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 ribosome subunit binding. We show that FMRP binds directly to eIF4E at the 5'-cap of its target mRNAs in competition with eIF4G1 binding to eIF4E, and that Ataxin-2-Like promotes FMRP binding to its target mRNAs, thereby promoting their protection from translation and decay. The KH1+KH2 domains of FMRP are critical for the co-immunoprecipitation of eIF4E, mRNA targets, PABPC1 and Ataxin-2-Like. Our findings supplement FMRP-mediated ribosome-stalling data, demonstrating that FMRP largely mediates the sequestration of its mRNA targets from translation initiation and degradation in a network of FMRP simultaneously associating with cap-bound eIF4E, GC-rich mRNA regions, and poly(A)-bound PABPC1.