Characterization of a membrane‐associated phosphotyrosyl protein phosphatase from the A431 human epidermoid carcinoma cell line

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Abstract
The A431 human epidermoid carcinoma cell line exhibits a 30‐100‐fold overexpression of the epidermal growth factor (EGF) receptor. We have characterized a membrane‐associated phosphotyrosyl‐protein phosphatase (PTPase) in these cells since it seemed reasonable that overexpression of the EGF‐receptor tyrosine kinase will be matched by high PTPase activity. Indeed, of 12 cell lines tested, the A431 cells had the highest specific PTPase activity. About 70% of the total cellular PTPase activity was found associated with membranes after cell fractionation. The membrane‐associated PTPase was hydrophobic as judged by its behaviour in Triton X‐114 phase partitioning. High‐performance liquid chromatography (HPLC) on a DEAE column revealed a single, homogeneous species of membrane‐associated PTPase with an apparent molecular mass of 43 kDa as determined by HPLC on a gel permeation column in the presence of Triton X‐100. Comparison of this PTPase with the membrane‐associated PTPase activities present in rat spleen and in the human chronic myelogenous leukemia cell line K562 revealed additional species resolvable by DEAE‐HPLC. These findings suggest that cells may possess different PTPase activites depending on their growth and differentiation states.

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