Subcellular localization of fragile X mental retardation protein with the I304N mutation in the RNA-binding domain in cultured hippocampal neurons.

1. Fragile X syndrome, the most common form of inherited mental retardation, is caused by the lack or dysfunction of fragile X mental retardation protein (FMRP). The 1304N mutation in the RNA-binding domain of FMRP results in an exceptionally severe form of mental retardation. 2. We have investigated the subcellular localization of FMRP and its 1304N-mutated form in cultured hippocampal neurons and PC12 cells, using immunofluorescence microscopy. In PC12 cells, FMRP was predominantly localized to the cytoplasm and also to the processes after differentiation by NGF. 3. In cultured hippocampal neurons, granular labeling was detected along the neuronal processes. 4. Double-labeling with synaptophysin antibody revealed FMRP at synaptic sites in neurons. 5. The 1304N mutation did not appear to affect the transport of FMRP to dendrites or its localization at synaptic sites. Thus, FMRP is a synaptic protein and the severe phenotype observed in the patient with the 1304N mutation is not produced by alterations in dendritic transport.