Receptor protein phosphotyrosine

Receptor Serine/Threonine Receptor Protein Kinase = Receptor Protein Phosphotyrosine Phosphatase = Receptor Tyrosine Kinase = Signal Transducer and Activator of Transcription = Transforming Growth Factor P = Tetradecanoylphorbolacetate = Tetradecanoylphorbolacetate Response Element = Change in Permeability of PM to specific solutes = Change in Transmembrane Potential. 

It is not clear whether V(V) or V(IV) (or both) is the active insulin-mimetic redox state of vanadium. In the body, endogenous reducing agents such as glutathione and ascorbic acid may inhibit the oxidation of V(IV). The mechanism of action of insulin mimetics is unclear. Insulin receptors are membrane-spanning tyrosine-specific protein kinases activated by insulin on the extracellular side to catalyze intracellular protein tyrosine phosphorylation. Vanadates can act as phosphate analogs, and there is evidence for potent inhibition of phosphotyrosine phosphatases (526). Peroxovanadate complexes, for example, can induce autophosphorylation at tyrosine residues and inhibit the insulin-receptor-associated phosphotyrosine phosphatase, and these in turn activate insulin-receptor kinase. 

As outlined above, protein phosphorylation is a key process involved in many signal transduction pathways and reversal of this process is catalyzed by a multiplicity of phosphoprotein phosphatases (PPs). Major PPs catalyzing dephosphorylation of phosphoserine or phosphothreonine residues on proteins include PP1 (inhibited by phosphorylated inhibitor protein I-1 and by okadaic acid and microsystins), PP2 (also inhibited by okadaic acid and microcystins), PP2B or calcineurin (CaM-activated and having a CaM-like regulatory subunit) and PP2C (Mg2+-dependent) [18]. These PPs have been found in all eukaryotes so far examined [18, 19]. In addition, a variety of protein phosphotyrosine phosphatases can reverse the consequences of RTK or JAK/STAT receptor activation [20]. 

Phosphotyrosine phosphatases are integrated just like tyrosine Idnases into signalling pathways. They interact with receptors and have recognition motifs that direct diem to their targets. 5 Protein phosphotyrosine phosphatases downregulate tyrosine phosphorylation and play a role in cellular regulation as important as that of protein tyrosine kinases. 

Fig. 9.9 Model of the function of Grb2-mSos and the adaptor protein She in the Ras pathway. The figure shows a highly simplified version of the two known pathways of involvement of the Grb2-mSos complex in signal transduction via the Ras protein. Phosphotyrosine residues of an activated, auto-phosphorylated receptor R may serve as attachment points forthe PTB domains of the She adaptor protein (a) or forthe SH2 domain of the Grb2-mSos...

Fig. 11.5 Steps involved in cytokine signaling. Binding of a cytokine to the extracelluar ligand binding domain of the cytokine receptor activates the tyrosine kinase activity of the associated tyrosine kinase (e. g. )akl). Tyrosine phosphorylation occurs in trans between neighbouring kinase molecules. The activated protein kinase also catalyzes Tyr phosphorylation ofthe cytoplasmic domain of the receptor. The phosphotyrosine residues serve as attachment points for adaptor proteins or other effector proteins containing phosphotyrosine-binding motifs (PTB or SH2). The signal is then transmitted further into the cytoplasm.

Although many different signal transducer proteins have SH2 domains, and many receptors have phosphotyrosine residues, each signal transducer protein is specific for one type of receptor. This specificity of binding results from the fact that each phosphotyrosine residue has a different amino acid sequence around it that forms the binding domain. Likewise, the SH2 domain of the transducer protein is only part of its binding domain. 

Stimulation of insulin receptor kinase by peroxovanadates (complexes of vanadate and H2O2) presumably represents a mechanism of insulin mimesis different from that of vanadate or vanadyl. ° Low concentrations (5-20 M) of peroxovanadate, but not vanadate, potentiated insulin-stimulated glucose uptake in rat adipocytes, an effect that correlated with an increase in protein phosphotyrosine content. Peroxovanadate inhibited lipolysis, stimulated protein synthesis and lipogenesis, and promoted autophosphorylation and activation of the IR tyrosine kinase, similarly to insulin. Although peroxovanadate, unlike vanadate, increased tyrosine kinase activity in intact cells, neither compound, unlike insulin, had any effect on the kinase activity of partially purified adipocyte IR preparations. ... 

The current preferred postulated mechanism of vanadium s in vitro effects is that vanadate stimulates specific protein-tyrosine phosphorylation by virtue of its inhibitory actions on appropriate PTPases. Routine addition of vanadate to cell lysates, particularly for tyrosine kinase assays, is testimony of the ability of vanadate to preserve the phosphotyrosine content of cells. The preponderance of evidence favors a post-receptor mechanism in stimulating glucose utilization, perhaps also involving a cytosolic i.e. non-receptor) protein tyrosine kinase that is stimulated preferentially by vanadium and may be insulin independent. Vanadyl is not a potent PTPase inhibitor. " Thus, whatever portion of intracellular vanadium is present as vanadyl is probably acting by some alternative mechanism. ... 

EGF to Ras All of the reactions up to Ras are stoichiometric (no amplification). An EGF/receptor complex autoactivates its ovm tyrosine kinase. The receptor s phosphotyrosine then recruits Grb-2, which recruits Sos, which binds and activates Ras. (Downstream from Ras, amplification does occur through a chain of protein phosphorylations [Figure 15.32].)... 

Besides cytoplasmic protein kinases, membrane receptors can exert protein kinase activity. These so-called receptor tyrosine kinases (RTK) contain a ligandbinding extracellular domain, a transmembrane motif, and an intracellular catalytic domain with specificity for tyrosine residues. Upon ligand binding and subsequent receptor oligomerization, the tyrosine residues of the intracellular domain become phosphory-lated by the intrinsic tyrosine kinase activity of the receptor [3, 4]. The phosphotyrosine residues ftmction as docking sites for other proteins that will transmit the signal received by the RTK. 

Protein-protein interaction domain that recognizes short sequences containing a phosphotyrosine. Hydrophobic residues N-terminal to the phosphotyrosine residue provide distinction from SH2 domains. Particularly important in assembling protein complexes at activated receptors. 

The endothelin B receptor is an example of characterization of a homogeneous, affinity purified protein (Roos et al., 1998). Significant progress has been made in the development of techniques for more high-throughput identification of phosphorlyation events. Analysis of large sets of phosphorylated proteins is facilitated by the availability of affinity purification methods such as anti-phosphotyrosine or anti-phosphoserine antibodies or metal affinity chromatography (Neubauer and Mann, 1999 Soskic et al., 1999). These methods are not specific to a particular protein but rather are used to fractionate all proteins that are phosphorylated. 

Dimerization allows the kinase activity of both intracellular chains to encounter target sequences on the other, linked receptor molecule. This enables the intermolecular cross-phosphorylation of several tyrosine residues (Figure 8.2). The phosphorylated dimer then constitutes the active receptor. It possesses an array of phosphotyrosines that enable it to bind proteins to form receptor signaling complexes. Additionally, the dimerized and phosphorylated receptor has the potential of phospho-rylating its targets. 

The catalytic pi 10 subunit has four isoforms, all of which contain a kinase domain and a Ras interaction site. In addition, the a, (3, and y isoforms possess an interaction site for the p85 subunit. The class I enzymes can be further subdivided class IA enzymes interact through their SH2 domains with phosphotyrosines present on either protein tyrosine kinases or to docking proteins such as insulin-receptor substrates (IRSs GAB-1) or linkers for activation of T cells (LATs in the case of T cells). 

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