Autophosphorylation protein tyrosine kinase

Like all immunoreceptor family members, FceRI lacks intrinsic tyrosine kinase activity. IgE and antigen-induced crosshnking of FceRI initiates a complex series of phosphate transfer events via the activation of non-receptor Src, Syk and Tec family protein tyrosine kinases (fig. 1). The Src family kinase Lyn, which associates with the FceRI p subunit in mast cells, transphosphorylates neighboring FceRI ITAMs after receptor aggregation [7, 26]. Once phosphorylated, the p chain ITAM binds to the SH2 domain of additional Lyn molecules, while the phosphorylated y chain ITAM recruits Syk to the receptor complex, where it is activated by both autophosphorylation and phosphorylation by Lyn [2, 7,15, 26]. 

Most protein serine-threonine kinases undergo autophosphorylation. The autophosphorylation of most protein kinases is associated with an increase in kinase activity [4, 10]. In some instances, such as with the RII subunit of PKA, autophosphorylation represents a positive feedback mechanism for kinase activation, in this case by enhancing the rate of dissociation of the RII and C subunits. In the case of CaMKII, autophosphorylation causes the catalytic activity of the enzyme to become independent of Ca2+ and calmodulin. This means that the enzyme, activated originally in response to elevated cellular Ca2+, remains active after Ca2+ concentrations have returned to baseline. By this mechanism, neurotransmitters that activate CaMKII can produce relatively long-lived alterations in neuronal function. In other instances, such as with the receptor-associated protein tyrosine kinases (discussed in Ch. 24), autophosphorylation is an obligatory step in the sequence of molecular events through which those kinases are activated and produce physiological effects. 

The cellular functions of protein tyrosine phosphatases are closely associated with signal transduction via protein tyrosine kinases. The growth and differentiation promoting signals mediated by protein tyrosine kinases include autophosphorylations and phosphorylation of effector proteins (see 8.1.4). According to current ideas, the activity of protein tyrosine phosphatases may have a negative or positive influence on signal transduction via protein tyrosine kinases. On the one hand, protein tyrosine... 

Phospholipases of type Cy are activated by receptor tyrosine kinases (see Chapter 8), and thus phospholipase Cy is involved in growth factor-controlled signal transduction pathways. The receptor tyrosine kinases (see Chapter 8) phosphorylate the enzyme at specific tyrosine residues and initiate activation of the enzyme. This activation mobilizes internal calcium stores and engages multiple protein kinase pathways. Characteristic for the structure of phospholipase Cy is the occurrence of SH2 and SH3 domains (see Chapter 8). These represent protein modules that serve to attach upstream and downstream partner proteins. The SH2 domains mediate binding to Tyr-phos-phates of the activated, autophosphorylated receptor tyrosine kinase. During this process, the phospholipase Cy enzymes are phosphorylated onTyr-residues and are thereby activated. 

Typically, the activated protein tyrosine kinases of growth factor receptors catalyze the autophosphorylation of a number of tyrosine residues on the cytoplasmic domains of the adjacent receptor molecule. These phosphorylated residues provide docking sites for both effector and adaptor proteins that mediate downstream signaling pathways. 

Structural studies of some growth factor receptor protein tyrosine kinases have shown that the autophosphorylation occurs at tyrosine residues in an activation loop of the polypeptide chain. When this loop is not phosphorylated, it restricts access of ATP to the active site of the kinase. Upon phosphorylation it moves, so as to allow ATP ready access to the site, leading to the generation of a high-activity form of the enzyme. 

A case in point is the structure determination of the insulin receptor substrate-1 (IRS-1). Insulin binds to a membrane-bound receptor that is a ligand-activated protein tyrosine kinase. Upon insulin binding there is an autophosphorylation of several tyrosine residues on the cytosolic side of the receptor. This enhances the tyrosine kinase activity of the insulin receptor towards other substrates and is required for signal transduction. A cascade of events is initiated, the first of which is the phosphorylation of IRS-1. This occurs when IRS-1 binds to the insulin receptor via a specific domain of the protein that is termed the phosphotyrosine binding (PTB) domain. 

Insulin Receptor. Figure 1 Structure and function of the insulin receptor. Binding of insulin to the a-subunits (yellow) leads to activation of the intracellular tyrosine kinase ((3-subunit) by autophosphorylation. The insulin receptor substrates (IRS) bind via a phospho-tyrosine binding domain to phosphorylated tyrosine residues in the juxtamembrane domain of the (3-subunit. The receptor tyrosine kinase then phosphorylates specific tyrosine motifs (YMxM) within the IRS. These tyrosine phosphorylated motifs serve as docking sites for some adaptor proteins with SRC homology 2 (SH2) domains like the regulatory subunit of PI 3-kinase. 

Seger, R., Ahn, N. G., Boulton, T. G., Yanopoulos, G., Panayotatos, N., Radzziejewska, E., Ericsson, L., Bratlien, R. L., Cobb, M. H., and Krebs, E. G. (1991). Microtubule-associated protein 2 kinases, ERK1 and ERK2, undergo autophosphorylation on both tyrosine and threonine residues implications for their mechanism of activation. Proc. Natl. Acad. Sci. USASS 6142-6146. 

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