Phosphorylation in signal transduction

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... 

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. 

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. 

GM-CSF and IL-3 have been shown to compete for receptors in some types of cells (e.g. eosinophils and KG-1 cells), indicating some structural homology between GM-CSF and IL-3 receptors, perhaps because they share certain subunits or adapter proteins. GM-CSF occupancy results in phosphorylation of certain proteins, and because the receptor possesses no inherent kinase activity, receptor occupancy must be linked to kinase activity via the generation of second messenger molecules. Pretreatment of cells with pertussis toxin abolishes the effects of GM-CSF, indicating the involvement of G-proteins in signal transduction. Priming of neutrophil functions with GM-CSF involves the activation of phospholipases A2 and D. 

Methylation plays an important role in transcriptional regulation and a lesser role in signal transduction. " Histones are heavily methylated proteins. Single, double, or triple methylated lysines play an important role on histones. Lysine methylation is a more subtle transcriptional control than acetylation. Lysine methylation has come to light in another protein known as p53. p53 is a protein expressed in low levels in the cell and stabilized by posttranslational modifications including phosphorylation, acetylation, and now N-methylation. There are several C-terminal lysines on p53 that increase its stability. The addition of the methylation modifications adds complexity to p5 3 and fine-tunes its activity and ultimately suppresses tumor formation. ... 

Smooth muscle effects. The opposing effects on smooth muscle (A) of a-and p-adrenoceptor activation are due to differences in signal transduction (p. 66). This is exemplified by vascular smooth muscle (A). ai-Receptor stimulation leads to intracellular release of Ca + via activation of the inositol tris-phosphate (IP3) pathway. In concert with the protein calmodulin, Ca + can activate myosin kinase, leading to a rise in tonus via phosphorylation of the contractile protein myosin. cAMP inhibits activation of myosin kinase. Via the former effector pathway, stimulation of a-receptors results in vasoconstriction via the latter, P2-receptors mediate vasodilation, particularly in skeletal muscle - an effect that has little therapeutic use. 

Src is the prototype of the superfamily of protein tyrosine kinases and was one of the first protein kinases to be characterized by various genetic, cellular, and structure-function studies to help imderstand its role in signal transduction pathways as well as in disease processes, including cancer, osteoporosis, and both tumor- and inflammation-mediated bone loss [28-38]. In fact, studies on Src provided some of the first evidence correlating protein kinase activity and substrate protein phosphorylation in the regulation of signal transduction pathways relative to normal cellular activity as well as mahgnant transformations. Src family kinases include Fyn, Yes, Yrk, Blk, Fgr, Hck, Lyn,... 

The final step in signal transduction is the action of cAMP on the regulatory subunit of the enzyme, protein kinase A. This ubiquitous enzyme then phosphorylates and activates enzymes with functions specific to different cells and organs. In fat cells, protein kinase A activates lipase, which mobilizes fatty acids in muscle and liver cells, it regulates glycogenolysis and glycogen synthesis. 

The phosphorylation of enzymes by specific protein kinases is a widespread mechanism for the regulation of enzyme activity. It represents a flexible and reversible means of regulation and plays a central role in signal transduction chains in eucaryotes. 

The extent of tyrosine phosphorylation of signal proteins is determined both by the activity of the tyrosine kinases and also the activity of tyrosine-specific protein phosphatases. If the total activity of both enzymes in the cell is considered, it is found that there is a preponderance of protein tyrosine phosphatase activity compared to tyrosine kinase activity. In contrast, the activities of the Ser/Thr-specific protein kinases and protein phosphatases are approximately balanced. It is estimated that the activity of the protein tyrosine phosphatases is about 3-4 orders of magnitude higher than the activity of the protein tyrosine kinases. With this relationship between the activities, it is not surprising that the net level of tyrosine phosphorylation in the cell is very low and that tyrosine phosphorylation is often only transient. Consequently, it took a relatively long time until the importance of tyrosine phosphorylation for signal transduction was assessed correctly. 

Fig. 11.6. Model of activation of Jak kinases. The Jak kinases (Jakl and Jak2 are shown as examples here) are attributed a two-fold function in signal transduction via cytokine receptors. On binding to the activated cytokine receptor, the Jak kinases are activated and phosphorylation of the Jak kinases takes place, probably by a trans mechanism (dashed arrow). The Jak kinases also catalyze Tyr phosphorylation of the cytoplasmic domain of the receptor (solid arrow). The phosphotyrosine residues serve as attachment points for adaptor proteins or other effector proteins.

Oncogenes code for oncoproteins, which are mostly involved in signal transduction controlling/regulating cell growth, differentiation, or apoptosis. These pathways often involve modification of proteins by phosphorylation (e.g., of serine, tyrosine, threonine) by kinases. 

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