Nuclear localization of the truncated hepatitis C virus core protein with its hydrophobic C terminus deleted

Abstract

The core protein of hepatitis C virus (HCV) is considered to be cleaved from the N terminus of the large precursor polyprotein by cellular signalase. The HCV cDNA encoding the core protein was expressed (i) in monkey COS cells by a plasmid expression vector driven by the SR~ promoter, and (ii) in insect cells by a recombinant baculovirus. The expressed product had an M r of 22 000 and was located in the cytoplasm. When the C-terminal hydrophobic domains were deleted, however, the truncated core proteins were translocated into the nucleus. The truncated core proteins were located in the nucleus even when they were expressed as a fusion protein with E. eoli fl-galactosidase, which is essentially localized in the cytoplasm. Plasmids containing HCV cDNAs with a deletion in one of the regions encoding clusters of basic amino acids were expressed in COS cells and the localization of the core protein was examined. The residues PRRGPR were suggested to play an important role in nuclear localization. HCV is an RNA virus and its life cycle was originally considered to be confined to the cytoplasm; the present study, however, suggests that the HCV core protein can translocate into the nucleus under certain circumstances.

Intramembrane Proteolysis and Endoplasmic Reticulum Retention of Hepatitis C Virus Core Protein

“…EGFP-Core 191 colocalized with ERDsRed to the ER (Fig. 2), whereas EGFP-Core 179 was localized primarily to the nucleus as reported previously (3,33,43,47), suggesting that the C-terminal signal sequence is essential for anchoring HCV core protein to the ER membrane. However, EGFP-Core 174-191 exhibited diffuse staining similar to that of EGFP, suggesting that the signal sequence alone is not sufficient for ER localization.…”

“…This is then cleaved by SPP into the mature core protein and localizes primarily to the ER. The mature core protein, further processed by an unidentified protease, is composed of amino acids 151 to 153 and is detected in the nucleus (33,47). Actually, HCV core protein is observed in the cytoplasm, nucleus, and nucleoli in transgenic mice expressing HCV core protein (35).…”

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“…It thus seems likely that, in certain circumstances, trun-that the truncated HCV core protein with its hydrophobic C-terminus deleted can translocate efficiently into the nucleus. We also identified the nuclear localization signal sequence in the core protein (21). Such study is preliminary and the translocation of the truncated core protein may be an unnatural phenomenon where replicated HCV RNA does not exist.…”

“…Within this region, there are three clusters of basic amino acids. Experiments with the deletion of one of each basic amino acid clusters and translocation of the fused protein with P-galactosidase of E. coli demonstrated that the sequence, PRRGPR (residues 38-43), plays an important role as a nuclear localization signal (21). This sequence PRRGPR is well conserved among various types of HCV isolates examined (28)(29)(30)(31)(32)(33) (Fig.…”

“…It is noteworthy that there are several clusters which may function to regulate gene expression in the amino acid sequence of HCV core protein (21,25). Within this region, there are three clusters of basic amino acids.…”