title: Novel isoforms of the fragile X related protein FXR1P are expressed during myogenesis.
abstract: 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 P78and P70isoforms are associated with mRNPs within translating polyribosomes, similarly to their closely related FMRP homologues. In muscle tissues as well as in murine myoblastic cell lines induced to differentiate into myotubes, FXR1P78and P70isoforms are replaced by novel unpredicted isoforms of 81-84 kDa and a novel FXR1 exon splice variant was detected in muscle RNA. While P81-84isoforms expressed after fusion into myotubes in murine myoblast cell lines grown in culture are associated with polyribosomes, this is not the case when isolated from muscle tissues since they sediment with lower S values. Immunohistochemical studies showed coexpression of FMRP and FXR1P70and P78in the cytoplasm of brain neurons, while in muscle no FMRP was detected and FXR1P81-84were mainly localized to structures within the muscle contractile bands. The complex expression pattern of FXR1P suggests tissue-specific expression for the various isoforms of FXR1 and the differential expression of FMRP and FXR1Ps suggests that in certain types of cells and tissues, complementary functions may be fulfilled by the various FMRP family members.
authors.affiliation: Unité de recherche en génétique humaine et moléculaire, Pavillon Saint François d'Assise du CHUQ, Département de biologie médicale, Faculté de médecine, Université Laval, Québec, Canada. edward.khandjian@crsfa.ulaval.ca
authors.forename: E W
authors.initials: EW
authors.lastname: Khandjian
authors.forename: B
authors.initials: B
authors.lastname: Bardoni
authors.forename: F
authors.initials: F
authors.lastname: Corbin
authors.forename: A
authors.initials: A
authors.lastname: Sittler
authors.forename: S
authors.initials: S
authors.lastname: Giroux
authors.forename: D
authors.initials: D
authors.lastname: Heitz
authors.forename: S
authors.initials: S
authors.lastname: Tremblay
authors.forename: C
authors.initials: C
authors.lastname: Pinset
authors.forename: D
authors.initials: D
authors.lastname: Montarras
authors.forename: F
authors.initials: F
authors.lastname: Rousseau
authors.forename: J
authors.initials: J
authors.lastname: Mandel
mesh_terms: D016475:3T3 Cells; D017398:Alternative Splicing; D000818:Animals; D001483:Base Sequence; D001921:Brain / Q000378:metabolism; D019556:COS Cells; D002460:Cell Line; D005591:Chemical Fractionation; D051860:Fragile X Messenger Ribonucleoprotein 1; D005600:Fragile X Syndrome / Q000235:genetics / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D007150:Immunohistochemistry; D051379:Mice; D008969:Molecular Sequence Data; D024510:Muscle Development; D009132:Muscles / Q000737:chemistry / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism; D011132:Polyribosomes / Q000378:metabolism; D012313:RNA / Q000235:genetics; D016601:RNA-Binding Proteins / Q000032:analysis / Q000235:genetics* / Q000378:metabolism; D012261:Ribonucleoproteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D051860:Fragile X Messenger Ribonucleoprotein 1; D009419:Nerve Tissue Proteins; D012313:RNA; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins; C497683:FMR1 protein, human; C096916:FXR1 protein, human; C096915:FXR2 protein, human; C497684:Fmr1 protein, mouse