Cleavage of hepatitis C virus nonstructural protein 5A by a caspase-like protease(s) in mammalian cells.

Nonstructural 5A protein (NS5A) of hepatitis C virus (HCV) is localized in the cytoplasm although it has a functional nuclear localization signal. To clarify the determinant of NS5A cytoplasmic localization, various N- or C-terminal deleted NS5A mutants were generated and their subcellular localization was analyzed in cell lines after transient expression. N-terminal deleted forms of NS5A were localized in the nucleus, and the sequence of the N-terminal 27 amino acids of NS5A had sufficient function to cause retention of a normally nuclear protein in the cytoplasm. These observations indicated that cytoplasmic localization of NS5A is determined primarily by the N-terminal region of the molecule. In addition, we found proteolytic processing of NS5A in transiently expressing cells. In these cells, cleavage occurred at a few sites located in the N- and C-terminal regions of NS5A. This cleavage in cells was enhanced by apoptotic stimuli and was inhibited by the caspase inhibitor Z-VAD-FMK, suggesting that a caspase-like protease(s) contributes to the cleavages of NS5A. Based on the results of mutational analysis of NS5A, we predicted one cleaved form, which had lost both the N- and the C-terminal portions of NS5A, to be composed of amino acid residues 155 to 389. Peptide containing the same amino acid sequence as this cleaved product was localized in the nucleus. Furthermore, we found that a fusion protein consisting of Gal4 DNA-binding domain fused with this cleaved form showed transcriptional activity only when the alpha-catalytic subunit of protein kinase A (PKA) was coproduced, suggesting that the transcriptional activity of this product was regulated by PKA. These results suggested that the cleavage product of NS5A by a caspase-like protease(s) plays a role in transcriptional regulation of the host cell gene(s) in HCV-infected cells.