Phosphorylation, in signaling

Pawson, T. and Scott, J. D. (2005) Protein phosphorylation in signaling—50 years and counting. Trends in Biochemical Sciences, 30, 286-290. 

Many hormones, such as epinephrine (adrenaline), alter the activities of enzymes by stimulating the phosphorylation of the hydroxyl amino acids serine and threonine phosphoserine and phosphothreonine are the most ubiquitous modified amino acids in proteins. Growth factors such as insulin act by triggering the phosphorylation of the hydroxyl group of tyrosine residues to form phosphotyrosine. The phosphoryl groups on these three modified amino acids are readily removed thus they are able to act as reversible switches in regulating cellular processes. The roles of phosphorylation in signal transduction will be discussed extensively in Chapter 14. 

B.N. Kholodenko, G.C. Brown, J.B. Hoek, Diffusion control of protein phosphorylation in signal transduction pathways, Biochem. J. 2000, 350, 901-907. 

Bourret, R.B., Hess, J.F. and Simon, M.I. (1990). Conserved aspartate residues and phosphorylation in signal transduction by the chemotaxis protein CheY. Proc. Natl. Acad. Sd. U.S.A. 87, 41-45. 

Rothman, D.M., Shults, M.D., and Imperial , B. (2005) Chemical approaches for investigating phosphorylation in signal transduction networks. Trends Cell... 

The abundance of many protein kinases in cells is an indication of the great importance of protein phosphorylation in cellular regulation. Exactly 113 protein kinase genes have been recognized in yeast, and it is estimated that the human genome encodes more than 1000 different protein kinases. Tyrosine kinases (protein kinases that phosphorylate Tyr residues) occur only in multicellular organisms (yeast has no tyrosine kinases). Tyrosine kinases are components of signaling pathways involved in cell-cell communication (see Chapter 34). 

Noncatalytic phosphotyrosine binding (PTB) domains are 100-150 residue modules, which bind Asn-Pro-X-Tyr motifs. PTB-domain binding specificity is determined by residues at the amino-terminal side of the phosphotyrosine. In most cases, the tyrosine residue must be phosphorylated in order to mediate binding. PTB domain containing proteins are often found in signal transduction pathways. 

When considering the role of phosphorylation in the regulation of the HS response, it is indeed curious that oxidative stress and heat induce a protein tyrosine phosphatase at the transcriptional level (Keyse and Emslie, 1992). Whether this phosphatase has any role in the regulation of HSF phosphorylation is not known, but it does indicate that both transcriptional and translational regulation of signaling... 

The phosphorylation and dephosphorylation of seryl, threonyl, and tyrosyl residues regulate the activity of certain enzymes of lipid and carbohydrate metabolism and the properties of proteins that participate in signal transduction cascades. 

All RTKs contain between one and three tyrosines in the kinase activation loop, which is composed of subdomains VII and VIII of the protein kinase catalytic core. Phosphorylation of these tyrosines has been shown to be critical for stimulation of catalytic activity and biological function for a number of RTKs, including insulin receptor, FGF receptor, VEGF receptor, PDGF receptor, Met (hepatocyte growth factor receptor), and TrkA (NGF receptor). A major exception is the EGF receptor, for which autophosphorylation of a conserved tyrosine in the activation loop does not seem to be involved in signaling. Substitution of tyrosine with phenylalanine has no effect on RTK activity or downstream signals. 

FIGURE 8.8 Mechanism of activation of protein kinase B (PKB). PI3-kinase is recruited to the membrane via direct association with the receptor PTK or via association with the docking protein Gab-1. It catalyzes the generation of phosphatidyl-3,4,5-inositolphosphate, which serves as a membrane-recruitment signal for PKB. Associated with the membrane, it is first phosphorylated in its catalytic domain by PDK1 and then by PDK2 in the hydrophobic motif. The activated PKB then detaches from the membrane. 

Two limitations of this approach are that (1) it requires both that the phosphorylation site(s) have been identified and that a truly phosphospecific antibody has been generated and (2) the method gives no information on the absolute levels of phosphorylation a ten-fold increase in signal might reflect a change from 1 to 10% phosphorylation, or from 10 to 100%. Such information is important in interpreting the likely impact of the change on the process of translation. 

Lee, J., Owens, J.T., Hwang, I., Meares, C., and Kustu, S. (2000) Phosphorylation-induced signal propagation in the response regulator NtrC. J. Bacteriol. 182, 5188-5195. 

Horrocks, L. A. and Farooqui, A. A. NMDA receptor-stimulated release of arachidonic acid mechanisms for the Bazan effect. In Municio, A. M. and Miras-Portugal, M. T. (eds), Cell Signal Transduction, Second Messengers, and Protein Phosphorylation in Health and Disease. New York Plenum Press, 1994, pp. 113-128. 

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