The 18 kDa cytosolic acid phosphatase from bovine liver has phosphotyrosine phosphatase activity on the autophosphorylated epidermal growth factor receptor

In this paper we demonstrate that the cytosofic low-M_r acid phosphatase purified from bovine liver has phosphotyrosine protein phosphatase acitivity on ³²P-autophosphorylated epidermal growth factor (EGF) receptor. This activity was significantly inhibited by orthovanadate and p-hydroxymercuribenzoate; the latter result indicates that free sulfhydryl groups are required for phosphotyrosine phosphatase activity. The enzyme was active in a broad pH range, with maximum activity between pH 5.5 and 7.5. The apparent K_m for ³²P-EGF receptor dephosphorylation was 4 nM. The enzyme appeared to be specific for phosphotyrosine in that it dephosphorylated the autophosphorylated EGF receptor and L-phosphotyrosine, but not ³²P-Ser-casein, L-phosphoserine or L-phosphothreonine. These data suggest that the cytosolic low-M_r acid phosphatase might play a regulatory role in EGF receptor-dependent transmembrane signalling.