Keiko Tanaka

Molecular Determinants for Subcellular Localization of Hepatitis C Virus Core Protein

ABSTRACT Hepatitis C virus (HCV) core protein is a putative nucleocapsid protein with a number of regulatory functions. In tissue culture cells, HCV core protein is mainly located at the endoplasmic reticulum as well as mitochondria and lipid droplets within the cytoplasm. However, it is also detected in the nucleus in some cells. To elucidate the mechanisms by which cellular trafficking of the protein is controlled, we performed subcellular fractionation experiments and used confocal microscopy to examine the distribution of heterologously expressed fusion proteins involving various deletions and point mutations of the HCV core combined with green fluorescent proteins. We demonstrated that a region spanning amino acids 112 to 152 can mediate association of the core protein not only with the ER but also with the mitochondrial outer membrane. This region contains an 18-amino-acid motif which is predicted to form an amphipathic α-helix structure. With regard to the nuclear targeting of the core protein, we identified a novel bipartite nuclear localization signal, which requires two out of three basic-residue clusters for efficient nuclear translocation, possibly by occupying binding sites on importin-α. Differences in the cellular trafficking of HCV core protein, achieved and maintained by multiple targeting functions as mentioned above, may in part regulate the diverse range of biological roles of the core protein.

Microtubule network facilitates nuclear targeting of human cytomegalovirus capsid

ABSTRACT We assessed the requirement of the host cytoskeleton for the intracytosolic transport of incoming human cytomegalovirus (HCMV) capsids. Treatments with microtubule (MT)-depolymerizing drugs nocodazole and colchicine led to a drastic decrease in levels of IE1 antigen, whereas cytochalasin B had no effect on the level of IE1 as determined by Western blot analyses. Sequential treatment including nocodazole washout and removal of cell surface virion revealed that HCMV entry into the cells occurred normally in the absence of the MT network. This finding was also supported by data obtained by monitoring pUL83 signals with an immunofluorescent assay (IFA). Furthermore, we demonstrated a close association of incoming HCMV capsids with MTs by IFA and ultrastructural analyses. In the absence of the MT network, the capsids which had entered the cytoplasm did not move to close proximity of the nucleus. These data suggest that HCMV capsids associate with the MT network to facilitate their own movement to the nucleus before the onset of immediate-early (IE) gene expression and that this association is required to start efficient IE gene expression.

Cross-reactivity among sapovirus recombinant capsid proteins

Summary.Sapovirus (SaV), a member of the genus Sapovirus in the family Caliciviridae, is an agent of human and porcine gastroenteritis. SaV strains are divided into five genogroups (GI–GV) based on their capsid (VP1) sequences. Human SaV strains are noncultivable, but expression of the recombinant capsid protein (rVP1) in a baculovirus expression system results in the self-assembly of virus-like particles (VLPs) that are morphologically similar to native SaV. In this study, rVP1 constructs of SaV GI, GII, and GV strains were expressed in a baculovirus expression system. The structures of the GI, GII, and GV VLPs, with diameters of 41–48 nm, were morphologically similar to those of native SaV. However a fraction of GV VLPs were smaller, with diameters of 26–31 nm and spikes on the outline. This is the first report of GII and GV VLP formation and the first identification of small VLPs. To examine the cross-reactivities among GI, GII, and GV rVP1, hyperimmune rabbit antisera were raised against Escherichia coli-expressed GI, GII, and GV N- and C-terminal VP1. Western blotting showed the GI antisera cross-reacted with GV rVP1 but not GII rVP1; GII antisera cross-reacted weakly with GI rVP1 but did not cross-react with GV rVP1; and GV antisera reacted only with GV rVP1. Also, hyperimmune rabbit and guinea pig antisera raised against purified GI VLPs were used to examine the cross-reactivities among GI, GII, and GV VLPs by an antigen enzyme-linked immunosorbent assay (ELISA). The ELISA showed that the GI VLPs were antigenically distinct from GII and GV VLPs.

