Mitotic Activation of Protein-tyrosine Phosphatase α and Regulation of Its Src-mediated Transforming Activity by Its Sites of Protein Kinase C Phosphorylation

Abstract

During mitosis, the catalytic activity of protein-tyrosine phosphatase (PTP) ␣ is enhanced, and its inhibitory binding to Grb2, which specifically blocks Src dephosphorylation, is decreased. These effects act synergistically to activate Src in mitosis. We show here that these effects are abrogated by mutation of Ser 180 Protein-tyrosine phosphatase (PTP)1 ␣ is an ϳ130-kDa transmembrane PTP (1, 2) that activates the cytoplasmic membrane-bound Src protein-tyrosine kinase by dephosphorylating Src Tyr(P) 527 (Refs. 3 and 4; see Ref. 5 for review). This releases Src from its negatively regulated conformation in which Tyr (P) 527 is bound intramolecularly to the Src SH2 domain (see Refs. 6 and 7 for review). Overexpression of PTP␣ results in dephosphorylation of Tyr(P) 527 and activation of Src in vivo (3, 4). Conversely, Src Tyr(P) 527 phosphorylation is higher and Src catalytic activity is about three times lower in cells from PTP␣ Ϫ/Ϫ knockout mice (8, 9) or following antisense-induced PTP␣ down-regulation (10), indicating that PTP␣ is a major physiological positive regulator of Src. Constitutive activation of the Src proto-oncoprotein by mutation increases the tyrosine phosphorylation of multiple signal transduction proteins and thereby neoplastically transforms a variety of cell types (see Ref. 7 for review). The fact that activation by overexpressed PTP␣ also causes transformation (3) is perhaps more surprising and suggests that PTP␣ activity is directed in vivo preferentially to Src (and Src family members), rather than to Src substrates.This substrate specificity is due, at least in part, to a phosphotyrosine displacement mechanism that selectively promotes dephosphorylation of Src by PTP␣: ϳ20% of PTP␣ in NIH3T3 cells is phosphorylated at Tyr 789 , a residue near its carboxyl terminus (11, 12). Tyr 789 phosphorylation does not affect PTP␣ dephosphorylation of nonspecific substrates such as myelin basic protein (MBP), whose phosphotyrosines are not bound, but is required for dephosphorylation of Src Tyr(P) 527 , which is protected against many phosphatases by its SH2 domain binding (13). Phosphorylated Tyr 789 can bind to the Src SH2 domain, thereby displacing and thus unprotecting Src Tyr(P) 527 . This also forms a transient bound state that additionally facilitates Tyr(P) 527 dephosphorylation (13). Tyr (P) 789 also binds the SH2 domain of the adapter protein Grb2 (11, 12), which participates in Ras activation following peptide growth factor stimulation (see Ref. 14 for review). Because of steric hindrance resulting from the interaction of one of the Grb2 SH3 domains with PTP␣, PTP␣-bound Grb2 is not able to bind Sos, the downstream protein in the Grb2-Ras signal transduction pathway. Thus, it does not appear that localization of Grb2 to the plasma membrane by binding to PTP␣ can activate the Ras signaling pathway (15, 16). Instead, control may flow in the other direction: Grb2 binding to Tyr(P) 789 blocks phosphotyrosine displacement and the ability of PTP␣ to dephosphorylate Src, so only Grb2-unbo...

Protein Kinase Cδ Induces Src Kinase Activity via Activation of the Protein Tyrosine Phosphatase PTPα

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The uncovering of a novel regulatory mechanism for PLD2: Formation of a ternary complex with protein tyrosine phosphatase PTP1B and growth factor receptor-bound protein GRB2

“…Presumably, this kinase could in turn activate the PTP1b in a signaling complex. There are many examples where a protein tyrosine phosphatase regulates kinase function [37][38][39][40][41][42][43][44][45][46].…”

Replication checkpoint control by a PTK/STAT3/Cyclin D1 axis

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