Kinase-associated protein phosphatase

Shah, K., Russinova, E., Gadella, T. W., Jr., Willemse, J. and De Vries, S. C. (2002). The Arabidopsis kinase-associated protein phosphatase controls internalization of the somatic embryogenesis receptor kinase 1. Genes Dev. 16,1707-20. 

The cAMP and Ca2+ pathways also interact at the level of protein kinases and protein phosphatases. This is illustrated by inhibitor-1 and DARPP-32, which are phosphorylated and activated by PKA and then inhibit PP1, which can dephosphorylate numerous substrates for Ca2+-dependent protein kinases. Another example is the physical association between PKA and PP2B (a Ca2+/ calmodulin-activated enzyme) via the AKAP-anchoring proteins. 

Sugiyama K, Sugiura K, Kara T, Sugimoto K, Shima H, Honda K, Furukawa K, Yamashita S, Urano T (2002) Aurora-B associated protein phosphatases as negative regulators of kinase activation. Oncogene... 

The extent and specificity of the reactions of protein kinases and protein phosphatases are extremely dependent on the degree to which substrate and enzyme are localized at the same place in the cell. Many substrates of protein kinases occur either as membrane associated or particle associated forms (see 7.6.1, enzymes of glycogen metabolism). For protein kinases or protein phosphatases to perform their physiological function in a signal transduction process, they must be transported to the location of then-substrate in many cases (review Hubbard and Cohen, 1992 Mochly-Rosen, 1995). This is vahd both for the Ser/Tbr-specific protein kinases as well as for many Tyr-speci-fic protein kinases. In the course of activation of signal transduction pathways, com-partmentahzation of protein kinases, redistributed to new subcellular locations, is often observed. 

In another mechanism, an associated subunit of the protein kinase or protein phosphatase determines in which compartment of the cell and at which membrane section... 

Fig. 7.22. The prindple of targeted localization of protein kinases and protein phosphatases. The spatial configuration between the catalytic subunit of a protein kinase or protein phosphatase and a membrane-associated substrate is mediated by localization subunits that specifically bind to membrane-localized anchor proteins. The specificity of co-localization is predominantly achieved at the level of binding of the localization subunit to the anchor protein. The co-localization is regulated, in particular, by the interaction of the catalytic subunit with the localization subunit. In the membrane-associated form, the catalytic subunit has increased activity towards membrane-bound substrates.

Another second-messenger-dependent kinase which interacts with targeting proteins, is the cAMP-dependent protein kinase A. PKA interacts with AKAPs (A-kinase-associated proteins). AKAPs bind to the dimeric form of the regulatory subunit of PKA. They are multivalent linkers, which bind not only to PKA, but also to other kinases, such as PKC, and the Ca +Z calmodulin-dependent kinase II and the phosphatase PP2B (Fig. 7.6). 

Fig. 7.20 The principle of targeted localization of protein kinases and protein phosphatases. The spatial configuration between the catalytic subunit of a protein kinase or protein phosphatase and a membrane-associated substrate is mediated by localization subunits that specifically bind to membrane-localized anchor proteins. The specificity of...

Other negative correlations are simpler to interpret. Two functionally antagonistic enzymes, namely, protein kinases and protein phosphatases, have, to date, not been found in the same protein. Similarly, WW and SH3 domains that both bind the similar polyproline-containing substrates are never found together. This last finding, however, is curious since 216 proteins that contain either two or more WW, or two or more SH3, domains are known. Finally, it appears that proteins with domains that bind phosphoserine or phosphothreonine (FHA, fork-head-associated domains) never contain domains that bind phosphoty-rosine (SH2, PTB, and PTBI domains). This indicates that cytoplasmic signaling via phosphoserine or phosphothreonine occurs via pathways distinct from those signaling via phosphotyrosine. 

