Fragile X Mental Retardation Protein Targets G Quartet mRNAs Important for Neuronal Function in proteins involved in many aspects of RNA metabolism including splicing factors and hnRNP proteins and are commonly found in conjunction with other RNA binding

RNA secondary structure, thereby allowing access to specific sequences by other RNA binding domains (Ghi-Summary et al., 2000b), the splicing factor SF1/BBP (Berglund et al., 1998), and hnRNPs K and E1/E2 (Ostareck et al., Loss of fragile X mental retardation protein (FMRP) function causes the fragile X mental retardation syn-1997; Thisted et al., 2001), the RNA specificity with which FMRP binds RNA has remained unexplored. This ques-drome. FMRP harbors three RNA binding domains, associates with polysomes, and is thought to regulate tion is of particular interest to the fragile X syndrome, since one severely affected individual harbors a single mRNA translation and/or localization, but the RNAs to which it binds are unknown. We have used RNA amino acid change (I304N) within the second KH domain of FMRP (DeBoulle et al., 1993). Some of the conse-selection to demonstrate that the FMRP RGG box binds intramolecular G quartets. This data allowed us quences of this mutation on FMRP, such as its inability to form a homodimer (Absher et al., 2001; Laggerbauer to identify mRNAs encoding proteins involved in syn-aptic or developmental neurobiology that harbor et al., 2001), failure of the mutant protein to interact with polyribosomes (Absher et al., 2001; Feng et al., 1997a), FMRP binding elements. The majority of these mRNAs have an altered polysome association in fragile X pa-and more rapid protein shuttling between the nucleocy-toplasmic space (Tamanini et al., 1999), have been tient cells. These data demonstrate that G quartets serve as physiologically relevant targets for FMRP and noted. Cocrystallization of the Nova KH3 domain with its RNA target predicts that the mutated isoleucine in identify mRNAs whose dysregulation may underlie human mental retardation. FMRP (I304N) lies at the core of the hydrophobic RNA binding pocket of the KH domain (Lewis et al., 2000). These observations suggest that identification of the Introduction RNA targets to which FMRP binds will yield biologically important insight into its function. The fragile X syndrome is the most common form of inherited mental retardation, manifested by mild to mod-The role of FMRP as an RNA binding protein has been most clearly established by the demonstration that erate cognitive and behavioral abnormalities accompanied by macroorchidism and subtle craniofacial dysmor-‫%58ف‬ of cellular FMRP is present on actively translating polyribosomes (Feng et al., 1997a). Interestingly, the phia. The syndrome usually results from the expansion and hypermethylation of a CGG repeat in the 5Ј UTR of I304N mutant …