Nuclear export is an important process that not only regulates the functions of cellular factors but also facilitates the assembly of viral nucleoprotein complexes. Chromosome region maintenance 1 (CRM1) that mediates the transport of proteins bearing the classical leucine-rich nuclear export signal (NES) is the best-characterized nuclear export receptor. Recently, several CRM1-independent nuclear export pathways were also identified. The nuclear export of the large form of hepatitis delta antigen (HDAg-L), a nucleocapsid protein of hepatitis delta virus (HDV), which contains a CRM1-independent proline-rich NES, is mediated by the host NES-interacting protein (NESI). The mechanism of the NESI protein in mediating nuclear export is still unknown. In this study, NESI was characterized as a highly glycosylated membrane protein.
Hepatitis delta virus (HDV) is a human pathogen associated with fulminant hepatitis and progressive chronic liver cirrhosis upon superinfection or coinfection with hepatitis B virus (HBV) (1). The HDV RNA genome contains only one open reading frame that encodes the viral structural protein, hepatitis delta antigen (HDAg). HDAg has two isoforms, small HDAg (HDAg-S; 195 amino acid [aa] residues) and large HDAg (HDAg-L; 214 amino acid residues). HDAg-S functions as a trans-activator in HDV replication (2, 3), whereas HDAg-L contains a 19-aminoacid C-terminal extension which is necessary for virion assembly (4-6). Both HDAg-S and HDAg-L possess nuclear localization signals (NLSs) and mainly localize in the nucleus. Nevertheless, the proline-rich nuclear export signal (NES), 198-ILFPADPPFSPQS-210, which is located at the C terminus of HDAg-L, triggers the nuclear export of HDAg-L essential for HDV morphogenesis (7).The nuclear-cytoplasmic transport of proteins and RNAs across the nuclear envelope occurs through channels formed by macromolecular structures known as nuclear pore complexes (NPCs) (8). The NPC in vertebrates contains about 50, or even more, different proteins. The NPC spans the dual membrane of the nuclear envelope and acts as a gateway for macromolecular trafficking. A general theme in the nuclear export of proteins is that specialized export receptors recognize cargoes harboring specific export signals. A well-known example is the recognition of the classical leucine-rich NES by the chromosome region maintenance 1 (CRM1) exportin (9-11).There are several NESs that contribute to CRM1-independent nuclear export pathways. The heterogeneous nuclear ribonucleoprotein (hnRNP) A1 and K proteins contain serine and acidic amino acid-rich NESs (12, 13). The HDAg-L protein (7) and the human cytomegalovirus transactivator protein pUL69 (14) both contain proline-rich NESs. However, the mechanisms of the involvement of these proteins in the CRM1-independent nuclear export pathways remain unclear.In our earlier study, a novel human NES-interacting protein, designated NESI, was identified. NESI protein interacts with the proline-rich NES of HDAg-L and plays a critical role in the assembly of HDV ...