Approximately one-third of cellular proteins contain phosphate and are subject to covalent modification by phosphorylation and dephosphorylation reactions. This reversible phosphorylation of proteins causes conformational changes in the protein that dramatically alters their properties, e.g. from an active to an inactive enzyme, or vice versa. The sites of protein phosphorylation are those amino acid residues that contain hydroxyl groups, most commonly serine but also tyrosine and threonine (Fig. 27.2) (Chapter 31). Phosphorylation uses protein kinase and dephosphorylation uses protein phosphatase. The importance of reversible protein phosphorylation to the living cell is emphasised by the fact that protein kinases and protein phosphatases comprise approximately 5% of the proteins encoded by the human genome. Current research is discovering abnormalities of protein phosphorylation that are associated with diseases, notably type 2 diabetes meUitus (T2DM) and cancer. In the future, the discovery of drugs that modify protein phosphorylation/dephosphorylation promises new therapies for the treatment of these diseases. 

Figure 2. Mechanism of PDH. The three different subunits of the PDH complex in the mitochondrial matrix (E, pyruvate decarboxylase E2, dihydrolipoamide acyltrans-ferase Ej, dihydrolipoamide dehydrogenase) catalyze the oxidative decarboxylation of pyruvate to acetyl-CoA and CO2. E, decarboxylates pyruvate and transfers the acetyl-group to lipoamide. Lipoamide is linked to the group of a lysine residue to E2 to form a flexible chain which rotates between the active sites of E, E2, and E3. E2 then transfers the acetyl-group from lipoamide to CoASH leaving the lipoamide in the reduced form. This in turn is oxidized by E3, which is an NAD-dependent (low potential) flavoprotein, completing the catalytic cycle. PDH activity is controlled in two ways by product inhibition by NADH and acetyl-CoA formed from pyruvate (or by P-oxidation), and by inactivation by phosphorylation of Ej by a specific ATP-de-pendent protein kinase associated with the complex, or activation by dephosphorylation by a specific phosphoprotein phosphatase. The phosphatase is activated by increases in the concentration of Ca in the matrix. The combination of insulin with its cell surface receptor activates PDH by activating the phosphatase by an unknown mechanism.

It is not clear whether V(V) or V(IV) (or both) is the active insulin-mimetic redox state of vanadium. In the body, endogenous reducing agents such as glutathione and ascorbic acid may inhibit the oxidation of V(IV). The mechanism of action of insulin mimetics is unclear. Insulin receptors are membrane-spanning tyrosine-specific protein kinases activated by insulin on the extracellular side to catalyze intracellular protein tyrosine phosphorylation. Vanadates can act as phosphate analogs, and there is evidence for potent inhibition of phosphotyrosine phosphatases (526). Peroxovanadate complexes, for example, can induce autophosphorylation at tyrosine residues and inhibit the insulin-receptor-associated phosphotyrosine phosphatase, and these in turn activate insulin-receptor kinase. 

Fig. 5.5. General functions of transmembrane receptors. Extracellular signals convert the transmembrane receptor from the inactive form R to the active form R. The activated receptor transmits the signal to effector proteins next in the reaction sequence. Important effector reactions are the activation of heterotrimeric G-proteins, of protein tyrosine kinases and of protein tyrosine phosphatases. The tyrosine kinases and tyrosine phosphatases may be an intrinsic part of the receptor or they may be associated with the receptor. The activated receptor may also include adaptor proteins in the signaling pathway or it may induce opening of ion channels.

Fig. 7.20. Regulation of glycogen-bound protein phosphatase I. Regulation of the activity of protein phosphatase I (PPI) takes place by phosphorylation of the G subunit. The G subunit is phos-phorylated at positions PI and P2, in the process of a signal chain mediated activation of protein kinase A. As a consequence of the phosphorylation, the catalytic subunit dissociates. The phosphatase activity of the free catalytic subunit is inhibited by association with a cytosohc protein phosphatase inhibitor (I), the binding of which is also controlled via a protein kinase A mediated phosphorylation. The phosphorylated G subunit can be dephosphorylated again by protein phosphatase 2A and may bind a catalytic PPI subunit once more.

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