Serine specific kinase

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. 

Cyclic nucleotides are made in response to receptor activation. The receptor activates a G-protein that, in turn, activates adenylyl cyclase to make the cyclic nucleotide. To complete the signaling, the increase in cAMP concentration activates a specific protein kinase (serine/threo-nine), cAMP-dependent protein kinase (A kinase) (Fig. 9-7). To turn off the signaling pathway, the cyclic nucleotides are destroyed by enzymes called phosphodiesterases. These cleave cAMP to AMP. 

Phosphorylation occurs in the Golgi membranes of the mammary cell, catalysed by two serine-specific casein kinases. Only certain serines are phosphorylated the principal recognition site is Ser/Thr.X.Y, where Y is a glutamyl and occasionally an aspartyl residue once a serine residue has been phosphorylated, SerP can serve as a recognition site. X may be any amino acid but a basic or a very bulky residue may reduce the extent of phosphorylation. However, not all serine residues in a suitable sequence are phosphorylated, suggesting that there may be a further topological requirement, e.g. a surface location in the protein conformation. 

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... 

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. 

Xenopus serine-specific protein kinase pp90-rsk... 

It has been known for a long time that a number of enzymes are regulated by insulin through phosphorylation and dephosphorylation at serine residues (Kahn, 1985). Therefore, a signal transduction from the tyrosine-specific insulin receptor kinase to a serine-specific kinase must occur. The serine kinase that might fulfil both functions in the insulin signal-transduction chain has not yet been identified however, there are several possible candidates for these so called switch kinases (Fig. 10). 

It is believed that the insulin receptor kinase might activate other serine-specific kinases which have a dual function, i.e. further transduction of the... 

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. 

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. 

The epinephrine receptor causes the synthesis of cyclic AMP, which is an activator of an enzyme, a protein kinase C (see Figure 12-11). Protein kinases transfer phosphate from ATP to the hydroxyl group on the side chain of a serine, threonine, or tyrosine. Protein kinase C is a serine-specific kinase. Protein kinase C is a tettamer composed of two regulatory (R) subunits and two catalytic (C) subunits. When it has cAMP bound to it, the R subunit dissociates from the C subunits. The C subunits are now catalytically active. 

Raya, A., Revert, F., Navarro, S., and Saus,J. (1999). Characterization of a novel type of serine/threonine kinase that specifically phosphorylates the human Goodpasture antigen./. Biol. Chem. 274, 12642-12649. 

Histone phosphorylation is a common posttranslational modification fond in histones, primarily on the N-terminal tails. Phosphorylation sites include serine and threonine residues, tyrosine phosphorylation has not been observed so far. Some phosphorylation events occur locally whereas others occur globally throughout all chromosomes during specific events like mitosis. Histone phosphorylation is catalyzed by kinases. Removal of the phosphoryl groups is catalyzed by phosphatases. 

MAPK cascades are composed of three cytoplasmic kinases, the MAPKKK, MAPKK, and MAPK, that are regulated by phosphorylation (Fig. 1) [1, 2]. The MAPKKK, also called MEKK for MEK kinase, is a serine/threonine kinase. Selective activation of MAPKKKs by upstream cellular stimuli results in the phosphorylation of MAPKK, also called MEK for MAP/ERK kinase by the MAPKKK. MAPKKK members are structurally diverse and are differentially regulated by specific upstream stimuli. The MAPKK is phosphorylated by the MAPKKK on two specific serine/ threonine residues in its activation loop. The MAPKK family members are dual specificity kinases capable of phosphorylating critical threonine and tyrosine residues in the activation loop of the MAPKs. MAPKKs have the fewest members in the MAPK signaling module. MAPKs are a family of serine/threonine kinases that upon activation by their respective MAPKKs, are capable of phosphorylating cytoplasmic substrates as well as... 

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. 

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