The Chironomus tentans translation initiation factor eIF4H is present in the nucleus but does not bind to mRNA until the mRNA reaches the cytoplasmic perinuclear region

Abstract

In the cell nucleus, precursors to mRNA, pre-mRNAs, associate with a large number of proteins and are processed to mRNA-protein complexes, mRNPs. The mRNPs are then exported to the cytoplasm and the mRNAs are translated into proteins. The mRNAs containing in-frame premature stop codons are recognized and degraded in the nonsense-mediated mRNA decay process. This mRNA surveillence may also occur in the nucleus and presumably involves components of the translation machinery. Several translation factors have been detected in the nucleus, but their functional relationship to the dynamic protein composition of pre-mRNPs and mRNPs in the nucleus is still unclear. Here, we have identified and characterized the translation initiation factor eIF4H in the dipteran Chironomus tentans. In the cytoplasm, Ct-eIF4H is associated with poly(A+) RNA in polysomes. We show that a minor fraction of Ct-eIF4H enters the nucleus. This fraction is independent on the level of transcription. CteIF4H could not be detected in gene-specific pre-mRNPs or mRNPs, nor in bulk mRNPs in the nucleus. Our immunoelectron microscopy data suggest that Ct-eIF4H associates with mRNP in the cytoplasmic perinuclear region, immediately as the mRNP exits from the nuclear pore complex.