title: Characterization of nuclear targeting signal of hepatitis delta antigen: nuclear transport as a protein complex.
abstract: Hepatitis delta antigen (HDAg) is the only protein encoded by hepatitis delta virus (HDV). HDAg has been demonstrated in the nuclei of HDV-infected hepatocytes, and its nuclear transport may be important for the replication of HDV RNA. In this report, we investigated the mechanism of nuclear transport of HDAg. By expressing fusion proteins consisting of the different portions of HDAg and alpha-globin, we have identified a nuclear localization signal (NLS) within the N-terminal one-third of HDAg. It consists of two stretches of basic amino acid domains separated by a short run of nonbasic amino acids. Both of the basic domains are necessary for the efficient nuclear transport of HDAg. The nonbasic spacer amino acids could be removed without affecting the nuclear targeting of HDAg significantly. Thus, the HDAg NLS belongs to a newly identified class of NLS which consists of two discontiguous stretches of basic amino acids. This NLS is separated from a stretch of steroid receptor NLS-like sequence, which is also present but not functioning as an NLS, in HDAg. Furthermore, we have shown that subfragments of HDAg which do not contain the NLS can be passively transported into the nucleus by a trans-acting full-length HDAg, provided that these subfragments contain the region with a leucine zipper sequence. Thus, our results indicate that HDAg forms aggregates in the cytoplasm and that the HDAg oligomerization is probably mediated by the leucine zipper sequence. Therefore, HDAg is likely transported into the nucleus as a protein complex.
authors.affiliation: Department of Microbiology, University of Southern California School of Medicine, Los Angeles 90033-1054.
authors.forename: Y P
authors.initials: YP
authors.lastname: Xia
authors.forename: C T
authors.initials: CT
authors.lastname: Yeh
authors.forename: J H
authors.initials: JH
authors.lastname: Ou
authors.forename: M M
authors.initials: MM
authors.lastname: Lai
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000956:Antigens, Viral / Q000032:analysis / Q000235:genetics*; D001483:Base Sequence; D002460:Cell Line; D002467:Cell Nucleus / Q000502:physiology* / Q000648:ultrastructure; D002678:Chimera; D002872:Chromosome Deletion; D003854:Deoxyribonucleotides; D005455:Fluorescent Antibody Technique; D005914:Globins / Q000235:genetics; D003698:Hepatitis Delta Virus / Q000235:genetics* / Q000502:physiology; D035923:Hepatitis delta Antigens; D016350:Leucine Zippers / Q000235:genetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D016133:Polymerase Chain Reaction; D021382:Protein Sorting Signals / Q000235:genetics*; D014162:Transfection; D014779:Virus Replication
chemical_list: D000956:Antigens, Viral; D003854:Deoxyribonucleotides; D035923:Hepatitis delta Antigens; D021382:Protein Sorting Signals; C442244:hepatitis delta virus large antigen; D005914:Globins