HBV Core Protein Is in Flux between Cytoplasmic, Nuclear, and Nucleolar Compartments.

Hepatitis B virus (HBV) core protein (Cp) can be found in the nucleus and cytoplasm of infected hepatocytes; however, it preferentially segregates to a specific compartment correlating with disease status. Regulation of this intracellular partitioning of Cp remains obscure. In this paper, we report that cellular compartments are filled and vacated by Cp in a time- and concentration-dependent manner in both transfections and infections. At early times after transfection, Cp, in a dimeric state, preferentially localizes to the nucleolus. Later, the nucleolar compartment is emptied and Cp progresses to being predominantly nuclear, with a large fraction of the protein in an assembled state. Nuclear localization is followed by cell-wide distribution, and then Cp becomes exclusively cytoplasmic. The same trend in Cp movement is seen during an infection. Putative nucleolar retention signals have been identified and appear to be structure dependent. Export of Cp from the nucleus involves the CRM1 exportin. Time-dependent flux can be recapitulated by modifying Cp concentration, suggesting transitions are regulated by reaching a threshold concentration. IMPORTANCE HBV is an endemic virus. More than 250 million people suffer from chronic HBV infection and about 800,000 die from HBV-associated disease each year. HBV is a pararetrovirus; in an infected cell, viral DNA in the nucleus is the template for viral RNA that is packaged in nascent viral capsids in the cytoplasm. Inside those capsids, while resident in cytoplasm, the linear viral RNA is reverse transcribed to form the circular double-stranded DNA (dsDNA) of the mature virus. The HBV core (or capsid) protein plays a role in almost every step of the viral life cycle. Here, we show the core protein appears to follow a programmed, sequential localization from cytoplasmic translation then into the nucleolus, to the nucleus, and back to the cytoplasm. Localization is primarily a function of time, core protein concentration, and assembly. This has important implications for our understanding of the mechanisms of antivirals that target HBV capsid assembly.
(Copyright © 2021 Nair and Zlotnick.)