FMRP regulates miR196a-mediated repression of HOXB8 via interaction with the AGO2 MID domain.

Fragile X syndrome (FXS) is caused by the loss of expression of fragile X mental retardation protein (FMRP), a selective RNA-binding protein that negatively regulates mRNA substrates. FMRP can regulate the translation via the cross-talk with the miRNA machinery, but the functional association among FMRP, miRNAs and mutual target mRNAs has rarely been studied. In this research, we find that HOXB8 mRNA is a target of FMRP associated with miR-196a-induced silencing, and discover that phosphorylation of FMRP promotes the miR-196a-mediated repression of HOXB8 without affecting the interaction between FMRP and mRNA. We further identify that the FMRP-binding site involved in the miR-196a-mediated repression of HOXB8 locates in the downstream neighbourhood of the miR-196a recognition element in the 3'UTR of HOXB8. Importantly, we reveal that FMRP faces toward the MID domain of AGO2 and interacts with a specific binding pocket (coordination with T544, K533 and K570) in the domain. Our research might provide new insights into both the cross-talk between FMRP and miRNA-mediated regulation of mRNA translation and the molecular pathogenesis of FXS.