REGULATION BY BIVALENT-CATIONS OF PHOSPHOLIPID-BINDING TO THE C2A DOMAIN OF SYNAPTOTAGMIN-III

Synaptotagmins are Ca2+-and phospholipid-binding proteins of synaptic vesicles that might function as Ca2+ receptors for neurotransmitter re lease via their first C2 (C2A) domain. Here we describe the effect of Mg2+ on phospholipid binding to the C2A domains of multiple synaptotag mins (II-VI), and demonstrate that only synaptotagmin III can bind neg atively charged phospholipids [phosphatidylserine (PS) and phosphatidy linositol] in a Mg2+-dependent manner. The Mg2+-dependent interaction with PS was found to have an EC50 of approx. 30 mu M Mg2+, which is co mparable to that of Sr2+ and Ba2+ (EC50 values of approx. 10 mu M). Th is binding property of the C2A domain is specific to synaptotagmin III , because none of the C2A domains of other proteins, such as rabphilin 3A, Doc2 alpha, Doc2 beta or Gap 1(m), showed phospholipid binding ac tivity in the presence of 1 mM Mg2+. Our results suggest that synaptot agmin III is involved in presynaptic functions different from those of synaptotagmins I and II.