A Secreted Phospholipase A 2 Binds to Calmodulin at Sub-micromolar Concentrations of Calcium

To determine the possibility that calmodulin, a cytosolic regulatory protein, and ammodytoxin, a neurotoxic secreted phospholipase A 2 , interact in vivo, we studied the dependence of their association on Ca 2+ . The interaction between the two proteins was positively dependent on Ca 2+ , and greatest at millimolar concentrations of this ion. Importantly, they interacted already in the presence of sub-micromolar concentrations of Ca 2+ , as demonstrated by affinity labelling, laser tweezers and surface plasmon resonance. Tight binding of the secreted phospholipase A2 to calmodulin is therefore expected on depolarization of the axolemma, when the concentration of free Ca 2+ at active zones rises to 100 µM. These results strengthen the proposal that binding of ammodytoxin to calmodulin is a step in the process of presynaptic toxicity of this and related phospholipases A 2 . Moreover, they suggest that some other (patho)physiological effects induced by endogenous secreted phospholipases A2 could be also due to their interaction with calmodulin in the cytosol of cells.