FMRP drives mRNP targets into translationally silenced complexes

Fragile X Syndrome (FXS) is due to a deficiency in the ubiquitously expressed RNA-binding protein FMRP. While the mechanism of FMRP-mediated translational repression has been attributed primarily to ribosome stalling, we show using immunoprecipitations and polysome profiling of nonpolar- and polar-cell lysates, together with LC-MS/MS analyses, that FMRP largely represses translation initiation by associating with granule constituents to preclude 40S ribosomal subunit binding. We show that FMRP associates with its target mRNAs by binding directly to eIF4E at the 5'-cap in competition with eIF4G1 binding to eIF4E, and that Ataxin-2-Like promotes FMRP binding to the transcribed body. Together with previous data demonstrating that FMRP binds directly to 3'-poly(A)-bound PABPC1, FMRP protects its target mRNAs from translation and decay. 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.