Autophosphorylation calmodulin-kinase

Fig. 7.14. Regulation of CaM kinase II. Scheme of regulation of CaM kinase II by Ca Vcalmodu-lin and by autophosphorylation. CaM kinase II is inactive in the unphosphorylated form and in the absence of Ca calmodulin. Binding of Ca Vcalmodulin activates the kinase for phosphorylation of protein substrates. In the process, autophosphorylation takes place at a conserved Thr residue that stabilizes the active state of the enzyme. In this state, significant residual activity is still present after dissociation of Ca Vcalmodulin and the enzyme remains in an active state for a longer time after the Ca signal has died away. The active state is only terminated when the activating phosphate residue is cleaved off by a protein phosphatase.

Hashimoto,Y., Sharma, R. K. and Soderling, T. R. Regulation of Ca+2/calmodulin-dependent cyclic nucleotide phosphodiesterase by the autophosphorylated form Ca+2/ calmodulin-dependent protein kinase II. J. Biol. Chem. 264 10884-10887,1989. 

Most protein serine-threonine kinases undergo autophosphorylation. The autophosphorylation of most protein kinases is associated with an increase in kinase activity [4, 10]. In some instances, such as with the RII subunit of PKA, autophosphorylation represents a positive feedback mechanism for kinase activation, in this case by enhancing the rate of dissociation of the RII and C subunits. In the case of CaMKII, autophosphorylation causes the catalytic activity of the enzyme to become independent of Ca2+ and calmodulin. This means that the enzyme, activated originally in response to elevated cellular Ca2+, remains active after Ca2+ concentrations have returned to baseline. By this mechanism, neurotransmitters that activate CaMKII can produce relatively long-lived alterations in neuronal function. In other instances, such as with the receptor-associated protein tyrosine kinases (discussed in Ch. 24), autophosphorylation is an obligatory step in the sequence of molecular events through which those kinases are activated and produce physiological effects. 

Lee, S.R Mahasneh, A. de la Roche, M. Cote, G.P. Regulation of the p21-activated kinase-related Dictyostelium myosin I heavy chain kinase by autophosphorylation, acidic phospholipids, and Ca " -calmodulin. J. Biol. Chem., 273, 27911-27917 (1998)... 

Picciotto, M., Czernik, A. and Nairn, A., 1993, Calcium/calmodulin-dependent protein kinase I. cDNA cloning and identification of autophosphorylation site., J. Biol. Chem., 268, pp 26512-26521. 

Saitoh, Y., Yamamoto, H., Fukunaga, K., Matsukado, Y. and Miyamoto, E., 1987, Inactivation and reactivation of the multifunctional calmodulin-dependent protein kinase from brain by autophosphorylation and dephosphorylation Involvement of protein phosphatases from brain, Journal of Neurochemistry, 49, pp 1286—1292. 

Strack S, Colbran RJ (1998) Autophosphorylation-dependent targeting of calcium/calmodulin-dependent protein kinase II by the NR2B subunit of the iV-methyl-D-aspartate receptor. J Biol Chem 275 20689-20692. 

The special feature of regulation of CaM kinase II is the memory effect within the activation process. Activation of the enzyme is initiated by a generally transient increase in cellular Ca2+. Ca2+ activates CaM kinase II in the form of the Ca2+/calmodulin complex the kinase remains active even after the Ca2+ signal has died away, because the enzyme is converted into an autonomous activated state upon autophosphorylation. Presumably the complex holoenzyme structure of CaM kinase II endows the kinase with this unique regulatory property, allowing it to function as a sensor of cellular Ca2+ oscillations. 

In addition to generating autonomous activity, autophosphorylation of CaM-kinase II on Thr ss has been shown to result in calmodulin "trapping" as a consequence of nearly a thousandfold decrease in calmodulin off-rate (Meyer etal., 1992). This activation scheme is summarized in Rg. 4. Autophosphoryl-ation-dependent calmodulin trapping and enzymatic activity in the absence of Ca +ZCaM provide mechanisms for prolonging kinase activity beyond the dura-... 

Once subunits are autophosphorylated on Thr ss in vitro, Ca2+ chelation and dissociation of CaM result in the autophosphorylation of additional serine and threonine residues, including Thr os and/or Thr oe in the calmodulin binding domain (Miller and Kennedy, 1986 Hashimoto et al., 1987 Patton et al, 1990). Phosphorylation of Thr305/306 blocks rebinding of CaM and this is reflected by a loss of total Ca +ZCaM-dependent activity to a level equal to the activator-independent activity. Autophosphorylation of Thr o in this manner also appears to be an intraholoenzyme intersubunit reaction (Mukherji and Soderling, 1994). Under certain experimental conditions it has been possible to demonstrate the slow autophosphorylation of Thr oe and concomitant loss of CaM-kinase II activity without addition of activators and prior autophosphorylation of Thr286 (Colbran, 1993). In this case, Thr o autophosphorylation appears to be catalyzed by an intrasubunit reaction (Mukherji and Soderling, 1994). 

---