Lobe-Specific Functions of Calcium.Calmodulin in Alpha-Calcium. Calmodulin-Dependent Protein Kinase II Activation

Abstract

N-methyl-D-aspartic acid receptor-dependent long-term potentiation (LTP), a model of memory formation requires calcium.calmodulin-dependent protein kinase II (αCaMKII) activity and Thr286-autophosphorylation via both global and local calcium signalling but the mechanisms of signal transduction are not understood. We tested the hypothesis that the calcium-binding activator-protein calmodulin (CaM) is the primary decoder of calmodulin signals thereby determining the output, e.g. LTP. Thus we investigated the function of CaM mutants, deficient in calcium binding at sites 1, 2 of the N terminal lobe or sites 3, 4 of the C terminal CaM lobe, in the activation of αCaMKII. Occupancy of CaM calcium binding sites 1, 3 and 4 is necessary and sufficient for full activation. Moreover, the N and C terminal CaM lobes have distinct functions: calcium binding to N lobe calcium binding site 1 increases the turnover rate of the enzyme 5-fold, while the C lobe plays a dual role: it is required for full activity, but in addition, via calcium binding site 3, it stabilizes ATP binding to αCaMKII 4-fold. Thr286-autophosphorylation too is dependent on calcium binding sites on both the N and C lobes of CaM. As the CaM C lobe sites are populated by low amplitude/low frequency (global) calcium signals, but occupancy of N lobe site 1 and thus activation of αCaMKII requires high amplitude/high frequency (local) calcium signals, lobe-specific sensing of calcium signalling patterns by CaM is proposed to explain the requirement for both global and local calcium signalling in the induction of LTP via αCaMKII.