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Synaptotagmin I (syt I) is an abundant integral membrane protein of the synaptic vesicle and the C2A domain is an important functional domain in the cytoplasmic part of syt I. C2A prefers to interact with plasmic membranes of neuron cells in vivo and such interaction is closely related to the syt I physiological function as a Ca2+ sensor in the Ca2+-regulated neurotransmitter release. However, the interaction nature between C2A and phospholipids is not well understood. Monolayers at an air /water interface were used to study the interaction between C2A and a phospholipid membrane. The results show that C2A preferentially inserts into the negatively charged phosphatidylserine monolayer and Ca2+ ions are required for the interaction. Electrostatic force is mostly responsible for the insertion of C2A into dipalmitoyl phosphatidylserine monolayers.