Calmodulin trapping by CaMKII
Author summaryCalcium/calmodulin-dependent kinase II (CaMKII) is a neuronal protein involved in learning and memory. Upon binding to its activator calmodulin, it can undergo phosphorylation, which has two main effects: First, it maintains CaMKII activity, thus rendering it calmodulin-independent. Second, it decreases calmodulin dissociation rates, thereby ``trapping'' calmodulin in the complex. The exact mechanism by which calmodulin trapping is achieved is so far poorly understood. In this paper, we combine stuctural modelling using the crystal structures of CaMKII and calmodulin with stochastic simulations of interactions between the two molecules. We argue that a single CaMKII unit has not one, but two binding sites for calmodulin, of which oine (the stronger one) becomes more easily accessible if CaMKII is phosphorylated. This hypothesis is consistent with existing biochemical data and with the available protein structures. A computer simulation based on this hypothesis can, indeed, reproduce calmodulin trapping by CaMKII and is the first computational model to do so.
Activation of CaMKII by calmodulin and the subsequent maintenance of constitutive activity through autophosphorylation at threonine residue 286 (Thr286) are thought to play a major role in synaptic plasticity. One of the effects of autophosphorylation at Thr286 is to increase the apparent affinity of CaMKII for calmodulin, a phenomenon known as "calmodulin trapping". It has previously been suggested that two binding sites for calmodulin exist on CaMKII, with high and low affinities, respectively. We built structural models of calmodulin bound to both of these sites. Molecular dynamics simulation showed that while binding of calmodulin to the supposed low-affinity binding site on CaMKII is compatible with closing (and hence, inactivation) of the kinase, and could even favour it, binding to the high-affinity site is not. Stochastic simulations of a biochemical model showed that the existence of two such binding sites, one of them accessible only in the active, open conformation, would be sufficient to explain calmodulin trapping by CaMKII. We can explain the effect of CaMKII autophosphorylation at Thr286 on calmodulin trapping: It stabilises the active state and therefore makes the high-affinity binding site accessible. Crucially, a model with only one binding site where calmodulin binding and CaMKII inactivation are strictly mutually exclusive cannot reproduce calmodulin trapping. One of the predictions of our study is that calmodulin binding in itself is not sufficient for CaMKII activation, although high-affinity binding of calmodulin is.