A new regulatory function of the region proximal to the RGG box in the fragile X mental retardation protein.

Fragile X mental retardation protein (FMRP) is required for normal cognition. FMRP has two autosomal paralogs, which although similar to FMRP, cannot compensate for the loss of FMRP expression in brain. The arginine- and glycine-rich region of FMRP (the RGG box) is unique; it is the high-affinity RNA-binding motif in FMRP and is encoded by exon 15. Alternative splicing occurs in the 5' end of exon 15, which is predicted to affect the structure of the distally encoded RGG box. Here, we provide evidence that isoform 3, which removes 25 amino acids from the 5' end of exon 15, has an altered conformation that reduces binding of a specific antibody and renders the RGG box unable to efficiently associate with polyribosomes. Isoform 3 is also compromised in its ability to form granules and to associate with a key messenger ribonucleoprotein Yb1 (also known as p50, NSEP1 and YBX1). Significantly, these functions are similarly compromised when the RGG box is absent from FMRP, suggesting an important regulatory role of the N-terminal region encoded by exon 15.