Receptor autophosphorylation

The receptor autophosphorylates tyrosine residues on the internal portion (p subunit), turning the receptor into active kinase. 

An important question arises about the effects of phospholipid composition and the function of membrane-bound enzymes. The phospholipid composition and cholesterol content in cell membranes of cultured cells can be modified, either by supplementing the medium with specific lipids or by incubation with different types of liposomes. Direct effects of phospholipid structure have been observed on the activity of the Ca2+-ATPase (due to changes in the phosphorylation and nucleotide binding domains) [37]. Evidence of a relationship between lipid structure and membrane functions also comes from studies with the insulin receptor [38]. Lipid alteration had no influence on insulin binding, but modified the kinetics of receptor autophosphorylation. 

Three Trks have been identified, Trk A, B and C. Ligand-binding induces Trk receptor homodimerization. This activates the intrinsic cytoplasmic tyrosine kinase activity, resulting in receptor autophosphorylation and initiation of an intracellular response. A characteristic feature of the Trk family of receptors is the presence of a leucine-rich region near the N-terminal (extracellular) end of the molecule. 

Sunahara GI, Nelson KG, Wong TK, et al. 1987. Decreased human birth weights after in utero exposure to PCBs and PCDFs are associated with decreased placental EGF-stimulated receptor autophosphorylation capacity. Mol Pharmacol 32 572-578. 

The lack of effectiveness of aptamers in some experimental studies, such as in the inhibition of PDGF-induced brain tumor cells in vitro, may be explained by the fact that, in contrast to classical tyrosine kinase inhibitors that enter the tumor cell and inhibit receptor autophosphorylation, the anti-PDGF aptamers do not recognize their binding motif on PDGF when PDGF is bound to its receptor and are therefore less active [73],... 

Half-maximal effects for insulin stimulation of the receptor tyrosyl kinase occurred at around 5-10 nM insulin and, from analysis of insulin specific-binding studies, it appears that there is a non-linear relationship between the stimulation of receptor autophosphorylation by insulin and receptor occupancy. In contrast, a linear relationship has been noted between steady-state insulin binding and the ability of solubilized receptor kinase preparations to phosphorylate various exogenous substrates. The kinetics of coupling between insulin binding and the ability to elicit kinase activation as regards both exogenous substrates and its autophosphorylative capability requires further study. 

In intact cells insulin has been shown to stimulate receptor autophosphorylation. However, much smaller amounts of phosphotyrosine were found compared to that seen using purified, solubilized receptor preparations and, indeed, the predominant phosphorylation actually occurred on serine residues [69,71]. It has been suggested that there may be an insulin-stimulated, phosphoseryl-specific kinase which is loosely associated with the receptor and is activated by the receptor tyrosyl kinase [61]. This might account for the insulin-stimulated phosphoseryl kinase activity observed in both intact cells and when using crude, solubilized receptor preparations. Such an activity might also provide a mechanism for insulin s ability to enhance serine phosphorylation on target proteins in plasma membranes and elsewhere in the cell [25,78]. Nevertheless, (auto)-phosphorylation of the insulin receptor on tyrosine residues has been shown to occur immediately upon receptor occupancy by insulin and to precede any phosphorylation on serine residues [69]. Indeed, it remains to be seen as to whether the seryl phosphorylation occurs as a direct insulin-stimulation event or mediated by other kinases (cAMP/C-kinase) as a consequence of insulin-stimulated autophosphorylation on tyrosyl residues. 

The reduced level of insulin-stimulated receptor autophosphorylation upon tyrosine residues in intact cells compared to that seen using solubilized receptors may also be due in part to the fact that additional sites become available on the solubilized receptors. Alternative explanations are that other membrane components might alter the specificity of the receptor-kinase in situ in the membrane [40] or that the action of other kinases on the receptor may attenuate the autophosphorylative activity. In any event a detailed analysis of the phosphorylation sites on the receptor in both solubilized and intact cell systems is required. 

Guyda HJ. Mathieu L. Lai W. et al. 1990. Benzo(a)pyrene inhibits epidermal growth factor binding and receptor autophosphorylation in human placental cell cultures. Mol Pharmacol 37(2) 137-143. 

McCallum CD, Epand RM. Insulin receptor autophosphorylation and signaling is altered by modulation of membrane physical properties. Biochemistry 1995 34 1815-1824. 

The substrates for neurotrophin Trk receptors-phos-pholipase C-y, PI3-K SHC and Grb2 adaptor proteins-are utilized by many tyrosine kinase receptors. This raises the question of how these phosphorylation events lead to different biological outcomes (Chao, 1992b). There are several possibilities. First, the strength and duration of the receptor autophosphorylation events may determine downstream signalling outcomes. Second, differential signalling may be controlled by specific dephosphorylation events. Third, there may be unique second messengers or substrates which determine the specific nature of the response. 

Apparent inhibition constant against purified kinases. Inhibition of ligand-induced receptor autophosphorylation in the cell lines indicated. 

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