Evidence for specific nucleocytoplasmic transport pathways used by leucine-rich nuclear export signals

Various proteins with different biological activities have been observed to be translocated from the nucleus to the cytoplasm in an energy- and signal -dependent manner in eukaryotic cells. This nuclear export is directed by n uclear export signals (NESs), typically characterized by hydrophobic, prima rily leucine, amino acid residues. Moreover, it has been shown that CRM1/ex portin I is an export receptor for leucine-rich NESs. However, additional N ES-interacting proteins have been described. In particular, eukaryotic init iation factor 5A (eIF-5A) has been shown to be a critical cellular cofactor for the nuclear export of the HIV type 1 (HIV-I) Rev trans-activator prote in. In this study we compared the nuclear export activity of NESs of differ ent origin. Microinjection of export substrates into the nucleus of somatic cells in combination with specific inhibitors indicated that specific nucl ear export pathways exist for different NES-containing proteins. In particu lar, inhibition of eIF-5A blocked the nuclear export of NESs derived from t he HIV-1 Rev and human T cell leukemia virus type I Rex transactivators, wh ereas nucleocytoplasmic translocation of the protein kinase inhibitor-NES w as unaffected. In contrast, however, inhibition of CRM/exportin 1 blocked t he nuclear export of all NES-containing proteins investigated. Our data con firm that CRM1/exportin 1 is a general export receptor for leucine-rich NES s and suggest that eIF-5A acts either upstream of CRM1/exportin 1 or forms a complex with the NES and CRM1/exportin 1 in the nucleocytoplasmic translo cation of the HIV-1 Rev and human T cell leukemia virus type I Rex RNA expo rt factors.