The fragile X mental retardation protein inhibits translation via interacting with mRNA.

Fragile X syndrome is a frequent form of inherited mental retardation caused by functional loss of the fragile X mental retardation protein, FMRP. The function of FMRP is unknown, as is the mechanism by which its loss leads to cognitive deficits. Recent studies have determined that FMRP is a selective RNA-binding protein associated with polyribosomes, leading to the hypothesis that FMRP may be involved in translational regulation. Here we show that purified recombinant FMRP causes a dose-dependent translational inhibition of brain poly(A) RNA in rabbit reticulocyte lysate without accelerated mRNA degradation. In our translation reaction FMRP interacts with other messenger ribonucleoproteins and pre-exposure of FMRP to mRNA significantly increased the potency of FMRP as a translation inhibitor. Translation suppression by FMRP is reversed in a trans-acting manner by the 3'-untranslated portion of the Fmr1 message, which binds FMRP, suggesting that FMRP inhibits translation via interacting with mRNA. Consistently FMRP suppresses translation of the parathyroid hormone transcript, which binds FMRP, but not the beta-globin transcript, which does not bind FMRP. Moreover, removing the FMRP-binding site on a translation template abolishes the inhibitory effect of FMRP. Taken together, our results support the hypothesis that FMRP inhibits translation via interactions with the translation template.