Threonine specific kinase

Raf kinases such as Rafl are serine/threonine-specific protein kinases which function in signal transduction... 

Cytoplasmic serine/threonine protein kinases catalyze the transfer of phosphate groups to serine and threonine residues of target proteins. Serine/threonine kinases have been recognized as the products of protooncogenes (e.g., c-mos, c-raj) or as kinases intimately involved with the regulation of serine/threonine kinase activity by cAMP. Some of these kinases specifically phosphorylate cellular structural proteins, such as histone, laminins, etc. Others phosphorylate still more kinases, resulting in either the activation or deactivation of downstream protein kinases. Specific examples in which serine/threonine kinases elicit specific cellular responses are discussed in this chapter. 

This set of enzymes [EC 3.1.3.16], also known as serine/ threonine-specific protein phosphatases, catalyzes the hydrolysis of a seryl- or threonyl-bound phosphate group from a wide range of phosphoproteins, including a number of enzymes which have been phosphorylated under the action of a kinase. Thus, phosphoprotein J- water yields protein J- orthophosphate. 

The cAMP-dependent protein kinases phosphorylate specific Ser (Thr) residues on target proteins. Given the availability of serine and threonine residues on the surface of globular proteins, how might a protein... 

A major signalling pathway involves activation of a protein kinase that phosphorylates inhibitor kB proteins (IkBs) that normally inhibit the function of the nuclear transcription factor NFkB. Phosphorylation of IkB by the serine/threonine-specific IkB kinases (IKKs) leads to NFkB de-inhibition, nuclear translocation and expression of pro-inflammatory proteins such as inducible cyclooxygenase (iCOX) (which generates prostaglandins), inducible nitric oxide synthase (iNOS) (which generates vasodilatory and toxic free radicalgenerating NO) and pro-inflammatory cytokines. 

In vitro, with CaATP as a substrate, E. coli dnaK autophosphorylates exclusively at Thr-199 (McCarty and Walker, 1991). It does not auto-phosphorylate when MgATP is used as a substrate. In vivo, dnaK is found to be phosphorylated on serine as well as on threonine residues (Rieul et al., 1987). Under normal growth conditions, phosphorylation is primarily on serine when E. coli is infected with bacteriophage M13, the phosphorylation shifts predominantly to threonine, with minor phosphorylation of serine. The specific target residues of in vivo phosphorylation of dnaK have not yet been determined. The disparity between the in vitro results (autophosphorylation exclusively at Thr-199 an absolute requirement for CaATP) and the in vivo results (phosphorylation on both serine and threonine residues, under conditions in which MgATP would be presumed to be the available intracellular substrate nucleotide) raises the questions of (1) whether the specific autophosphorylation of Thr-199 observed in vitro also occurs in vivo, or whether it may be an artifactual side reaction when the larger Ca ion is substituted for Mg" at the active site of the protein, and (2) whether there is a serine/threonine protein kinase that specifically phosphorylates dnaK in vivo. 

The. superfamily of protein kinases can be classified by the nature of their substrate in two groups kinases that phosphorylate serine or/and threonine residues, and kinases that phosphorylate tyrosine aminoacids. The serine-and threonine-specific protein kinases can be further classified by the nature of their activators. For example, cAMP-dependent, cGMP-dependent, Ca Vcalmodulin-dependent and Ca 7 phospholipid-dependent protein kinases have been identified. Many tyrosine protein kinases are intrinsic parts of the cytoplasmic domains of growth factor receptors. 

The activity of the protein kinases are regulated allosterically. The serine- and threonine-specific protein kinases have a regulatory domain which, in the resting state, keeps the catalytic part of the enzyme inactive. When a second messenger or activator (cAMP, cGMP, Ca or DAG) binds to the regulatory domain, the enz)m e is activated. 

Under the influence of insulin (and other hormones or growth factors), a signaling chain is activated that results in the activation of the serine/threonine-specific protein kinase Akt/PKB (see Section6.6.3) and in the phosphorylation of 4E-BP1. It is assumed that the phosphorylation of 4E-BP1 represents the signal for the release of... 

The final link in the kinase cascade activated by Ras-GTP emerged from studies in which scientists fractionated extracts of cultured cells searching for a kinase activity that could phosphorylate MAP kinase and that was present only in cells stimulated with growth factors, not quiescent cells. This work led to Identification of MEK, a kinase that specifically phosphorylates one threonine and one tyrosine residue on MAP kinase, thereby activating its catalytic activity. (The acronym MEK comes from MAP and ERK kinase.) Later studies showed that MEK binds to the C-terminal catalytic domain of Raf and is phosphorylated by the Raf serine/ threonine kinase this phosphorylation activates the catalytic activity of MEK. Hence, activation of Ras induces a kinase cascade that Includes Raf, MEK, and MAP kinase activated RTK Ras Raf MEK MAP kinase. 

Once Ras is in its active form, it binds to another oncogene product called Raf, which localizes Raf to the membrane and activates its serine/threonine specific kinase activity. Among the many phosphorylation targets of Raf is another kinase called MAP kinase kinase (also called MEK), which is activated by Raf and in turn phosphorylates cytosolic MAP kinases on threonine and tyrosine residues. Activated MAP kinases (also called ERK1 and ERK2) are serine/threonine kinases that, once activated, can localize to the nucleus where they phosphorylate transcription factors such as Fos and Jun. MAP kinases also phosphorylate and activate other kinases (such as Rsk) which then pass the signal on to additional transcription factors (e.g., SRF, serum response factor). Therefore, downstream of Ras, a series of phosphorylation reactions culminate in the activation or inactivation of a variety of transcription... 

After activation, protein kinase C phosphorylates specific serine and threonine residues in target proteins. As with cAMP-stimulated protein kinase, the specific cellular responses to protein kinase C activation depend on the ensemble of target proteins that become phosphorylated in a given cell. Known target proteins include calmodulin, the insulin receptor, / -adrenergic receptor, glucose transporter, HMG-CoA reductase, cytochrome P-450, and tyrosine hydroxylase. 

---