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; EC 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 [ (PUBMED:9818190) (PUBMED:14625689) ]. The PTP superfamily can be divided into four subfamilies [ (PUBMED:12678841) ]:

• (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 [ (PUBMED:16672235) ]
• Non-receptor (intracellular) PTPases [ (PUBMED:8948575) ]

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 [ (PUBMED:9646865) ]. Functional diversity between PTPases is endowed by regulatory domains and subunits.

This entry repesents the PTP-signature motif that characterises the catalytic site, and which encompasses only part of the PTPase domain structure.