Translation is required to remove Y14 from mRNAs in the cytoplasm

Y14 is an RNA binding protein which is part of a multiprotein complex, the exon-exon junction complex (EJC), that assembles on the exon-exon junctions of mRNAs produced by splicing. The position-specific binding of Y14 persists on mRNAs after their export to the cytoplasm. Thus, Y14, together with its interacting proteins, has the capacity to communicate to the cytoplasm the processing history of the mRNA, including the position of the removed introns, information that is likely to be important for defining premature termination codons. How Y14 and other components of the EJC are removed from mRNAs into the cytoplasm has not been determined.

We show that Y14 but not another EJC component, Aly/REF, is present in polysome profile fractions containing one ribosome per mRNA. Using reporter constructs in an in vitro splicing/translation-coupled system, we show that Y14 remains associated with untranslated mRNAs but is removed from translationally active mRNAs. Importantly, mRNAs whose translation in vivo is prevented by the presence of strong secondary 5' UTR structure retain Y14 in the cytoplasm.

These findings indicate that Y14 remains associated with mRNAs in the cytoplasm until they are translated, and translation is required to remove Y14 from mRNAs. Thus, the process of translation removes the splicing-dependent EJC protein imprints, which most likely function in the surveillance of mRNAs to define premature termination codons and possibly also in modulating the translation activity of cytoplasmic mRNAs.