Localization and subcellular distribution of N-copine in mouse brain

N-Copine is a novel protein with two C2 domains. Its expression is brain sp ecific and up-regulated by neuronal activity such as kainate stimulation an d tetanus stimulation evoking hippocampal CA1 long-term potentiation. We ex amined the localization and subcellular distribution of N-copine in mouse b rain. In situ hybridization analysis showed that N-copine mRNA was expresse d exclusively in neurons of the hippocampus and in the main and accessory o lfactory bulb, where various forms of synaptic plasticity and memory format ion are known to occur. In immunohistochemical analyses, N-copine was detec ted mainly in the cell bodies and dendrites in the neurons, whereas presyna ptic proteins such as synaptotagmin I and rab3A were detected in the region s where axons pass through. In fractionation experiments of brain homogenat e, N-copine was associated with the membrane fraction in the presence of Ca 2+ but not in its absence. As a GST-fusion protein with the second C2 domai n of N-copine showed Ca2+-dependent binding to phosphatidylserine, this dom ain was considered to be responsible for the Ca2+-dependent association of N-copine with the membrane. Thus, N-copine may have a role as a Ca2+ sensor in postsynaptic events, in contrast to the known roles of "double C2 domai n-containing proteins," including synaptotagmin I, in presynaptic events.