like ISO1 and ISO7 (13,30). In addition, ISO 18 is able to bind the 30-UTR of FMR1 mRNA (36), that, conversely, is not bound by ISO1 and ISO7 (13,30). In adult muscle, where both FMRP and FXR2P are absent, the FXR1P isoforms encountered both contain exon 15 sequences and correspond to a doublet of 82 (Isog) and 84 kDa (Isoe) (20,21,37). Their ability to bind G-quartet RNA structure suggests that, in this tissue, speciﬁc RNAs might be recognized by FXR1P via the interaction with G-quartet forming structure. Since FXR1P absence has a strong impact during muscle development (8,9), its RNA binding capacities are critical per se, independently from FMRP’s fonction. Indeed, a recent knock-down analysis for xFxr1 produced a list of putative FXR1P target RNAs. Interestingly, several of their human homologues harbor a putative G-quartet structure [(9), and our unpublished data]. In addition, very little is known concerning the precise function of FXR1P in muscle, and it is possible that muscle- speciﬁc FXR1P interacting proteins might modulate its afﬁnity for RNA. Our analysis to dissect the binding capacities to G-quartet RNA FXR1P and its heterodimer with FMRP yielded unexpected ﬁndings. We observed a dramatic effect of the FMRP/FXR1P heterodimer on the dynamics of complex formation with G-quartet RNA. This effect was not observed when FMRP and its other partner MSP58 were mixed together, suggesting that MSP58 can probably compete for the same binding site as FMRP. In addition, the interaction of FMRP with FXR1P-Isoa or Isod strongly reduced FMRP speciﬁcity for G-quartet RNA. These different behaviors of the two FMRP-interacting proteins illustrate the complexity of the functions and interactions that take place in FMRP- containing mRNPs and in different tissues. FXR1P-Isod and Isoa are the FXR1P isoforms with the highest expression in brain, suggesting that in neurons FMRP interacts mostly with these two isoforms that might regulate negatively its action. Since in brain and cerebellum FXR1P-Isoe mRNA is expressed at a low level as revealed by RT–PCR (our unpublished data), probably only a very small portion of FMRP may be regulated by FXR1P-Isoe. Conversely, FMRP and FXR1P-Isoe and Isog isoforms are co-expressed in myoblasts and in myotubes, suggesting a particular regula- tion of G-quartet containing target mRNAs during muscle differentiation but not in adult muscle, where FMRP is not expressed anymore (21). The present study highlights the functional differences between FXR1P isoforms and there- fore emphasizes the importance of the extensive tissue- speciﬁc alternative splicing undergone by FXR1 mRNA. In view of these results, it is clear that in each mRNP the ratio between FMRP and FXR1P different isoforms becomes important to precisely regulate FMRP function. The modula- tion of the afﬁnity and/or of the dynamics observed for the FXR1P/FMRP heterodimer may reﬂect a regulation of the exchange of mRNAs between mRNPs or trafﬁcking granules and polyribosomes. The interaction domain of the two FXR proteins is local- ized in the N-terminal region of both proteins. This domain mediates the interaction between FMRP and several other proteins (FXR2P, CYFIP1, CYFIP2, NUFIP and 82-FIP) (10,33). On the other hand, despite the high level of homo- logy, the N-terminal region of FXR1P seems to interact Nucleic Acids Research, 2007, Vol. 35, No. 1 305 only with CYFIP2 and FXR2P (18). CYFIP2, together with CYFIP1, that only interacts with FMRP, belongs to a small family of proteins linking FMRP to the Rac pathway (18,33,38). We have previously proposed that the CYFIP pro- teins might modulate the ability of the FXR family members to homo and/or heterodimerize (18). FXR1P and FXR2P are believed to have distincts but overlapping function in conjunction with FMRP, with the possibility to partially compensate for its absence. Our results show here a completely different function for two different FXR1P isoforms, which modulate the action of FMRP. This data reveals how a full understanding of FMRP function may be achieved through the deciphering of the global action of FMRP-containing mRNP complexes. ACKNOWLEDGEMENTS The authors are grateful to Alexandra Charlesworth for help. B.B. is supported by CNRS, Fondation J. Lejeune, GIS Maladies Rares, Fondation pour la Recherche Me´dicale, FRAXA Foundation and ‘Contrat d’interface’ from INSERM and CHU de Nice. E.B. is recipient of a fellowship from Fondation pour la Recherche Me´dicale. E.L. is supported by CNRS, Fondation pour la Recherche Me´dicale and ARC. L.D. is supported by the FRAXA Foundation and by a short term fellowship from the Boeringher Ingelheim Fund and the Conquer Fragile X Foundation. E.W.K. is supported by the NSERC. The authors also thank the support of the French Embassy in Canada for their Travelling Exchange Programm. Funding to pay the Open Access publication charges for this article was provided by CNRS. Conflict of interest statement. None declared. REFERENCES 1. Bardoni,B., Mandel,J.L. and Fisch,G.S. (2000) FMR1 gene and fragile X syndrome. Am. J. Med. Genet., 97, 153–163. 2. Bardoni,B., Davidovic,L., Bensaid,M. and Khandjian,E.W. (2006) The fragile X syndrome: exploring its molecular basis and seeking a treatment. Expert Rev. Mol. Med., 8, 1–16. 3. Siomi,M.C., Siomi,H., Sauer,W.H., Srinivasan,S., Nussbaum,R.L. and Dreyfuss,G. (1995) FXR1, an autosomal homolog of the fragile X mental retardation gene. EMBO J., 14, 2401–2408. 4. 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