Protein Tyrosine Phosphatase

Definition
Protein tyrosine phosphatases (PTPases) are the enzymes responsible for the selective dephosphorylation of tyrosine residues.

Discovery
Soon after the discovery of protein tyrosine kinases (PTKs), the presence of PTPases was deduced from experiments using cell lines transformed with temperature-sensitive v-src mutants. At the permissive temperature (36 ºC) the virus-transformed cells rapidly incorporated phosphate into tyrosine residues of proteins. However, when the temperature was raised to (41 ºC ) where the temperature-sensitive v-src kinase was inactive, the level of cellular tyrosine phosphates declined very rapidly to the basal level, presumably due to the activities of cellular PTPases 1. This important observation was followed by numerous attempts to identify and characterize PTPases 2.

Classification
The human genome encodes approximately 100 phosphatases that belong to the protein tyrosine phosphatase (PTP) superfamily, whose substrates range from proteins to phosphoinositides and mRNAs. The hallmark for this superfamily is the active site sequence C(X)5R, also known as the PTP signature motif. The PTPs are key regulatory components in signal transduction pathways and the importance of PTPs in the control of cellular signaling is well established. Based on structure and substrate specificity, the PTP super-family is divided into four distinct subfamilies: 1). pTyr specific PTPs, 2). Dual specificity phosphatases, 3). Cdc25 phosphatases, and 4). LMW PTPs. The PTPs have similar core structures made of a central parallel beta-sheet with flanking alpha-helices containing a beta-loop-alpha loop that encompasses the PTP signature motif

Structural Characteristics
The PTP family of enzymes dephosphorylates target signaling proteins and are involved in the diverse regulation of numerous cell functions. PTPs comprise a large gene-family with the minimal catalytic motif CX5R, where C is the cysteine nucleophile that attacks the phosphate group, R is the arginine residue that binds phosphate and stabilizes the transition state, and X represents any amino acid. A large subgroup of the PTPs are capable of efficient hydrolysis of both phosphotyrosine and phosphothreonine/serine residues and are often referred to as dual-specificity PTPs. Members of the PTP family can be soluble or membrane-associated proteins, as in the receptor-like PTPs. The common feature of these phosphatases appears to be the basic catalytic mechanism involving the formation of a phospho-cysteinyl enzyme intermediate, using the conserved cysteine, arginine, and general acid/base aspartate residue. The catalytic domain of PTPs consists of an a/ß fold composed of a highly twisted core of ß-strands flanked by a-helices. Domains outside of the catalytic fold serve as regulatory and/or targeting modules 4, 5.

Mode of Action
PTKs and their associated signaling pathways are crucial for the regulation of numerous cell functions including growth, mitogenesis, motility, cell-cell interactions, metabolism, gene transcription, and the immune response. Since tyrosine phosphorylation is reversible and dynamic in vivo, the phosphorylation states of proteins are governed by the opposing actions of PTKs and PTPs. In this light, both PTKs and PTPs play equally important roles in signal transduction in eukaryotic cells, and comprehension of mechanisms behind the reversible pTyr-dependent modulation of protein function and cell physiology must necessarily encompass the characterization of PTPs as well as PTKs 4.

Functions
Protein tyrosine phosphatases as targets of the combined insulinomimetic effects of zinc and oxidants: Zinc ions have an insulin-like (insulinomimetic) effect. A particularly sensitive target of zinc ions is protein tyrosine phosphatase 1B (PTP 1B), a key regulator of the phosphorylation state of the insulin receptor. Tyrosine phosphatases seem to be regulated jointly by insulin-induced redox (hydrogen peroxide) signaling, which results in their oxidative inactivation, and by their zinc inhibition after oxidative zinc release from other proteins. In diabetes, the significant oxidative stress and associated changes in zinc metabolism modify the cell’s response and sensitivity to insulin. Zinc deficiency activates stress pathways and may result in a loss of tyrosine phosphatase control, thereby causing insulin resistance 6.

Protein tyrosine phosphatases and the immune response: Reversible tyrosine phosphorylation of proteins is a key regulatory mechanism for numerous important aspects of eukaryotic physiology and is catalyzed by kinases and phosphatases. Together, cells of the immune system express at least half of the 107 PTP genes in the human genome, most of which encode multidomain proteins that contain protein- and phospholipid-interaction domains. Recent evidence show, that even subtle alteration in PTPs can cause immune dysfunction and human disease 7.

References
1. Friis RR, Jockusch BM, Boschek CB, Ziemiecki A, Rübsamen H, Bauer H (1979). Transformation-defective, temperature sensitive mutants of Rous sarcoma virus have a reversibly defective src-gene product. Cold. Spring. Harbor. Symp Quant. Biol., 44:1007-1012.
2. Saito H, Streuli M (1991). Molecular Characterization of Protein Tyrosine. Cell Growth & Differentiation, 2:59-65.
3. Wang WQ, Sun JP, Zhang ZY (2003). An overview of the protein tyrosine phosphatase superfamily. Curr Top Med Chem., 3(7):739-748.
4. Burke TR Jr, Zhang ZY (1998). Protein-tyrosine phosphatases: structure, mechanism, and inhibitor discovery. Biopolymers, 47(3):225-241.
5. Zhang ZY (1998). Protein-tyrosine phosphatases: biological function, structural characteristics, and mechanism of catalysis. Crit. Rev. Biochem. Mol. Biol., 33(1):1-52.
6. Haase H, Maret W (2005). Protein tyrosine phosphatases as targets of the combined insulinomimetic effects of zinc and oxidants. BioMetals, 18:333–338.
7. Mustelin T, Vang T, Bottini N (2005). Protein tyrosine phosphatases and the immune response. Nat. Rev. Immunol., 5(1):43-57.