PURIFICATION AND CHARACTERIZATION OF A RAT-LIVER PROTEIN-TYROSINE PHOSPHATASE WITH SEQUENCE SIMILARITY TO SRC-HOMOLOGY REGION-2

Utilizing three proteins plus tyrosine-glutamate copolymer as substrates, all of which are subjected to (near) stoichiometrical phosphorylation exclusively on tyrosine residues, we partially purified four different protein-tyrosine phosphatases (PTPases) from rat liver cytosol which differed in substrate preference. Of the four PTPases, tentatively termed L1, L2, L3, and L4, PTPase L1 was purified to apparent homogeneity by a procedure involving chromatography on DEAE-cellulose at pH 7.0, Blue Sepharose, DEAE-cellulose at pH 7.6, hydroxyapatite, Phenyl Sepharose, Mono Q, and TSKgel Heparin. PTPase L1 was purified about 7000-fold from the extract and 0.27 mg was isolated from 1000 g liver corresponding to a yield of 13% from the Blue Sepharose step where it had become freed from any other PTPases detectable by our assay procedure. The purified PTPase L1 showed a major protein band of 67 kDa on SDS/PAGE. Catalytically, PTPase L1 had a specific activity of about 6500 nmol P(i) released min-1 mg-1 toward tyrosine-glutamate copolymer phosphorylated on tyrosine residues. PTPase L1 exhibited very low sensitivities to PTPase inhibitors such as zinc acetate, sodium vanadate, and acidic compounds as compared with those of most of the PTPases purified thus far. Amino acid sequence analysis of the purified PTPase L1 revealed a partial peptide sequence showing similarity to the catalytic domain core sequences conserved in the PTPase family. PTPase L1 was most similar to a PTPase termed PTP1C encoded by a human breast carcinoma cDNA but the identity was 55% over 117 residues spanning nearly half of the catalytic domain of PTP1C. The analysis also revealed another partial peptide sequence (113 residues) 70% identical with the sequence corresponding to 68% of two adjacent copies of the src homology region 2 (SH-2 domain) identified in PTP1C. Besides those peptide sequences, PTPase L1 had regional sequences which were 70 - 90% identical with the residues lying between the two SH-2 domains or between the more C-terminal SH2 domain and the catalytic domain of the carcinoma PTPase.