The 5' cap structure m7GpppN (where N is any nucleotide) is a ubiquitous feature of cellular eukaryotic mRNAs. The cap is multifunctional as it is involved in translation, nucleocytoplasmic transport, splicing, and stabilization of mRNA against 5' exonucleolytic degradation. The cap binding protein, eukaryotic initiation factor 4E (eIF-4E), is a translation initiation factor that binds to the cap structure and is part of a complex (eIF-4F) that promotes mRNA binding to ribosomes. Overexpression of eIF-4E in fibroblasts results in cell. transformation. To test the hypothesis that some of the biological effects of eIF-4E might be effected by a nuclear function, we determined the cellular distribution of eIF-4E. By means of indirect immunofluorescence experiments using polyclonal and monoclonal antibodies against eIF-4E as well as transfected epitope-tagged eIF-4E, we demonstrate that a fraction ofeIF-4E localizes to the nucleus. These results suggest that eIF-4E is also involved in a nuclear function.The cap structure of eukaryotic mRNAs is added early during the transcription of RNA polymerase II genes in the nucleus (1) and plays an important role in several cytoplasmic and nuclear biochemical processes: (i) translational initiation (2), (ii) precursor mRNA splicing (3, 4), (iii) mRNA 3'-end processing (5, 6), (iv) mRNA nucleocytoplasmic transport (7), and (v) protection of mRNA against 5' exonucleolytic degradation in the cytoplasm and nucleus (8). Several proteins that recognize and bind the cap have been identified in the cytoplasm and the nucleus. These proteins are likely to mediate the disparate functions of the cap. Indeed, a large body of evidence shows that a polypeptide of 24 kDa, termed eukaryotic initiation factor 4E (eIF-4E), mediates cap function during translation initiation (9, 10). However, relatively little is known about the proteins that function in capdependent nuclear processes. Several nuclear cap-binding proteins have been identified by UV-induced cross-linking to the cap structure. These include polypeptides of 20 and 115 kDa (11) as well as 80, 89, and 120 kDa (12). More recently, an 80-kDa cap-binding protein has been purified from HeLa nuclear extracts (13). The function(s) of the nuclear capbinding proteins is not known.eIF-4E was initially identified and purified from the highsalt wash of ribosomes prepared from rabbit reticulocyte lysate (14, 15) and subsequently was detected in the postribosomal supernatant (16). It has been purified from different species including plants (17,18), yeast (19), and Drosophila (20). eIF4E can be purified as a single polypeptide or as part of a complex (termed eIF4F) with one or two other polypeptides. In all species, a subunit of 150-220 kDa is associated with eIF-4E in the complex (17,(21)(22)(23)(24) (30). In addition, eIF-4E cooperates with nuclear oncogenes such as c-myc or EJA to transform primary cells (31). The mechanism by which eIF-4E transforms cells in culture is not clear. To test the hypothesis that eIF-4E might have nuclear fu...
