Nuclear Export Signal-Interacting Protein Forms Complexes with Lamin A/C-Nups To Mediate the CRM1-Independent Nuclear Export of Large Hepatitis Delta Antigen

Abstract

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 ...

Dual roles of the N‐terminal coiled‐coil domain of an Aplysia sec7 protein: homodimer formation and nuclear export

“…Therefore, reverse sequences of NESs (Class 1a-R, 1b-R, 1c-R, and 1d-R) may also be involved in forming functional NESs (Fung et al 2015). In addition, CRM1-independent NESs exist, which are LMB insensitive (Sato et al 2006;Thomas et al 2011;Huang et al 2013). Thus, the molecular mechanisms of NESs are highly diverse.…”

The synthetic triterpenoids CDDO-TFEA and CDDO-Me, but not CDDO, promote nuclear exclusion of BACH1 impairing its activity

“…Many nuclear export substrates contain a nuclear export signal (NES) that binds the export receptor CRM1. However, not all proteins that shuttle between the nucleus and cytoplasm use CRM1 to do so, and CRM1-independent nuclear export pathways have been identified [ [45] , [46] , [47] , [48] , [49] ]. To address whether the changes in nuclear and cytoplasmic BACH1 in response to CDDO-TFEA/Me are related to a CRM1-dependent nuclear export mechanism (as the most common mechanism reported for BACH1 regulation) we tested the effect of two CRM1 inhibitors (leptomycin B and selinexor) (Suppl.…”

Comprehensive Analysis of Hepatitis Delta Virus Assembly Determinants According to Genotypes: Lessons From a Study of 526 Hepatitis Delta Virus Clinical Strains

“…Among them, the unique C-terminal 197-213 amino-acids of the L-HDAg, resulting from an editing process of the HDV antigenome replicative intermediate which cumulates major functional and structural features involved in interactions with the HBsAg. They include a nuclear export signal (NES) between amino acids 197 to 210 via a chromosome region maintenance-1 independent pathway ( Lee et al, 2001 ; Huang et al, 2007 , 2009 , 2013 ); the farnesylation site, 211-CXPQ-214 ( Glenn et al, 1992 ). Moreover, within the DPD, a clathrin box between amino-acids 199 to 203 has been described.…”