A Secreted Phospholipase A2 Binds to Calmodulin at Sub-micromolar Concentrations of Calcium

Uroš Logonder,1 Jernej Jorgačevski,1 Gregor Anderluh,2 Uroš Petrovič,1 Igor Poberaj3 and Igor Križaj1,*

1Department of Molecular and Biomedical Sciences, Jožef Stefan Institute, Jamova cesta 39, SI-1000 Ljubljana, Slovenia
2Department of Biology, Biotechnical Faculty, University of Ljubljana, Večna pot 111, SI-1000 Ljubljana, Slovenia
3Department of Physics, Faculty of Mathematics and Physics, University of Ljubljana, Jadranska cesta 19, SI-1000 Ljubljana, Slovenia
* Corresponding author: E-mail: igor.krizaj@ijs.si,
Tel: +386 1 477 36 26, Fax: +386 1 477 39 84

To determine the possibility that calmodulin, a cytosolic regulatory protein, and ammodytoxin, a neurotoxic secreted phospholipase A2, interact in vivo, we studied the dependence of their association on Ca2+. The interaction between the two proteins was positively dependent on Ca2+, and greatest at millimolar concentrations of this ion. Importantly, they nteracted already in the presence of sub-micromolar concentrations of Ca2+, 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 Ca2+ 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 A2. 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.

Keywords: Secreted phospholipase A2, Ammodytoxin, Calmodulin, Neurotoxicity, Ca2+-dependence, Cytosol