Identification of a minimal segment of complexin II essential for preferential distribution in axons.

Complexin II (CPLX2) is a soluble pre-synaptic protein believed to regulate neurotransmitter release from pre-synaptic terminals. CPLX2 is localized in pre-synaptic terminals in mature brain, but the mechanism of selective localization remains unclear. Here we identified an essential segment of CPLX2 for preferential axonal distribution. Myc-tagged CPLX2 was expressed in cultured rat hippocampal neurons and its distribution between axons and dendrites was compared by immunocytochemistry and image analysis. Fluorescence signals were detected in both axons and dendrites; however, their respective distribution varied significantly. Despite the fact that signal intensity decreased almost linearly from the base to the tip of the dendrite, a substantial level was sustained along the axon, even at a position near the tip. Image analyses using a series of mutants indicated that the deletion of 19 amino acid residues, G71-P89, within the 'central core' for binding to soluble N-ethylmaleimide sensitive factor attachment protein receptor proteins resulted in the loss of preferential axonal distribution. The enhanced green fluorescent protein derivative fused with the G71-P89 fragment exhibited a similar localization to that of wild type CPLX2, indicating that the G71-P89 region of CPLX2 is essential and sufficient for preferential axonal distribution.