Adenomatous polyposis coli protein (APC) is highly expressed in the nervous tissue, but its function there is not yet known. We previously found that the microtubule-bundling activity of APC is stimulated by its interaction with PSD-95, a neuronal scaffolding protein, in cultured COS-7 cells. In the present study, we investigated the distribution and localization of the intrinsic APC and PSD-95 in both cultured rat hippocampal neurons and in the rat cerebellum by immunofl uorescence and immunoelectron microscopy. In cultured neurons, most of the PSD-95 immunofl uorescence puncta were colocalized with APC, and the APC and PSD-95 immunogolds were colocalized in the nerve fi bers as well as in the postsynaptic site, but not in the presynaptic site. In the molecular layer of the rat cerebellum, colocalization of APC and PSD-95 was also detected in the nerve fi bers and the postsynaptic site, but not in the presynaptic site. Based on these results, we conclude that APC and PSD-95 colocalize and bind to form a protein complex in nerve fi bers as well as in postsynaptic sites in matured neurons, suggesting the involvement of the APC/ PSD-95 complex in the microtubule functions within the nerve fi bers and the synapse functions at the postsynaptic site.
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