title: Purification and characterization of a higher-molecular-mass form of protein phosphotyrosine phosphatase (PTP 1B) from placental membranes.
abstract: Purification of a major placental membrane protein phosphotyrosine phosphatase (PTP-I) through the use of a nonhydrolysable phosphotyrosine analogue affinity ligand has enabled identification of the enzyme as a single polypeptide of at least 46 kDa. This phosphatase specifically dephosphorylates phosphotyrosine-containing substrates, including the src peptide, the epidermal-growth-factor receptor tyrosine kinase and the non-receptor tyrosine kinase p56lck. The p56lck can be dephosphorylated by PTP-I at two tyrosine residues (Tyr-394 and Tyr-505), which are differentially phosphorylated in vitro and in vivo and have been suggested to modulate kinase activity. The activity of PTP-I towards these substrates indicates a possible function of regulation of cellular tyrosine phosphorylation pathways at the level of growth-factor receptor and/or oncogene/proto-oncogene tyrosine kinases. Kinetic analyses show that PTP-I exhibits a Km value of about 2 microM with either src peptide or reduced, carboxyamidomethylated and maleylated (RCM)-lysozyme as substrate, and is inhibited in a mixed competitive manner by the polyanions heparin and poly(Glu4,Tyr1). Sequencing of PTP-I peptides reveals almost complete identity with sequences within the N-terminal half of the 37 kDa non-receptor tyrosine phosphatase 1B. However, the size and amino acid composition of PTP-I are similar to that of a higher-molecular-mass form of PTP 1B predicted from cDNA cloning. These results suggest that the 37 kDa PTP 1B is a proteolysed form of PTP-I, and provide evidence that a larger form of PTP 1B exists in vivo, at least in association with placental membranes.
authors.affiliation: Institute of Molecular and Cell Biology, National University of Singapore.
authors.forename: C J
authors.initials: CJ
authors.lastname: Pallen
authors.forename: D S
authors.initials: DS
authors.lastname: Lai
authors.forename: H P
authors.initials: HP
authors.lastname: Chia
authors.forename: I
authors.initials: I
authors.lastname: Boulet
authors.forename: P H
authors.initials: PH
authors.lastname: Tong
mesh_terms: D000595:Amino Acid Sequence; D002462:Cell Membrane / Q000201:enzymology; D002846:Chromatography, Affinity / Q000379:methods; D002850:Chromatography, Gel; D002852:Chromatography, Ion Exchange; D005260:Female; D006801:Humans; D007527:Isoenzymes / Q000302:isolation & purification* / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010455:Peptides / Q000138:chemical synthesis; D010748:Phosphopeptides / Q000302:isolation & purification; D010749:Phosphoprotein Phosphatases / Q000302:isolation & purification* / Q000378:metabolism; D010920:Placenta / Q000201:enzymology*; D011247:Pregnancy; D017027:Protein Tyrosine Phosphatases; D000090063:Proto-Oncogene Mas; D013379:Substrate Specificity
chemical_list: D007527:Isoenzymes; C000715470:MAS1 protein, human; D010455:Peptides; D010748:Phosphopeptides; D000090063:Proto-Oncogene Mas; D010749:Phosphoprotein Phosphatases; D017027:Protein Tyrosine Phosphatases