The conformation of Ca2+/calmodulin changes from extended when free in solution to compact when bound in peptide complexes. The extent and kinetics of calmodulin compaction in association with Ca2+/calmodulin-dependent protein kinases (CaMKs), as well as target peptides, were investigated by fluorescence, resonance energy transfer and stopped-flow kinetics. Compaction of Ca2+/ calmodulin labelled with resonance energy-transfer probes in association with target peptides was rapid (>350 s−1). With the target enzymes smooth-muscle myosin light-chain kinase, CaMKIV and αcCaMKII, the rates of calmodulin compaction were one-two orders of magnitude lower compared with those of the peptides and in the case of αCaMKII, ATP binding and Thr286 auto-phosphorylation were required for calmodulin compaction. In the absence of nucleotides, Ca2+/calmodulin bound to αCaMKII in extended conformations, initially probably attached by one lobe only. Kinetic data suggest that in the activation process of Ca2+/ calmodulin-dependent protein kinases, productive as well as unproductive complexes are formed. The formation of productive complexes with Ca2+/calmodulin thus may determine the rate of activation.

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