Human papillomavirus 16 minor capsid protein L2 helps capsomeres assemble independently of intercapsomeric disulfide bonding.

The human papillomavirus (HPV) capsomeres (pentamers of major capsid protein L1), which constitute along with L2 the virion capsid, can assemble themselves alone into the L1-capsid particles in vivo and in vitro, depending on intercapsomeric disulfide bonds. To study a possible role of L2 in capsid assembly, we examined the interaction between HPV16 L2 and capsomeres under the conditions that inhibit the formation of disulfide bonds in vitro and in vivo. The purified L2 bound to free capsomeres prepared by disassembling L1-capsids but not to the L1-capsids in vitro. And the L2 was found to help capsomeres assemble into smaller capsid-like particles independently of intercapsomeric disulfide bonding. Similar particles were obtained from the Sf9 cells co-infected with baculoviruses expressing L2 and an L1 mutant that lacks a C-terminal cysteine (C428S) and can form capsomeres but no capsids when expressed alone. These findings suggest that L2, which is known to bind both viral DNA and L1, may contribute to the formation of the virion by linking viral DNA and capsomeres and by helping capsomeres assemble before the virion capsid structure is completed by dintercapsomeric disulfide bonding.

Inhibition of nuclear entry of HPV16 pseudovirus-packaged DNA by an anti-HPV16 L2 neutralizing antibody.

Rabbit anti-HPV16 L2 serum (anti-P56/75) neutralizes multiple oncogenic human papillomaviruses (HPVs). We inoculated HeLa cells with HPV16 pseudovirus (16PV) and with anti-P56/75-bound 16PV (16PV-Ab). Both 16PV and 16PV-Ab attached equally well to the cell surface. However, the cell-attached L1 protein of 16PV became trypsin-resistant after incubation at 37°C, whereas approximately 20% of the cell-attached 16PV-Ab L1 remained trypsin-sensitive. Confocal microscopy of HeLa cells inoculated with 16PV revealed packaged DNA in the nucleus at 22h after inoculation; however, nuclear DNA was not detected in cells inoculated with 16PV-Ab. Electron microscopy of HeLa cells inoculated with 16PV showed particles located in multivesicular bodies, lamellar bodies, and the cytosol after 4h; no cytosolic particles were detected after inoculation with 16PV-Ab. These data suggest that anti-P56/75 inhibits HPV infection partly by blocking viral entry and primarily by blocking the transport of the viral genome to the nucleus.

Isolation and Characterization of Mumps Virus Strains in a Mumps Outbreak with a High Incidence of Aseptic Meningitis

In 1993, mumps with a high incidence of aseptic meningitis became prevalent in Akita prefecture, Japan. Three mumps virus isolates obtained from the nonvaccine‐associated cases lacked the BamHI restriction cleavage site of the P gene, like the Urabe strain (Yamada, A. et al, Vaccine 8: 553‐557). However, four additional nucleotide substitutions were found in the determined region of 157 bp. Fourteen of 19 cases from which mumps virus showing the Urabe‐like RFLP profile was detected were complicated with symptomatic meningitis, whereas there were only four cases of meningitis among 23 individuals infected with the wild type showing no Urabe‐like RFLP profile (non‐“Urabe‐like” wild‐type). The incidence of meningitis was over 70% among patients infected with the “Urabe‐like” wild‐type virus. The “Urabe‐like” wild‐type disappeared after February 1994 in the epidemic area and was replaced by the non‐“Urabe‐like” wild‐type. Patients infected with the “Urabe‐like” wild‐type lived in a closed colony, in which there were two instances of transmission between siblings. Thus this outbreak was transient and narrowly localized.

Effects of the number of amino acid residues in the signal segment upstream or downstream of the NS2B-3 cleavage site on production and secretion of prM/M-E virus-like particles of West Nile virus.

