 7.3). 

CG10417 Ser/Thr phosphatase CG6180 kinase inhibitor Synaptic Ras-GTPase activating protein 1... 

Fig. 4.3 Phosphorylated and non-phos-phorylated forms of PLDl shown by gel electrophoresis and phosphatase treatment. Panel (A) The N-terminal half of rat PLDl (rPLDlNT) was expressed in COS7 cells and immunoprecipitated. The immunoprecipitates were treated with a Ser/Thr phosphatase in the absence and presence of a phosphatase inhibitor (mi-crocystin) and then subjected to gel electrophoresis and Western blotting. The figure shows that the phosphatase eliminated the upper (phosphorylated) band, whereas microcystin restored it. (B) The same experiment utilizing full-length rat PLDl (rPLDl).

Brautigan DL (2013) Protein Ser/Thr phosphatases— the ugly ducklings of cell signalling. FEBS J 280(2) 324-345... 

Fostriecin is another phosphatase inhibiting compound that binds the catalytic subunit of Ser/Thr phosphatases and triggers premature entry in mitosis. It has a low toxicity and a high specificity for PP2A and the PP2A-like phosphatase PP4, but unfortunately,... 

Thus, despite the existence of several different classes ofPP2A inhibitors, some of which are selective for PP2A (vs. PPl), none of them can be considered as 100 % specific. Moreover, when used in vivo or in cells as a means to discriminate between the activities of different Ser/Thr phosphatases, one has to realize their effects are not only dependent on the specificity of the inhibition itself, but also on the permeability of the cells (or tissues) for these inhibitors, which can significantly differ for distinct cell types and inhibitors [2],... 

Janssens V, Rebollo A (2012) The role and therapeutic potential of Ser/Thr phosphatase PP2A in apoptotic signaling networks in human cancer cells. Curr Mol Med 12 268-287... 

In mammals, at least eight types of Ser/Thr-specific protein phosphatases have been identified, and these are classified according to their substrate specificities, metal requirements, and sensitivities to natural or synthetic inhibitors. Four groups of the Ser/ Thr phosphatases, the protein phosphatases (PP) 1, 2A, 2B and C2, have been studied in detail. PP-1, PP-2A and PP-2B, which is also known as calcineurin, are highly homo-... 

Calcineurin (protein phosphatase 2B) is a Ser/Thr phosphatase that is controlled by cellular calcium and regulates a large number of biological responses including lymphocyte activation, neuronal and muscle development, and the development of vertebrate heart valves. 

The introduction of a classification system for PP has been useful in characterization of the most abundant enzymatic forms from various tissues. However, cloning of cDNA for the PP catalytic subunits has led to the conclusion that PP is an expanding family, and other types of Ser/Thr phosphatases (PPX, PPZ, etc.) exist and are distinct from PPl and PP2 (Cohen and Cohen, 1989). Recognition of the dual specificity (Ser/Thr and Tyr) has been a significant development and the Tyr phosphatases also represent a new class (Guan et al., 1991). Thus the application of a rigid classification may not be appropriate and the use of a more flexible system may be imposed by future research. 

Gauss, C.M. et al., A molecular modeling analysis of the binding interactions between the okadaic acid class of natural product inhibitors and the ser/thr phosphatases, PPl and PP2A, Bioorg. Med. Chem., 5, 1751, 1997. 

Protein tyrosine phosphatases. Phosphoprotein phosphatases are integral components of the signahng systems operated by protein kinases (Sun and Tonks, 1994). Cloning data show the protein tyrosine phosphatases (PTPs) to be a family of multidomain proteins having exceptional diversity. They can be broadly divided into two groups, the transmembrane or receptor-like PTPs and the cytosolic PTPs. None of these are related to the serine-threonine specific phosphatases. This is in contrast to the protein kinases (Seer-Thr and Tyr specific), which share a common ancestry. Unlike the Ser-Thr phosphatases, in which substrate specificity is determined by associated targeting subunits, the Tyr phosphatases are all monomeric enzymes. 

Treatment of cells with inhibitors of ser/thr phosphatases 1 and 2A (calyculin A and okadaic acid) but not 2B (cypermethrin) prevent N-formyl peptide—induced moesin dephosphorylation, rear retraction, and cell migration, suggesting that activation of phosphatases is stimulated by N-formyl peptides and calyculin A-sensitive phosphatases play a crucial role in rear release by dephosphorylating ERM. 

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