FMRP drives mRNP targets into translationally silenced complexes.

Fragile X syndrome (FXS) results from a deficiency of the ubiquitously expressed RNA-binding protein fragile X protein (FMRP). While FMRP-mediated translational repression has been attributed primarily to ribosome stalling, using immunoprecipitations and polysome profiling of non-polar- and polar-cell lysates and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses, we show that mammalian FMRP largely represses translation initiation by associating with granule constituents to preclude 40S ribosomal subunit binding. We demonstrate that FMRP associates with its target mRNAs by binding directly to eukaryotic translation initiation factor 4E (eIF4E) at the 5' cap in competition with eIF4G1 and that ataxin-2-like promotes FMRP binding to the transcribed body. The KH1 + KH2 domains of FMRP are critical for the co-immunoprecipitation of eIF4E, mRNA targets, ataxin-2-like, and PABPC1. Our findings supplement FMRP-mediated ribosome-stalling data, suggesting that FMRP largely mediates the sequestration of its mRNA targets from translation initiation and degradation in a network of FMRP molecules that simultaneously associate with cap-bound eIF4E, GC-rich mRNA regions, and poly(A)-bound PABPC1.