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

Smita Nair,Adam Zlotnick,Ralf Bartenschlager,Stephen P Goff

doi:10.1128/mbio.03514-20

Abstract

ABSTRACTHepatitis 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.

Highlights

Hepatitis B virus (HBV) core protein (Cp) can be found in the nucleus and cytoplasm of infected hepatocytes; it preferentially segregates to a specific compartment correlating with disease status
To provide a basis for comparing different Cp mutants that modulate assembly, we first examined the intracellular distribution of Cp when expressed using a genomic clone, LJ144, that is mutated to delete expression of the HBsAg proteins [33]
We observed that in PY63F-transfected cells, with exclusively immature and empty capsids, 60% of transfected cells had most of their Cp in the cytoplasm (Fig. S1a and b in the supplemental material)

Summary

Introduction

Hepatitis B virus (HBV) core protein (Cp) can be found in the nucleus and cytoplasm of infected hepatocytes; it preferentially segregates to a specific compartment correlating with disease status. Regulation of this intracellular partitioning of Cp remains obscure. HBV is a small enveloped DNA virus that has a 3.2-kb circular, partially doublestranded DNA genome packaged in an icosahedral shell made of 240 copies of the core protein (Cp) [1] This 21-kDa protein, a dimer in solution, serves as the structural framework for the virus, but has indispensable roles at almost every step of the viral life cycle [4]. During an infection, Cp shuttles between the nuclear and cytoplasmic compartments of a hepatocyte

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Conclusion