Cofactor requirements for nuclear export of Rev response element (RRE)- and constitutive transport element (CTE)-containing retroviral RNAs: An unexpected role for actin

Nuclear export of proteins containing leucine-rich nuclear export signals ( NESs) is mediated by the export receptor CRM1/exportin1. However, additiona l protein factors interacting with leucine-rich NESs have been described. H ere, we investigate human immunodeficiency virus type 1 (HIV-1) Rev-mediate d nuclear export and Mason-Pfizer monkey virus (MPMV) constitutive transpor t element (CTE)-mediated nuclear export in microinjected Xenopus laevis ooc ytes. We show that eukaryotic initiation factor 5A (eIF-5A) is essential fo r Rev and Rev-mediated viral RNA export, but not for nuclear export of CTE RNA. In vitro binding studies demonstrate that eIF-5A is required for effic ient interaction of Rev-NES with CRM1/exportinl and that eIF-5A interacts w ith the nucleoporins CAN/nup214. nup153, nup98, and nup62. Quite unexpected ly, nuclear actin was also identified as an eIF-5A binding protein. We show that actin is associated with the nucleoplasmic filaments of nuclear pore complexes and is critically involved in export processes. Finally, actin- a nd energy-dependent nuclear export of HIV-1 Rev is reconstituted by using a novel in vitro egg extract system. In summary, our data provide evidence t hat actin plays an important functional role in nuclear export not only of retroviral RNAs but also of host proteins such as protein kinase inhibitor (PKI).