Fragile X syndrome is caused by the absence of the fragile X
mental-retardation protein (FMRP). FMRP and the fragile X-related proteins
1 and 2 (FXR1P and FXR2P) form a gene family with functional similarities,
such as RNA binding, polyribosomal association and nucleocytoplasmic
shuttling. In a previous study, we found that FMRP and FXR1P shuttle
between cytoplasm and nucleoplasm, while FXR2P shuttles between cytoplasm
and nucleolus. The nuclear and nucleolar-targeting properties of these
proteins were investigated further. Here, we show that FXR2P contains in
its C-terminal part, a stretch of basic amino acids 'RPQRRNRSRRRRFR' that
resemble the nucleolar-targeting signal (NoS) of the viral protein Rev.
This particular sequence is also present within exon 15 of the FXR1 gene.
This exon undergoes alternative splicing and is therefore only present in
some of the FXR1P isoforms. We investigated the intracellular distribution
of various FXR1P isoforms with (iso-e and iso-f) and without (iso-d) the
potential NoS in transfected COS cells treated with the nuclear export
inhibitor leptomycin-B. Both iso-e and iso-f showed a nucleolar
localization, as observed for FXR2P; iso-d was detected in the
nucleo-plasm outside the nucleoli. Further, when a labelled 16-residue
synthetic peptide corresponding to the NoS of FXR1P was added to human
fibroblast cultures a clear nucleolar signal was observed. Based on these
data we argue that the intranuclear distribution of FXR2P and FXR1P
isoforms is very likely to be mediated by a similar NoS localized in their
C-terminal region. This domain is absent in some FXR1P isoforms as well as
in all FMRP isoforms, suggesting functional differences for this family of
proteins, possibly related to RNA metabolism in different tissues