Expression of genes for precursor M (prM) and envelope (E) proteins of West Nile virus (WNV) leads to the production of small, capsidless, and non-infectious virus-like particles (VLPs) possessing the E antigen which is responsible for viral entry and immune protection. It has been reported that processing of the secretion signal affects viral release. We examined the secretion efficiency of VLPs into the culture medium from RK13 or 293T cells transfected with expression vectors for prM and E proteins of WNV which were constructed to comprise different lengths of signal peptides upstream of the prM-E domain. The number of amino acid residues present in the segment markedly affected the production, processing, and secretion of VLPs. Secreted VLPs possessed both the processed M protein and the glycosylated E protein. In addition, immunization with VLPs induced neutralizing antibodies in C3H/HeN mice. These results indicate that the number of amino acid residues comprising the N-terminus of the signal segment controls the efficiency of assembly, maturation, and release of VLPs in the absence of viral protease, which in turn indicates the potential of VLPs as a candidate for an effective WNV subunit vaccine.

Cloning and Characterization of the Genomic RNA Sequence of the Mumps Virus Strain Associated with a High Incidence of Aseptic Meningitis

cDNA clones of the mumps virus wild‐type strain, associated with a high incidence of aseptic meningitis (ODATE‐1 strain), were isolated and analyzed from genomic nucleotide position 22 to 8520 containing the NP, P, M., F, SH and HN protein coding region. The ODATE‐1 strain exhibited a RFLP profile identical to that of the Urabe vaccine strain in spite of the fact that the virus was isolated from non‐vaccinated cases. However, a comparison of nucleotide and amino acid sequences among the ODATE‐1 strain, Urabe strain and Miyahara strain revealed that the ODATE‐1 strain was not related to the Urabe strain.

Synthetic fibril peptide promotes clearance of scrapie prion protein by lysosomal degradation

Transmissible spongiform encephalopathies are infectious and neurodegenerative disorders that cause neural deposition of aggregates of the disease‐associated form of PrPSc. PrPSc reproduces by recruiting and converting the cellular PrPC, and ScN2a cells support PrPSc propagation. We found that incubation of ScN2a cells with a fibril peptide named P9, which comprises an intrinsic sequence of residues 167–184 of mouse PrPC, significantly reduced the amount of PrPSc in 24 hr. P9 did not affect the rates of synthesis and degradation of PrPC. Interestingly, immunofluorescence analysis showed that the incubation of ScN2a cells with P9 induced colocalization of the accumulation of PrP with cathepsin D‐positive compartments, whereas the accumulation of PrP in the cells without P9 colocalized mainly with lysosomal associated membrane proteins (LAMP)‐1‐positive compartments but rarely with cathepsin D‐positive compartments in perinuclear regions. Lysosomal enzyme inhibitors attenuated the anti‐PrPSc activity; however, a proteasome inhibitor did not impair P9 activity. In addition, P9 neither promoted the ubiquitination of cellular proteins nor caused the accumulation of LC3‐II, a biochemical marker of autophagy. These results indicate that P9 promotes PrPSc redistribution from late endosomes to lysosomes, thereby attaining PrPSc degradation.

Covalent bonded Gag multimers in human immunodeficiency virus type-1 particles

The oligomerization of HIV‐1 Gag and Gag‐Pol proteins, which are assembled at the plasma membrane, leads to viral budding. The budding generally places the viral components under non‐reducing conditions. Here the effects of non‐reducing conditions on Gag structures and viral RNA protection were examined. Using different reducing conditions and SDS‐PAGE, it was shown that oligomerized Gag possesses intermolecular covalent bonds under non‐reducing conditions. In addition, it was demonstrated that the mature viral core contains a large amount of covalent bonded Gag multimers, as does the immature core. Viral genomic RNA becomes sensitive to ribonuclease in reducing conditions. These results suggest that, under non‐reducing conditions, covalent bonded Gag multimers are formed within the viral particles and play a role in protection of the viral genome.