Cytosolic protein tyrosine phosphatases

Carton AJ, Flint AJ, Tonks NK (1996) Identification of pl30(cas) as a substrate for the cytosolic protein tyrosine phosphatase PTP-PEST. Mol Cell Biol 16 6408-6418... 

Aoki N, Matsuda T A cytosolic protein-tyrosine phosphatase PTPIB specifically dephos-phorylates and deactivates prolactin-activated STATSa and STATSb. J Biol Chem 2000 275 39718-39726. 

Vanadate stimulates protein kinases in the cytosol, as demonstrated in adipose cells and extracts. The activation of a membrane and cytosolic protein tyrosine kinase have been demonstrated in adipocytes, and the membranous enzyme has been postulated to be a way to involve PI-3K actions without activation of insulin receptor substrate-1 (IRS-1) in the insulin signal transduction pathway [140], It is always difficult to determine if protein kinase activation is direct or the result of stimulation of a protein phosphatase. The fact that kinase stimulation was seen in isolated extracts after cell disintegration in this adipocyte cell system supports the idea that vanadium addition to cells could directly stimulate kinases via an as-yet-undetermined mechanism. In other experiments with 3T3-L1 adipocytes bis(acetylacetonato)oxovana-dium (IV) BMOV and bis(l-N-oxide-pyridine-2thiolato)oxovanadium (TV) caused increased tyrosine phosphorylation of both the insulin receptor and IRS-1 in a synergistic way with insulin, as measured by antibodies to phosphotyrosine residues [141]. 

The best-studied protein tyrosine phosphatases are the high molecular weight cytoplasmic enzymes of the FTP family. X-ray structures of the human FTP IB cytosolic tyrosine phosphatase, have been solved by David Barford et and that of a Yersinia tyrosine phosphatase by Fauman et In Fig. 3.9a and b the structures of the... 

Many members of this class of receptors have an enzymatic activity known as a protein tyrosine kinase within their cytoplasmic segment. This kinase phosphorylates tyrosine residues in the receptors themselves (autophosphory lation), and in other proteins to initiate biochemical cascades. Phosphorylatipn of tyrosine can be reversed by protein tyrosine phosphatases, which are also present in all cells (Shenolikar and Naim, 1990). Tyrosine phosphatases form a diverse family of proteins, some of which are cytosolic while others are transmembrane molecules analogous to receptors. Some members of the transmembrane class may be involved in the mechanism of bacterial and viral infections (Tonks, 1991). Thus, kinases and phosphatases together act as on-off switches in the a ctivation of receptors and other proteins. 

Not so much is known about the significance of protein tyrosine phosphatases (PTP). hihibition of PTPs (by vanadate) leads to increase in tyrosine phosphorylation, secretion and aggregation (biazuer ail, 1990 Pumigliae/a/., 1992). At least two PTPs appear to be involved in platelet aggregatioa PTPIB is proteolytically cleaved and activated by calpain and released into the cytosol during aggregation. SH-PTPl associates with the... 

Receptor tyrosine kinases (RTKs) are activated (phosphorylated) by inhibition of a negatively regulating phosphatase upon treatment with UV (A, B, or C), hydrogen peroxide, or iodoacetamide. The phosphatase activity, (i.e., dephosphorylation and inactivation of RTKs) is restored upon the addition of thiol-regenerating agents, if not inhibited irreversibly by iodoacetamide [20]. H2O2 not only inactivates membrane-bound phosphatases but also diminishes cytosolic general protein tyrosine phosphatase activity in mouse fibroblasts [21]. Further, the activation of JNK by sodium arsenite, which is reactive towards thiols (especially vicinal dithiols), is by inactivation of a JNK phosphatase [22]. 

Figure 1 Insulin signal transduction cascade (simplified). Intracellular kinases affected by tyrosine phosphorylation activation/deactivation under phosphotyrosine phosphatase (PTPase) regulation (vanadium-inhibitable) include (especially) IRS-I, IRS-2, she, and MAPK. V indicates possible sites of vanadium s mechanism of action. Cytosolic protein tyrosine kinase (CytPTK, not shown) stimulation by phosphatase inhibition is independent of the insulin cascade, but is also multi-step, and is particularly susceptible to vanadyl stimulation...

Protein tyrosine phosphatases. Phosphoprotein phosphatases are integral components of the signahng systems operated by protein kinases (Sun and Tonks, 1994). Cloning data show the protein tyrosine phosphatases (PTPs) to be a family of multidomain proteins having exceptional diversity. They can be broadly divided into two groups, the transmembrane or receptor-like PTPs and the cytosolic PTPs. None of these are related to the serine-threonine specific phosphatases. This is in contrast to the protein kinases (Seer-Thr and Tyr specific), which share a common ancestry. Unlike the Ser-Thr phosphatases, in which substrate specificity is determined by associated targeting subunits, the Tyr phosphatases are all monomeric enzymes. 

Gu, M., Warshawsky, I., and Majerus, P. W. (1992) Cloning and expression of a cytosolic megakaryocyte protein-tyrosine-phosphatase with sequence homology to retinaldehyde-binding protein and yeast SE14p Proc. Natl. Acad Sci f/SA89, 2980-2984... 

The insulin receptor itself is a protein kinase, which phosphorylates susceptible tyrosine residues in proteins. When insulin binds to the external part of the receptor, there is a conformational change in the whole of the protein, resulting in activation of the protein kinase region at the inner face of the membrane. This phosphorylates, and activates, cytosolic protein kinases, which, in turn, phosphorylate target enzymes, including phosphoprotein phosphatase (see Figure 10.6), cAMP phosphodiesterase (see Figure 10.8) and acetyl CoA carboxylase. 

Receptor tyrosine kinases are integral membrane proteins that have a ligand-binding domain on the extracellular side and a tyrosine kinase domain on the cytosolic side (see Fig. 8.1). The transmembrane portion is made up of just one structural element thus it is assumed that it crosses the membrane in an a-helical form. On the cytoplasmic side, in addition to the conserved tyrosine kinase domain, there are also further regulatory sequence portions at which autophosphorylation, and phosphorylation and dephosphorylation by other protein kinases and by protein phosphatases can take place. 

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