Tyrosine Phosphatase Proteins


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 Tyrosine Phosphatase Proteins Background

Protein Tyrosine Phosphatases

The phosphorylation of proteins on tyrosine residues plays important roles in the regulation of cellular processes including cell migration, differentiation, proliferation, growth, metabolism, gene transcription, cell cycle, cell-cell communication, response to environmental changes, and cell survival via signal transduction pathways. Modification of proteins by phosphorylation of tyrosine residues by tyrosine-specific kinases was not discovered until 1979, when researchers working with the tumor virus proteins middle T antigen from polyomavirus, v-Src protein from Rous sarcoma virus, and Abl protein from the Abelson leukemia virus, found these proteins had the ability to phosphorylate tyrosine residues in vitro. These findings suggested that tyrosine phosphorylation of substrate proteins could be involved in morphological changes and unregulated growth of tumor cells. Tyrosine phosphatases were discovered somewhat later: one of the first protein tyrosine phosphatases found, PTP1B, was first purified and sequenced in 1988.

Tyrosine phosphorylation of proteins is mediated by the balanced action of protein tyrosine kinases and protein tyrosine phosphatases; this reversible process of protein phosphorylation is highly regulated and requires both a protein kinase and a protein phosphatase to work as a team. Resting cells typically have low levels of protein phosphotyrosines. The pattern of cellular proteins with phosphorylated tyrosine residues results from the combined action of these kinases and phosphatases. Theoretically, phosphorylation at a particular tyrosine residue can be regulated by changing the activity of its kinase or phosphatase, or both. The disregulation of phosphorylation control can disrupt cellular processes, resulting in pathological situations, such as cancer, if the kinase or phosphatase is mutated or anomalously expressed, resulting in abnormal phosphorylation patterns of their substrates. Many dominant oncogenes are mutated versions of protein tyrosine kinases or phosphatases. One of the best known of these is the proto-oncogene kinase Src, originally identified in its oncogenic form, v-Src, which could form tumors in birds infected with the Rous sarcoma virus. v-Src is missing its C-terminal segment, which contains a negatively regulatory tyrosine residue that when phosphorylated, puts the enzyme in an inactive state. The v-Src can no longer be switched off, so its action can no longer be regulated. An example of a disregulated phosphatase causing disease is the over-expression of PRL3 in metastatic colon cancer.

The Protein Tyrosine Phosphatase Superfamily

Protein tyrosine phosphatases make up a large family of enzymes that are vital in regulating other proteins in signal transduction pathways by catalyzing the removal of a phosphate group (dephosphorylation) from a tyrosine residue. All protein tyrosine phosphatases (PTPs) contain the conserved motif CX5R, comprising the enzyme active site. The active site, also called the PTP loop or P-loop, binds the phosphate group of the phosphotyrosine.

The PTP superfamily is divided into three class types based on the substrate specificities and structures of the members; they all share the same catalytic mechanism that depends upon the formation of a thiol-phosphate intermediate and general acid-general base catalysis, which requires there be a conserved cysteine residue in the active site. However, beyond the conservation of the active site, these three class types are only distantly related and have little sequence similarity.

Type I PTPs includes the dual-specificity phosphatases, which dephosphorylate phosphoserine and phosphothreonine residues in addition to phosphotyrosine residues, and also includes the classical PTPs, which are subdivided into a non-receptor cytosolic group and a receptor-like group whose members have a transmembrane domain. Each of these is further divided into subgroups. Subgroups in the dual-specificity protein phosphatase group include MAPK phosphatases, Slingshots, Cdc14s, PTENs, myotubularins, and atypical dual-specificity phosphatases. Subgroups in the classical protein tyrosine phosphatase non-receptor group include PTP1B, SHPs, TC-PTP, PTP-PEST, and PTPL1. Subgroups in the classical receptor-like group (few ligands have been yet identified) include CD45, LAR, PTP , RPTP , and RPTP; most members of this group contain one or two intracellular catalytic domains, a single transmembrane domain, and a variable extracellular ligand-binding domain. The Type III PTP class contains only the Cdc25 protein tyrosine phosphatases: Cdc25A, Cdc25B, and Cdc25C. The Type II PTP class also contains only one member: the low molecular weight protein tyrosine phosphatases (LMW-PTPs).

Substrate Catalysis by Protein Tyrosine Phosphatases

All the three-dimensional structures that have been solved for the PTP catalytic domains share the same features, even though the phosphatase signature motif is localized at different positions in PTP proteins. This is because all protein tyrosine phosphatases share a common mechanism to catalyze the removal of the phosphoryl group from their phosphotyrosine substrates. Three key structural features are conserved: the catalytic cysteine and arginine in the active site, along with the general acid/base containing surface loop, which contains the conserved aspartic acid residue. The common catalytic strategy involves a two-step mechanism: first, the thiol side chain of the invariant active site catalytic cysteine serves as a nucleophile and attacks the phosphoryl group from the substrate, forming a thiol-phosphate intermediate. The invariant active site arginine residue forms hydrogen bonds with the substrate phosphoryl group via its guanidinium group, and stabilizes the transition state during hydrolysis. Binding of substrate promotes conformational change of the general acid/base containing surface loop, which brings the conserved aspartic acid residue into the active site, positioning its carboxyl group close to the leaving group oxygen of the substrate, allowing for proton transfer. The aspartic acid first acts as a general acid by providing a proton to the oxygen of the tyrosine residue leaving group, enhancing formation of the enzyme-product complex. In the second step, the aspartic acid acts as a general base by accepting a proton from water, and allows the hydrolysis of the enzyme-product complex, releasing inorganic phosphate and returning the enzyme to its resting state. It is unusual to have a phosphocysteine intermediate in a phosphatase-catalyzed hydrolysis reaction; in the same reaction catalyzed by serine/threonine phosphatases, an activated water molecule attacks the phosphorus substrate atom. The enzyme may have evolved to form a cysteine phosphate intermediate in catalysis because the bond energy, 45- 50 kcal/mol, of the P-S bond is less than the 95-100 kcal/mol of the P-O bond, making the P-S bond easier to cleave.