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The fragile X mental retardation protein is a ribonucleoprotein containing both nuclear localization and nuclear
D E Eberhart1, H E Malter, Y Feng
1Howard Hughes Medical Institute, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
Abstract:
Fragile X syndrome is a frequent cause of mental retardation resulting from the absence of FMRP, the protein encoded by the FMR1 gene. FMRP is an RNA-binding protein of unknown function which is associated with ribosomes. To gain insight into FMRP function, we performed immunolocalization analysis of FMRP truncation and fusion constructs which revealed a nuclear localization signal (NLS) in the amino terminus of FMRP as well as a nuclear export signal (NES) encoded by exon 14. A 17 amino acid peptide containing the FMRP NES, which closely resembles the NES motifs recently described for HIV-1 Rev and PKI, is sufficient to direct nuclear export of a microinjected protein conjugate. Sucrose gradient analysis shows that FMRP ribosome association is RNA-dependent and FMRP is found in ribonucleoprotein (RNP) particles following EDTA treatment. These data are consistent with nascent FMRP entering the nucleus to assemble into mRNP particles prior to export back into the cytoplasm and suggests that fragile X syndrome may result from altered translation of transcripts which normally bind to FMRP.
Insights
Fragile X syndrome, caused by FMR1 gene absence, stems from FMRP protein dysfunction. Research reveals FMRP
Area of Science:
- Molecular Biology
- Neuroscience
- Genetics
Background:
- Fragile X syndrome is a common genetic cause of intellectual disability.
- It results from the absence of the Fragile X mental retardation protein (FMRP).
- FMRP's function and cellular localization were previously unclear.
Purpose of the Study:
- To investigate the function and localization of FMRP.
- To understand the cellular mechanisms underlying Fragile X syndrome.
Main Methods:
- Immunolocalization analysis of FMRP constructs.
- Nuclear localization signal (NLS) and nuclear export signal (NES) identification.
- Peptide-based nuclear export assays.
- Sucrose gradient analysis of FMRP-ribonucleoprotein (RNP) interactions.
Main Results:
- FMRP possesses both an NLS in its amino terminus and an NES in exon 14.
- A specific FMRP NES peptide can mediate nuclear export.
- FMRP associates with ribosomes in an RNA-dependent manner, forming RNP particles.
- FMRP is found in nuclear mRNP particles before cytoplasmic export.
Conclusions:
- Nascent FMRP translocates from the nucleus to the cytoplasm, assembling into mRNP particles.
- Fragile X syndrome may arise from disrupted translation of transcripts bound by FMRP.
- Understanding FMRP's nucleocytoplasmic transport is crucial for Fragile X syndrome research.