Phosphatase/kinase

Smooth muscle contractions are subject to the actions of hormones and related agents. As shown in Figure 17.32, binding of the hormone epinephrine to smooth muscle receptors activates an intracellular adenylyl cyclase reaction that produces cyclic AMP (cAMP). The cAMP serves to activate a protein kinase that phosphorylates the myosin light chain kinase. The phosphorylated MLCK has a lower affinity for the Ca -calmodulin complex and thus is physiologically inactive. Reversal of this inactivation occurs via myosin light chain kinase phosphatase. 

The classical PTPs can be subdivided into receptorlike PTPs and nonreceptor, cytosolic PTPs. The second category of PTPs are broadly defined as dual specificity phosphatases (DSPs), which dephosphorylate pSer/ pThr as well as pTyr. MAP kinase phosphatases (MKPs) ( MAP kinase cascades) and PTEN are examples of DSP family members. Remarkably, PTEN also has lipid phosphatase activity that is specific for phosphatidylinositol-3,4,5-trisphosphate generated in response to the actions of PI3K. Finally, the class of low molecular mass (LM-) PTPs and that of CDC25 PTPs accomplish the cells repertoire of PTPs (Fig. 3). 

Among the substrates of Src are other nonreceptor PTKs (e.g., Fak, Syk, and Tec kinases), RTKs (e.g. EGF and PDGF receptors), phospholipase Cy, PI3-kinase, phosphatases (e.g., SHP-2 and PP2A), and adaptor (e.g., She and Cbl) as well as focal adhesion proteins (e.g., paxillin, pl30Cas andtensin). Src-mediated phosphorylation either modulates enzymatic activity of... 

G-Protein-Coupled/Integrin Receptors Receptor/Nonreceptor Kinases/Phosphatases Transcription Factors/Proteases... 

Examples of such systems include the reactions of kinases, phosphatases, hydroxylases, acetylases, ubiquitin transferases, and many other enzyme classes that represent attractive targets for drug discovery. There are several mechanisms by which an enzyme can catalyze these types of reactions, and the details of the mechanism are important in determining the best approach to designing activity assays for the enzyme and for proper evaluation of inhibitors that are identified through those activity assays. 

Takaki, M., Ujike, H., Kodama, M. et al. Two kinds of mitogen-activated protein kinase phosphatases, MKP-1 and MKP-3, are differentially activated by acute and chronic methamphetamine treatment in the rat brain. J. Neurochem. 79 679, 2001. 

Mitogen-activated protein kinase phosphatases are dual-function protein phosphatases 401... 

Mitogen-activated protein kinase phosphatases are dual-function protein phosphatases. Just as the MAPK kinases (e.g. MEKs) are unique as dual-functioning kinases in that they phosphorylate MAPKs on threonine and tyrosine residues, there are unique dual-function ing protein phosphatases that reverse the phosphorylation and activation of MAPKs [43], Such MAPK phosphatases (MKPs) were first identified as a product of vaccinia virus (VH1) and later found in all eukaryotic cells. There are now numerous members of this VH1 family of dual-functioning protein phosphatases. 

Keyse, S. M. An emerging family of dual specificity MAP kinase phosphatases. Biochim. Biophys. Acta 1265 152-160, 1995. 

Brondello, J. M. et al., Constitutive MAP kinase phosphatase (MKP-1) expression blocks... 

Bhalla, U. S., Ram, P. T., and Iyengar, R. (2002) MAP kinase phosphatase as a locus of flexibility in a mitogen-activated protein kinase signaling network. Science 297, 1018-1023. 

Figure 1 The MAPK pathway and its connections to other signals A negative feedback loop connects the phosphorylated endpoint of the pathway ERK (Extracellular-signal Regulated Kinase) to the transcriptionally-driven synthesis of the phosphatase, MKP MAP kinase phosphatase. MKP then de-phosphorylates ERK to shut down the signaling cascade. The positive feedback loop again starts with the terminal kinase ERK which activates cPLA2 (cytosolic phospholipase A2). This leads to the synthesis of arachidonic acid, which, in turn activates protein kinase C (PKC). PKC is a positive regulator of RAS (Please see Color Plate Section in the back of this book).

In contrast to kinases, phosphatases catalyse the removal of phosphoryl groups, again, either from phosphorylated metabolites such as glucose-6-phosphate or fructose-1,6-bisphosphate... 

Bunyapaiboonsri, T Ramstrom, H. Ramstrom, O. Haiech, J. Lehn, J.-M. Generation of bis-cationic heterocyclic inhibitors of Bacillus subtilis HPr kinase/phosphatase from a ditopic dynamic combinatorial library. J. Med. Chem. 2003,46, 5803-5811. 

Another mechanism of regulation of protein tyrosine phosphatases is via Ser/Thr phosphorylation. Specific phosphorylation of protein tyrosine phosphatases by Ser/ Thr-specific protein kinases of types A and C has been reported (see Neel and Tonks, 1997). This observation indicates the possibility that signal transductions via Ser/Thr kinases and via Tyr kinases/phosphatases may cooperate and that different signal pathways may be crosslinked in this way. 

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