A common event in cellular signal transduction pathways is the phosphorylation of proteins on tyrosine residues. Tyrosine phosphorylation is reversible. The forward reaction is mediated by protein tyrosine kinases. By contrast, the reverse reaction is performed by protein tyrosine phosphatases (PTP). The PTP family consists of 107 genes whose protein products are diverse in form and specificity (Alonso et al. 2004). PTPN3 and PTPN4 constitute two members of this family that were initially identified by PCR amplification using primers specific to conserved regions of the catalytic domain of canonical PTP. PTPN3 was initially cloned from a HeLa cell cDNA library, whereas PTPN4 was cloned from a megakaryoblastic cell line (Gu et al. 1991; Yang and Tonks 1991). PTPN3 and PTPN4 are 50% identical and 67% homologous at the amino acid level.
Structure and Expression Studies
PTPN3 and PTPN4 are cytosolic proteins that localize to the plasma...
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