Threonine dephosphorylation

The last part of this account will be devoted to protein kinases and protein phosphatases and some recent results we have obtained for them. Protein kinases and phosphatases are signaling biomolecules that control the level of phosphorylation and dephosphorylation of tyrosine, serine or threonine residues in other proteins, and by this means regulate a variety of fundamental cellular processes including cell growth and proliferation, cell cycle and cytoskeletal integrity. 

Phosphorylation is the reversible process of introducing a phosphate group onto a protein. Phosphorylation occurs on the hydroxyamino acids serine and threonine or on tyrosine residues targeted by Ser/Thr kinases and tyrosine kinases respectively. Dephosphorylation is catalyzed by phosphatases. Phosphorylation is a key mechanism for rapid posttranslational modulation of protein function. It is widely exploited in cellular processes to control various aspects of cell signaling, cell proliferation, cell differentiation, cell survival, cell metabolism, cell motility, and gene transcription. 

PTEN is a phosphatase, which is a product of a tumor suppressor gene. This phosphatase has an unusual broad specificity and can remove phosphate groups attached to serine, threonine, and tyrosine residues. It is believed that its ability to dephosphorylate phosphati-dylinositol (PI) 3,4,5-triphosphate, the product of PI-3 kinase, is responsible for its tumor suppressor effects. 

This intermediate MAPK activator (MAPK kinase, MAPKK) is a 45 kDa phosphoprotein capable of phosphorylating MAPK on serine/threonine and tyrosine residues (Matsuda et al., 1992 Nakielny et al., 1992a Kosako et al., 1993). Like MAPK, the activity of MAPKK is regulated by phosphorylation. During oocyte maturation MAPKK is phosphorylated on threonine residues (Kosako et al., 1992), and this phosphorylation is required for its activity (Ahn et al., 1991 Gomez and Cohen, 1991 Kosako et al., 1992 Matsuda et al., 1992). MPF can activate both MAPKK and MAPK in vitro, with the activation of MAPK lagging behind that of MAPKK however, MPF cannot activate either purified MAPKK or MAPK that has been dephosphorylated by phosphatases (Matsuda et al., 1992). MAPKK and MAPK are therefore believed to function downstream of MPF (Fig. 3). 

Lorca, T., Labbe, J-C., Devault, A., Fesquet, Capony, J-P., Cavadore, J-C., Le Bouffant, F., and Doree, M. (1991). Dephosphorylation of cdc2 on threonine is required for cdc2 kinase inactivation and normal anaphase. EMBO J. 11 2381-2390. 

Sebastian, B., Kakizuka, A., and Hunter, T. (1993). Cdc2M2 activation of cyclin-dependent kinase by dephosphorylation of threonine-14 and tyrosine-15 Proc. Natl. Acad. Sci. USA 90 3521-3524. 

Protein dephosphorylation is catalyzed by phospho-hydrolases called protein phosphatases. While the number of protein tyrosine kinases is roughly comparable to the number of protein tyrosine phosphatases, protein serine-threonine kinases vastly outnumber the protein serine-threonine phosphatases, of which about 25 different species are known to exist. This relative under-representation may be accounted for by the alternative diversification... 

Phosphorylation of serine, threonine, or tyrosine residues by protein kinases, and their dephosphorylation by protein phosphatases, are critical mechanisms by which information-relaying signals are transduced in eukaryotic cells. Although protein kinases are by no means an eukaryotic invention (see Leonard et al., 1998 for details), the large numbers of protein kinases in eukaryotes (118 in. S . cerevisiae and 435 in C. elegans (Chervitz et al., 1998)) reflect their importance in a multitude of diverse cellular processes. Eukaryotes have evolved signaling pathways that exploit the dual state of an amino acid, dependent on its state of phosphorylation, both as a signaling mechanism and as a means of colocalization of molecules within multimolecular complexes. 

The switch in the action of the enzyme between its kinase and phosphatase activities is brought about by phosphorylation mediated by the serine/threonine protein kinase A (PKA), the same cAMP dependent enzyme which plays a role in the control of glycogen metabolism. In its kinase form, PFK-2 is dephosphorylated but phosphorylated in the phosphatase form. 

Ohtake K, Maeno T, Ueda H, Ogihara M, Natsume H, Morimoto Y (2003) Poly-L-arginine enhances paracellular permeability via serine/threonine phosphorylation of ZO-1 and tyrosine dephosphorylation of occludin in rabbit nasal epithelium. Pharm Res 20 1838-1845. 

Phosphorylation also plays an evident part in cell division. Specifically, cytoskeleton-associated protein 2 (CKAP2) has a phosphorylation site at threonine 596. This threonine has been tracked and gets phosphorylated between prophase and metaphase, and dephosphorylated a short time later before anaphase, suggesting that the protein is phosphorylated during the formation of mitotic spindles. ... 

The most common form of such modifications is phosphorylation and dephosphorylation of the hydroxyl group of a serine, threonine or tyrosine amino acid in the protein. The reactions are catalysed by kinases (protein kinases) or phosphatases (protein phosphatases), respectively. 

Phosphorylation of serine, threonine, and tyrosine side-chain OH groups of proteins by kinases and their dephosphorylation by protein phosphatases provides an important mechanism for biological regulation. Tyrosine phosphatases are not metalloenzymes but the serine/threonine phosphatases contain a bimetallic site. 

Kinase inhibitors such as H7, which inhibits serine threonine kinases, resulted in sustained JAK2/STAT activation, suggesting that besides protein dephosphorylation, protein phosphorylation is also required. Furthermore, both inhibition of protein synthesis with cycloheximide and inhibition of... 

Our results demonstrated that the identified subsets of the activated protein kinases significantly increased the accuracy of clinical outcome predictions. Most notably in the study, we evaluated protein phosphorylation levels instead of total protein expression levels. Protein phosphorylation and dephosphorylation are well-characterized biochemical processes for protein kinases to conduct cellular signal transduction. Phosphorylation at certain tyrosine, serine, or threonine residues in kinases is a key step for their activation, and the measurement of these phosphorylations reflects their functional status in vivo. Thus, the protein kinase phosphorylation-based tissue microarray more accurately reveals the molecular mechanisms of breast cancers, and more accurately predicts the individualized survival and treatment response. 

Many enzymes are regulated by covalent modification, most frequently by the addition or removal of phosphate groups from specific serine, threonine, or tyrosine residues of the enzyme. In the fed state, most of the enzymes regulated by covalent modification are in Ihe dephosphorylated form and are active (see Figure 24.2). Three exceptions are glycogen phosphorylase (see p. 129), fructose bis-phosphate phosphatase-2 (see p. 98), and hormone-sensitive lipase of adipose tissue (see p. 187), which are inactive in their dephosphorylated state. 

Phosphorylations of serine side chains in enzymes are catalyzed by kinases, and dephosphorylation by phosphatases. Threonine and tyrosine side chains undergo similar reactions, but these are less common than the phosphorylation of serine. 

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