Protein tyrosine phosphorylation receptor activation

FIGURE 8.3 Domain organization of proteins that associate with phosphorylated tyrosine kinase (PTK)-containing receptors. Proteins that associate with tyrosine-phosphorylated receptors contain SH2 or PTB domains, which recognize specific amino-acid stretches in the vicinity of phosphorylated tyrosine residues. Unlike the enzymes, the adaptors lack intrinsic catalytic activity but serve to link phosphorylated receptors with other effector proteins. Some of the proteins presented in this figure are discussed in this chapter. 

MAPK kinase (MAPKK). MAPK kinase itself is activated by phosphorylation by still another protein kinase, termed MAPK kinase kinase (MAPKKK). MAPK kinase kinase is activated upon interaction with a member of the Ras superfamily of small G proteins, which are bound to the plasma membrane (see Ch. 19). The exact mechanism of activation remains unknown, but it is believed that Ras and related proteins, in the activated GTP-bound form, can bind MAPK kinase kinase and thereby draw the kinase to the plasmalemma, where it is activated by as yet unknown factors, perhaps even an additional kinase, MAPK kinase kinase kinase (MAPKKKK). The mechanism governing the activation of Ras and related proteins by extracellular signals is quite complex and involves numerous Tinker proteins, for example She, Grb and Sos, that couple Ras to a variety of plasmalemma-associated growth factor-protein tyrosine kinase receptors (see Chs 20,24 and 27). 

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. 

In comparison to the level of cellular serine or threonine phosphorylation, protein tyrosine phosphorylation occurs at quite low levels in normal cells but dramatically increases upon oncogenic transformation or stimulation. Since the first discovery in 1978 that the transforming protein from Rous sarcoma virus (pp60vsrc) exhibited intrinsic kinase activity/5 protein kinase activity has also been shown to be inherent to other growth factor receptors such as epidermal growth factor receptor and the insulin receptor,[6 91 and to involve autophosphorylation processes. The diverse biochemical activity exhibited by protein tyrosine phosphorylation has stimulated the development of chemical methods for the preparation of phosphorylated peptides for use as substrates in elucidating the biochemical and physiological activity of phosphorylated site(s). 

Bading, H., and Greenberg, M. E. (1991). Stimulation of protein tyrosine phosphorylation by NMDA receptor activation. Science 253, 912-914. 

Similar to the D2 and D4 receptors, the D3 receptor inhibits cAMP accumulation through coupling to G proteins. In addition, the D3 receptor inhibits Ca2+ currents and promotes mitogenesis, probably via tyrosine phosphorylation and activation of mitogen-activated protein kinases (Sokoloff and Schwartz, 2003). 

In this section we review studies performed in our laboratory that strongly suggest that tyrosine kinase activity and protein tyrosine phosphorylation are important mechanisms for regulating receptor-activated increases in [Ca +Jj in smooth muscle. 

Activation of serotonin receptors in canine femoral VSMC evoked tyrosine phosphorylation of a group of substrates similar to those that were tyrosine phosphorylated during stimulation of a -adrenergic receptors with phenylephrine (Fig. 8A). Preincubation of the cells with 110 xM genistein suppressed tyrosine phosphorylation that was evoked by stimulation of either serotonin or adrenergic receptors (Fig. 8A, lanes 4 and 5). The same concentration of genistein that inhibited receptor-activated increases in [Ca +Jj (Fig. 5) also inhibited receptor-activated increases in protein tyrosine phosphorylation. In contrast, preincubation of the VSMC with 110 xM diadzein, a structural analog... 

Di Salvo J Nelson SR, Kaplan N (1997) Protein tyrosine phosphorylation in smooth muscle a potential coupling mechanism between receptor activation and intracellular calcium. Proc Soc Bxp Biol Med 214 285-301 Erdodi F, Ito M, Hartshome DJ. (1996) Myosin light chain phosphatase. In Bdrany M (ed) Biochemistry of smooth muscle contraction. Academic Press, San Diego, pp 13M42... 

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