We recently described an RNA-binding protein, Y14, that binds preferentiall
y to spliced mRNAs and persists in the cytoplasm, Y14 is part of a multi-pr
otein complex that also contains the mRNA export factor TAP. This suggests
that splicing imprints the mRNA with a unique set of proteins that communic
ate the history of the transcript to the cytoplasm, Here, using microinject
ion of pre-mRNAs into Xenopus oocyte nuclei followed by immunoprecipitation
of RNase-fragmented mRNAs from the cytoplasm, we show that Y14 is stably b
ound to sequences immediately upstream of exon-exon junctions. This feature
appears to be unique to Y14, Using monoclonal antibodies that we produced
against Aly/REF, another component recently reported to be an mRNA export f
actor, we show that Aly/REF is associated with spliced mRNAs in the nucleus
but is not detectable on mRNAs in the cytoplasm, Thus, we propose that the
splicing-dependent binding of Y14 provides a position-specific molecular m
emory that communicates to the cytoplasm the location of exon and intron bo
undaries. This novel mechanism may play an important role in post-splicing
events.