ABSTRACT
Alternative
splicing of fibroblast growth factor receptor 2 (FGFR2) occurs in a
cell-type-specific manner with the mutually exclusive use of exon IIIb
or exon IIIc. Specific inclusion of exon IIIb is observed in epithelial
cells, whereas exon IIIc inclusion is seen in mesenchymal cells.
Epithelium-specific activation of exon IIIb and repression of exon IIIc
are coordinately regulated by intronic activating sequence 2 (IAS2) and
intronic splicing activator and repressor (ISAR) elements in FGFR2
pre-mRNA. Previously, it has been suggested that IAS2 and a
20-nucleotide core sequence of ISAR form a stem structure
that allows for the proper regulation of FGFR2 alternative splicing.
Replacement of IAS2 and the ISAR core with random sequences
capable of stem formation resulted in the proper activation of exon
IIIb and repression of exon IIIc in epithelial cells. Given the high
degree of phylogenetic conservation of the IAS2-ISAR core structure and
the fact that unrelated stem-forming sequences could functionally
substitute for IAS2 and ISAR elements, we postulated that the stem
structure facilitated the approximation of intronic control elements.
Indeed, deletion of the entire stem-loop region and juxtaposition of
sequences immediately upstream of IAS2 with sequences immediately
downstream of the ISAR core maintained proper cell-type-specific
inclusion of exon IIIb. These data demonstrate that IAS2 and the ISAR
core are dispensable for the cell-type-specific activation of exon
IIIb; thus, the major, if not the sole, role of the IAS2-ISAR stem in
exon IIIb activation is to approximate sequences upstream of IAS2 with
sequences downstream of the ISAR core. The downstream sequence is very
likely a highly conserved GCAUG element, which we show was required for
efficient exon IIIb
activation.
Association of the Signaling Adaptor FRS2 with Fibroblast Growth Factor Receptor 1 (Fgfr1) Is Mediated by Alternative Splicing of the Juxtamembrane Domain
