Protein tyrosine (pTyr) phosphorylation is a common post-translational modification which can create novel recognition motifs for protein interactions and cellular localisation, affect protein stability, and regulate enzyme activity. Consequently, maintaining an appropriate level of protein tyrosine phosphorylation is essential for many cellular functions. Tyrosine-specific protein phosphatases (PTPase; 3.1.3.48) catalyse the removal of a phosphate group attached to a tyrosine residue, using a cysteinyl-phosphate enzyme intermediate. These enzymes are key regulatory components in signal transduction pathways (such as the MAP kinase pathway) and cell cycle control, and are important in the control of cell growth, proliferation, differentiation and transformation [1, 2]. The PTP superfamily can be divided into four subfamilies [3]:

• (1) pTyr-specific phosphatases
• (2) dual specificity phosphatases (dTyr and dSer/dThr)
• (3) Cdc25 phosphatases (dTyr and/or dThr)
• (4) LMW (low molecular weight) phosphatases

Based on their cellular localisation, PTPases are also classified as:

• Receptor-like, which are transmembrane receptors that contain PTPase domains [4]
• Non-receptor (intracellular) PTPases [5]

All PTPases carry the highly conserved active site motif C(X)5R (PTP signature motif), employ a common catalytic mechanism, and share a similar core structure made of a central parallel beta-sheet with flanking alpha-helices containing a beta-loop-alpha-loop that encompasses the PTP signature motif [6]. Functional diversity between PTPases is endowed by regulatory domains and subunits.

This entry includes proteins of two subfamilies: Ser/Thr (3.1.3.16) and Tyr dual specificity protein phosphatase and tyrosine specific protein phosphatase (3.1.3.48). Both of these subfamilies may also have inactive phosphatase domains, and dependent on the domain composition this loss of catalytic activity has different effects on protein function. Inactive single domain phosphatases can still specifically bind substrates, and protect against dephosphorylation, while the inactive domains of tandem phosphatases can be further subdivided into two classes. Those which bind phosphorylated tyrosine residues may recruit multi-phosphorylated substrates for the adjacent active domains and are more conserved, while the other class have accumulated several variable amino acid substitutions and have a complete loss of tyrosine binding capability. The second class shows a release of evolutionary constraint for the sites around the catalytic centre, which emphasises a difference in function from the first group. There is a region of higher conservation common to both classes, suggesting a regulatory centre [7].

Ser/Thr and Tyr dual specificity phosphatases are a group of enzymes with both Ser/Thr (3.1.3.16) and tyrosine specific proteinphosphatase (3.1.3.48) activity able to remove both the serine/threonine or tyrosine-bound phosphate group from a widerange of phosphoproteins, including a number of enzymes which have been phosphorylated under the action of a kinase. Dual specificity protein phosphatases (DSPs) regulate mitogenic signal transduction and control the cell cycle. Tyrosine specific protein phosphatases catalyze the removal of a phosphate group attached to a tyrosine residue. They are also very important in the control of cell growth, proliferation, differentiation and transformation.

This domain is common to both dual-specificity protein phosphatases and protein-tyrosine phosphatase.

1. Protein tyrosine phosphatases: mechanisms of catalysis and regulation. Cell. Mol. Life Sci. 2, 633-41
2. Receptor and nonreceptor protein tyrosine phosphatases in the nervous system. Protein Sci. 60, 2465-82
3. An overview of the protein tyrosine phosphatase superfamily. Dev. Biol. 3, 739-48
4. The crystal structure of human receptor protein tyrosine phosphatase kappa phosphatase domain 1. null 15, 1500-5
5. The nonreceptor protein tyrosine phosphatase corkscrew functions in multiple receptor tyrosine kinase pathways in Drosophila. null 180, 63-81
6. The structure and mechanism of protein phosphatases: insights into catalysis and regulation. null 27, 133-64
7. Evolution of the multifunctional protein tyrosine phosphatase family. Mol. Biol. Evol. 21, 625-31

Protein tyrosine (pTyr) phosphorylation is a common post-translational modification which can cre
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