THE GROWTH FACTOR-INDUCIBLE IMMEDIATE-EARLY GENE 3CH134 ENCODES A PROTEIN-TYROSINE-PHOSPHATASE

Stimulation of fibroblasts with serum growth factors results in the ra pid activation of a set of immediate-early genes, among them 3CH134. W e have purified a bacterially expressed form of the 3CH134-encoded pol ypeptide and demonstrated that it has intrinsic protein-tyrosine-phosp hatase (PTPase; protein-tyrosine-phosphate phosphohydrolase, EC 3.1.3. 48) activity in vitro. This activity is optimal at pH 7.5, is sensitiv e to vanadate and cysteinyl modifying agents, and is insensitive to a panel of serine/threonine phosphatase inhibitors. Purified 3CH134 prot ein displays a high degree of selectivity among the tyrosine-phosphory lated polypeptide substrates tested. Under our assay conditions, the r ates of dephosphorylation are in the order EDNDYINASL peptide < myelin basic protein < reduced, carboxyamidomethylated, and maleylated lysoz yme (RCML) < p42mapk. There is a 200-fold range in rates for these sub strates, with p42mapk dephosphorylated 15-fold more rapidly than RCML. Although 3CH134 is most closely related to the tyrosine/serine dual-s pecificity phosphatase VH1, we failed to detect any 3CH134-directed ac tivity on casein or RCML phosphorylated on serine/threonine residues b y cAMP-dependent protein kinase. Since 3CH134 expression is controlled transcriptionally and posttranscriptionally, it may represent a class of PTPases whose activity is regulated at the level of protein synthe sis and degradation.