Identification of a rat liver protein-tyrosine phosphatase similar to human placental PTPase-lB using quantitatively phosphorylated protein substrates

Erythrocyte Band 3 protein (Band 3), brain microtubule associated protein 2 (MAP2), and tubulin were phosphorylated to high stoichiometries (1-6 mol P₁/mol protein) on tyrosine residues using a rat spleen protein-tyrosine kinase in the presence of polylysine. After total removal of polylysine, the quantitatively phosphorylated proteins as well as tyrosineglutamate copolymer [Poly(Glu₄, Tyr₁)], which was also phosphorylated (1.5 mol/mol) by the kinase, were employed to assay rat liver protein-tyrosine phosphatases (PTPases). Of the four partially purified PTPases termed LI, L2, L3, and L4, PTPase LI was previously purified to homogeneity and demonstrated to be a novel enzyme with sequence similarity to src-homology region 2 [Hiraga, A. et al. (1992) Eur. J. Biochem. 209, 195-206]. In the present work PTPase L2 was purified to near homogeneity by a procedure involving chromatography on DEAE-cellulose, Blue Sepharose CL-6B, hydroxylapatite, Phenyl Sepharose CL-4B, and TSKgel Heparin-5PW. PTPase L2 was purified 20,000-fold with a recovery of 0.9% from the extract and 0.005 mg was isolated from 300 g of liver. The highly purified PTPase L2 showed a major protein band of 36 kDa on SDS/polyacrylamide gel electrophoresis. PTPase L2 had a specific activity of about 6,000 nmol of P₁ released min^-1.mg^-1 toward either Band 3 or poly(Glu₄, Tyr₁), the values being within the range of those obtained for PTPases purified thus far. PTPase L2 dephosphorylated Band-3 9-fold and 5-fold faster than tubulin and MAP2, respectively, under the assay conditions employed. PTPase L2 was highly sensitive to PTPase inhibitors such as zinc acetate, sodium vanadate, and non-phosphorylated poly(Glu₄, Tyr₁) as compared with rat liver PTPase LI and L3. PTPase L2 was most similar to human placental PTPase IB in molecular mass and sensitivity to the three inhibitors but appeared to be significantly different in sensitivity to heparin and affinity for DEAE-cellulose from the placental enzyme. 1993