Novel isoforms of the fragile X related protein FXR1P are expressed during myogenesis

The fragile X syndrome results from transcriptional silencing of the FMR1 gene and the absence of its encoded FMRP protein. Two autosomal homologues of the FMR1 gene, FXR1 and FXR2, have been identified and the overall structures of the corresponding proteins are very similar to that of FMRP. Using antibodies raised against FXR1P, we observed that two major protein isoforms of relative MW of 78 and 70 kDa are expressed in different mammalian cell lines and in the majority of mouse tissues. In mammalian cells grown in culture as well as in brain extracts, both P78 and P70 isoforms are associated with mRNPs within translating poly-ribosomes, similarly to their closely related FMRP homologues. In muscle tissues as well as in murine myoblastic cell lines induced to differentiate int