Functions of Protein Kinase

The members of the protein kinase C family are central signal proteins and as such, are involved in the regulation of a multitude of cellular processes. A problem in the identification of substrates of protein kinase C is its low substrate specificity which often cannot be differentiated from that of protein kinase A, particularly in in vitro experiments. The consensus sequence of the phosphorylation sites in substrate proteins are similar to those of protein kinase A, in that basic amino acids are required in the neighborhood of the Ser/Thr residue to be phosphorylated. The following consensus sequences may be formulated for phosphorylation by protein kinase C ( = phosphorylation site) S /T XK/R K/RXXS /T K/RXXS /T XK/R K/RXS /T K/RXS /T XK/R (Pearson and Kemp, 1991). 

Of the many substrates of protein kinase C, the MARCKS proteins are highhghted as very well characterized and specific substrates of protein kinase C (review Aderem, 1995). The abbreviation MARCKS stands for myristoylated, alanine-rich C-kinase substrate. 

The MARCKS proteins are a family of proteins that are involved in physiologically important processes such as cell mobility, secretion, membrane transport and in regulation of the cell cycle. All these processes are associated with changes and restructuring of the actin cytoskeleton. The role of converting extracellular signals into changes in the structure of the actin cytoskeleton is attributed to the MARCKS proteins. A 

The MARCKS proteins are acidic proteins with a high content of the amino acids Ala, Gly, Pro and Glu. An N-terminal domain carries a lipid anchor in the form of myristinic acid, from which it is assumed that it mediates the association with the membrane. A basic effector domain is important for regulation of the MARCKS proteins a binding site for Ca calmoduhn and the phosphorylation site for protein kinase C are located in this domain (see Fig. 7.12). 

In the imphosphorylated form and in the absence of Ca, the MARCKS proteins bind to actin filaments and bring about crosshnking of the latter. Binding of Ca Vcal-modulin or phosphorylation by protein kinase C inhibits the crosslinking activity. The MARCKS proteins can thus modulate the aggregation status of actin filaments and function as effectors for the conversion of extracellular signals that are carried into the cell via G-protein-coupled receptors and/or tyrosine kinase receptors. 

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EXPRESSION AND FUNCTION OF PROTEIN KINASES DURING MAMMALIAN GAMETOGENESIS... 

Of the protein kinases, protein kinase A is the best investigated and characterized (review Francis and Corbin, 1994). The functions of protein kinase A are diverse. Protein kinase A is involved in the regulation of metabolism of glycogen, lipids and sugars. Substrates of protein kinase A may be other protein kinases, as well as enzymes of intermediary metabolism. Protein kinase A is also involved in cAMP-stimulated transcription of genes that have a cAMP-responsive element in their control region (review Montminy, 1997). An increase in cAMP concentration leads to activation of protein kinase A which phosphorylates the transcription factor CREB at Ser 133. CREB only binds to the transcriptional coactivator CBP in the phosphorylated state and stimulates transcription (see Chapter 1.4.4.2). 

Many functions of protein kinase C in signaling pathways are closely linked with the membrane association of the enzyme. Activation of protein kinase C, initiated by addi-... 

Fig. 7.16. The dual function of protein kinases and protein phosphatases. Phosphorylation of proteins (PI, P2) can fix the latter into an active or inactive state. In the case of PI, protein kinases have an activating effect and protein phosphatases are inactivating the reverse is trne for P2.

Hie subcdlular localization of the kinase or phosphatase also plays a crucial role for the activity of protein kinases and protein phosphatases. Many physiological functions of protein kinases and protein phosphatases depend on the enzyme being brought, with the help of specific protein-protein interactions, to certain subceUular locations in the vicinity of its substrate. 

Fig. 1. The function of protein kinases [ADP (4), adenosine diphosphate ATP (3), adenosine triphosphate].

I.IW3J4 ln4olifvon derivative as tyrosine kinase inhibitor. User Harr ill m treating < Auction path) iseasts by modulating the function of protein kinases nays ... 

K. Mackay and D. Mochly-Rosen, Localization, anchoring and functions of protein kinase C isozymes in the heart, J. Mol. Cell. Cardiol. 33, 1301-1307 (2001). 

The common catalytic function of protein kinases is the covalent phosphorylation of substrate proteins via transfer of the y-phosphate of ATP to the OH group of serine, threonine or tyrosine residues. This catalytic function is carried out by a catalytic do-... 

Many functions of protein kinase C in signaling pathways are closely linked with the membrane association of the enzyme. Signal-dependent association of PKC with the cell membrane is therefore another major regulatory aspect of this enzyme class. Activation of protein kinase C, initiated by addition of phorbol esters, for example, is associated with a redistribution of the enzyme from the cytosol to the membrane. 

The enzymatic function of protein kinases is carried out by phosphorylation of serine, threonine, or tyrosine residues on target proteins. As an estimated 30% of human proteins are thought to be phosphorylated, identification of the direct substrates of all human protein kinases is a daunting challenge. Although a wide range of methods have been developed for isolating the... 

Sign in at www.thomsonedu.com/login and explore a Biochemistry Interactive tutorial on the structure and function of protein kinase C. 

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