Human Molecular Genetics, Vol 7, 2121-2128, Copyright © 1998 by Oxford University Press
EW Khandjian, B Bardoni, F Corbin, A Sittler, S Giroux, D Heitz, S Tremblay, C Pinset, D Montarras, F Rousseau and J Mandel
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.
ARTICLES
Novel isoforms of the fragile X related protein FXR1P are expressed during myogenesis
Unite de recherche en genetique humaine et moleculaire, Pavillon Saint Francois d'Assise du CHUQ, Departement de biologie medicale, Faculte de medecine, Universite Laval, Quebec, Canada. edward.khandjian@crsfa.ulaval.ca
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