Regulation of Calmodulin-Kinase II Activity

The of purified CaM-kinase II for calmodulin has been reported to be in the range of 20-100 nM at saturating [Ca +J with one calmodulin bound per sub- 

Early studies of CaM-kinase II indicated that the kinase subunits autophosphorylated with stoichiometries estimated to be as high as 4 mol Pj/mol subunit (Ahmad et al., 1982). Ca +ZCaM-dependent autophosphorylation was subsequently shown to result in the generation of activator-independent kinase activity to a maximum level of 40-70% of total Ca +Z CaM-stimulated activity (Miller and Kennedy, 1986 Schworer et al., 1986 Lou et al., 1986 Lai et al, 1986). Once CaM-kinase II is activator independent or autonomous, additional autophosphorylation can take place in the absence of Ca +ZCaM (Miller and Kennedy, 1986 Hashimoto et al., 1987). These properties distinguish CaM-kinase II from other Ca +ZCaM- 

It is now known that generation of activator-independent CaM-kinase II activity requires autophosphorylation of a specific threonine residue (Thr286) in the autoinhibitory domain (Schworer et al., 1988 Thiel et al., 1988 Hanson et al., 1989). Autophosphorylation on this residue is via a highly cooperative intraholoenzyme intersubunit reaction that occurs only between activated (Ca +ZCaM bound) subunits (Hanson etal., 1994 Mukherji and Soderling, 

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- 

FIGURE 4 Model for activation and autophosphorylation of a CaM-kinase II holoenzyme based on Hanson ct al. (1994). 1-2 Multi-mers of CaM-kinase II subunits are activated by increasing concentrations of free intracellular Ca + [(Ca +J,) and association of Ca +/CaM complexes. 2-3 Activated subunits (solid) with bound Ca +/CaM are autophosphorylated on Thr (P) in a cooperative intraholoenzyme intersubunit reaction. 3-4 Phosphorylated subunits have an increased affinity for CaM and are partially active (shaded circles) in the absence of bound Ca +/CaM. 4-1 Dissociation of Ca +/CaM and the action of phosphoprotein phosphatases return autophosphorylated subunits to the inactive state. 

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