Calcium/calmodulin-dependent protein kinases subunits

FIGURE 1 2-2 Schematic diagram of the phosphorylation sites on each of the four 60kDa subunits of tyrosine hydroxylase (TOHase). Serine residues at the N-terminus of each of the four subunits of TOHase can be phosphorylated by at least five protein kinases. (J), Calcium/calmodulin-dependent protein kinase II (CaM KII) phosphorylates serine residue 19 and to a lesser extent serine 40. (2), cAMP-dependent protein kinase (PKA) phosphorylates serine residue 40. (3), Calcium/phosphatidylserine-activated protein kinase (PKC) phosphorylates serine 40. (4), Extracellular receptor-activated protein kinase (ERK) phosphorylates serine 31. (5), A cdc-like protein kinase phosphorylates serine 8. Phosphorylation on either serine 19 or 40 increases the activity of TOHase. Serine 19 phosphorylation requires the presence of an activator protein , also known as 14-3-3 protein, for the expression of increased activity. Phosphorylation of serines 8 and 31 has little effect on catalytic activity. The model shown includes the activation of ERK by an ERK kinase. The ERK kinase is activated by phosphorylation by PKC. (With permission from reference [72].)... 

Strack S, McNeill RB, Colbran RJ. Mechanism and regulation of calcium/ calmodulin-dependent protein kinase II targeting to the NR2B subunit of the N-methyl-D-aspartate receptor. J. Biol. Chem. 275 23798-806... 

Omkumar RV, Kiely MJ, Rosenstein AJ, Min KT, Kennedy MB (1996) Identification of a phosphorylation site for calcium/calmodulin dependent protein kinase II in the NR2B subunit of the 7 1-methyl-D-aspartate receptor. J Biol Chem 277 31670-31678. 

Major changes in phosphorylation induced by ischaemia and subsequent reperfusion in rat striatum were observed for a 130-kDa protein, tentatively identified as the Ca "" transport ATPase, and calcium/calmodulin-dependent protein kinase II (Sankaran etal. 1997). A 200-300% increase in [ PjSNjATP photoinsertions was observed in the striatum and hippocampus region of a 43-kDa protein with an isoelectric point of 6.8. This protein was identified as glutamine synthetase and the increase in binding was found to be due to both increased copy number and activation by Mn ". An increase in [ PjSNjATP photoinsertions into a 55-kDa protein, identified as the P-subunit of tubulin, was found only in the striatum and hippocampus indicating the depolymerization of microtubulin in these tissues. 

In brain, a large number of particulate soluble proteins are phosphory-lated in the presence of Mg " -ATP and Ca +—calmodulin. These phosphorylations can be blocked by phenothiazines and do not occur in the presence of cAMP or cGMP. Furthermore, they are unaltered by the Walsh inhibitor, which is specific for the cAMP catalytic subunit. Hence, investigators have concluded that there is present in both particulate and soluble fractions of brain a Ca " -calmodulin-dependent protein kinase that is insensitive to cyclic nucleotides and that contains its own set of protein substrates. A few reports have appeared in the literature claiming purification of a calcium-calmodulin-dependent kinase from mammalian brain. The en-... 

Phosphorylase kinase is one of the best characterized enzyme systems to illustrate the role of calcium ions in regulation of intermediary metabolism. Phosphorylase kinase is composed of four different subunits termed a (Mr 145000), /3 (MT 128000), y (A/r 45000) and 5 (Mr 17000) and has the structure (a/3y8)A [106]. Only one of its four subunits actually catalyses the phosphorylation reaction the other three subunits are regulatory and enable the enzyme complex to be activated both by calcium and cyclic AMP. The y subunit carries the catalytic activity the 8 subunit is the calcium binding protein calmodulin and is responsible for the calcium dependence of the enzyme. The a and /3 subunits are the targets for cyclic-AMP mediated regulation, both being phosphorylated by the cyclic-AMP dependent protein kinase. Calmodulin appears to interact with phosphorylase kinase in a different manner from other enzymes, since it is an integral component of the enzyme. Phosphorylase kinase has an absolute requirement for calcium, and is inactive in its absence. 

Phosphorylase kinase activity has an absolute requirement for Ca +, which binds to the 5-subunit. The amino acid sequence of this subunit is nearly identical to that of calmodulin, with four calcium binding sites, but unlike calmodulin, the 5-subunit is an integral part of the enzyme and does not dissociate from it in the absence of Ca +. In the presence of Ca +, kinase activity is further increased by phosphorylation of the a- and j6-subunits, catalyzed by cAMP-dependent protein kinase and several other kinases. Phosphorylation may activate the enzyme by disinhibiting... 

Another advantage of a cascade is the possibility of intervening anywhere along the cascade, not only at the initiation. In the case of the phosphorylase cascade, phosphorylase kinase in the inactive (i.e., nonphosphorylated) form can be activated allosterically by calcium, thus causing phosphorylase kinase to catalyze the phosphorylation of phosphorylase to the active form, causing increased glycogenolysis. One of the subunits of phosphorylase kinase is a polypeptide known as calmodulin. This polypeptide occurs in many proteins that require or have Ca2+ as an effector. This polypeptide in phosphorylase kinase results in the ca2+ activation of this enzyme. The binding of Ca2+ by the calmodulin subunit of phosphorylase kinase also facilitates a more rapid phosphorylation of this enzyme by the cAMP-dependent protein kinase. 

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