From the Cytoplasm into the Nucleus-Hepatitis B Virus Travel and Genome Repair

Abstract

Hepatitis B virus (HBV) is a major global health concern, affecting millions of people worldwide. HBV is part of the hepadnaviridae family and one of the primary causes of acute and chronic liver infections, leading to conditions such as cirrhosis and hepatocellular carcinoma (HCC). Understanding the intracellular transport and genome repair mechanisms of HBV is crucial for developing new drugs, which-in combination with immune modulators-may contribute to potential cures. This review will explore the current knowledge of HBV intracytoplasmic and nuclear transport, as well as genome repair processes, while drawing comparisons to other viruses with nuclear replication.

Keywords: capsid; core protein phosphorylation; genome release; genome repair; hepatitis B virus; intracellular transport; nuclear import; nuclear pore complex (NPC).

Figure 1

Schematic overview of the HBV life cycle. The cell membrane is shown as a bold blue line; the nuclear envelope is indicated by a double blue line. Cytoplasm: white; nucleoplasm: yellow; ER/Golgi: light red. HBV surface proteins: red; viral polymerase: green. The different steps are indicated in the figure. For further details, see text.

Figure 2

HBV genome maturation. PG: black wave line; minus-strand DNA: blue line; plus-strand DNA: violet line. TP: terminal protein (=pol priming domain); pol: catalytic pol domain; RH: pol RNase H domain. The linker between TP and the catalytic domain is shown as a green line of variable length. DR1: direct repeat 1 present in 2 copies on the PG. DR2: direct repeat 2. (A) Pol binds to e and synthesizes the first nucleotides, causing a covalent link to the TP of pol. This complex translocates to DR1. (B) Pol synthesizes the minus-strand DNA until the 5′ end of the PG, which has the DR1 sequence. Synthesis is combined with degrading the RNA from the RNA–DNA hybrid. The first 11–16 nucleotides of the RP (including cap) stay non-degraded. (C) left: The RNA fragment translocates to DR2 upstream of DR1, where pol initiates plus-strand DNA synthesis. Right: the RNA primer translocation does not occur (approx. in 10%) and pol synthesizes the plus-strand DNA, resulting in dslDNA. (D) When reaching the end of the minus-strand DNA, the 5′ end of the plus strand anneals to the complementary DR1 on the 5′ end of the minus-strand DNA. (E) Pol continues with plus-strand DNA synthesis, creating a circular form of the genomes (rcDNA). The plus-strand DNA stays incomplete.

Figure 3

Domains of the core protein. Assembly domain: light green; CTD: orange. Red P: phosphorylation sites (only shown for internal CTD localization). Orange-filled P: major phosphorylation sites. The figure shows the interaction partners and functions of the CTD for its internal and external location in the assembled capsid. For the effect of phosphorylation, see text.

Figure 4

Schematic overview of cellular capsid transport. The boxes indicate capsids having other properties. (A) Cytoplasmic transport. CTD exposure is shown as red CTD, and the dynein chains are indicated. Importins are shown as grey ovals. (B) Passage through the nuclear pore, filled with FG repeats (brown lines). (C) Arrest in the nuclear basket by interaction with Nup153 (purple). (D) Dissociation of importin α/β from the capsid and capsid binding to Nup153. (E) Capsid disassembly and genome release. Released Cps are depicted as dimers or hexamers. Released rcDNA: blue line; minus-strand DNA: black wavy line; RNA primer for plus-strand DNA synthesis: violet line; plus-strand DNA with a variable 3′ end (dotted line). Pol: green oval.

Figure 5

Schematic structure of the rcDNA and the repair processes leading to cccDNA. Minus-strand DNA: blue line; “r”: terminal redundancy of the minus-strand DNA; RNA primer for plus-strand DNA synthesis: black wave line; plus-strand DNA: violet line with a variable 3′ end indicated by a dotted line. TP: terminal protein (=pol priming domain); pol: catalytic pol domain; RH: pol RNase H domain. The linker between TP and the catalytic domain is shown as a green line of variable length. (A) rcDNA. 1. Removal of the minus-strand DNA redundancy “r”. 2. Removal of the RNA primer. 3. Completion of the plus-strand DNA. 4. Ligation of 5′ and 3′ ends of the minus-strand DNA. 5. Ligation of 5′ and 3′ ends of the plus-strand DNA. (B) dslDNA. Removal of pol, RNA primer as in A. Circularization of the molecule, mediated by KU70, is indicated by a dotted orange line.