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J. Biol. Chem., Vol. 276, Issue 16, 12493-12496, April 20, 2001

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ACCELERATED PUBLICATION
A High Affinity Acceptor for Phospholipase A₂ with Neurotoxic Activity Is a Calmodulin^*

Jernej ribar^§, Alenka opi^§, Alenka Pari, Nicholas E. Sherman^¶, Franc Gubenek, Jay W. Fox^¶, and Igor Kriaj^**

From the  Department of Biochemistry and Molecular Biology, Joef Stefan Institute, Jamova 39, Slovenia, the  Department of Chemistry and Biochemistry, Faculty of Chemistry and Chemical Technology, Akereva 5, University of Ljubljana, 1000 Ljubljana, Slovenia, and the ^¶ W. M. Keck Biomedical Mass Spectrometry Laboratory and the University of Virginia Biomedical Research Facility, University of Virginia Medical School, Charlottesville, Virginia 22908

One of the high affinity binding proteins for ammodytoxin C, a snake venom presynaptically neurotoxic phospholipase A₂, has been purified from porcine cerebral cortex and characterized. After extraction from the membranes, the toxin-binding protein was isolated in a homogenous form using wheat germ lectin-Sepharose, Q-Sepharose, and ammodytoxin-CH-Sepharose chromatography. The specific binding of ¹²⁵I-ammodytoxin C to the isolated acceptor was inhibited to different extents by some neurotoxic phospholipases A₂, ammodytoxins, bee venom phospholipase A₂, agkistrodotoxin, and crotoxin; but not by nontoxic phospholipases A₂, ammodytin I₂, porcine pancreatic phospholipase A₂, and human type IIA phospholipase A₂; suggesting the significance of the acceptor in the mechanism of phospholipase A₂ neurotoxicity. The isolated acceptor was identified as calmodulin by tandem mass spectrometry. Since calmodulin is generally considered as an intracellular protein, the identity of this acceptor supports the view that secretory phospholipase A₂ neurotoxins have to be internalized to exert their toxic effect. Moreover, since ammodytoxin is known to block synaptic transmission, its interaction with calmodulin as an acceptor may constitute a valuable probe for further investigation of the role of the latter in this Ca²⁺-regulated process.

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