Phosphorylation influences the translation state of FMRP-associated polyribosomes

Area of Science:

• Neuroscience
• Molecular Biology
• Genetics

Background:

• Fragile X syndrome is a common cause of mental retardation, characterized by the absence of Fragile X mental retardation protein (FMRP).
• FMRP, an RNA-binding protein, is crucial for neuronal development and function, primarily by regulating mRNA translation.
• The precise regulatory mechanisms governing FMRP activity remain largely unknown.

Purpose of the Study:

• To investigate the post-translational modifications of FMRP, specifically phosphorylation.
• To determine the functional consequences of FMRP phosphorylation on its interaction with mRNAs and polyribosomes.
• To elucidate the role of FMRP phosphorylation in the context of fragile X syndrome pathogenesis.

Main Methods:

• Mass spectrometry was employed to identify phosphorylation sites on FMRP.
• Site-directed mutagenesis was used to study the impact of specific phosphorylation events.
• Cellular and biochemical assays were performed to analyze FMRP association with polyribosomes and mRNAs in vivo.

Main Results:

• FMRP is phosphorylated on serine 499 and nearby residues in both mouse brain and cultured cells.
• Phosphorylation occurs within hours of protein synthesis but does not affect FMRP half-life.
• Unlike its Drosophila ortholog, mammalian FMRP phosphorylation does not alter mRNA binding but influences its association with polyribosomes, with unphosphorylated FMRP on active polysomes and phosphorylated FMRP on stalled polysomes.

Conclusions:

• FMRP phosphorylation is a key regulatory mechanism impacting its function in translational control.
• The phosphorylation status of FMRP influences its localization on translating and stalled polyribosomes.
• Dephosphorylation may be involved in releasing FMRP-mediated translational suppression, offering potential therapeutic targets for fragile X syndrome.