Molecular cloning and characterization of PTP pi, a novel receptor-like protein-tyrosine phosphatase

Novel cDNAs encoding a receptor-like protein-tyrosine phosphatase (rPTP) have been isolated from human breast tumour cells and foetal brain. The predicted protein of approximately 160 kDa, called PTP pi, comprises an extracellular portion with a MAM (meprin-A5 antigen-PTP mu) domain, an IgG-like domain and four fibronectin III-like repeats, a hydrophobic transmembrane domain and an intracellular portion consisting of two PTP catalytic units. The predicted amino acid sequence shows high identity with those of the two homophilic binding rPTPs, PTP mu and PTP kappa. A variant of PTP pi potentially encoding a protein lacking three amino acids within the N-terminal tyrosine phosphatase domain has been identified. Reverse transcription-PCR has been used to confirm the expression of the variant in human foetal brain tissue. Expression analysis has shown that PTP pi is expressed in a variety of tissue types. Both forms of the N-terminal catalytic domain, the C-terminal catalytic domain and both catalytic domains in tandem were expressed in bacteria as fusion proteins. Intrinsic phosphatase activity was detected for all protein products with an artificial substrate. The fusion protein comprising both domains in tandem was also shown to dephosphorylate purified autophosphorylated epidermal growth factor receptor in vitro.