Nucleoporin 153 Arrests the Nuclear Import of Hepatitis B Virus Capsids in the Nuclear Basket

Abstract

Virtually all DNA viruses including hepatitis B viruses (HBV) replicate their genome inside the nucleus. In non-dividing cells, the genome has to pass through the nuclear pore complexes (NPCs) by the aid of nuclear transport receptors as e.g. importin β (karyopherin). Most viruses release their genome in the cytoplasm or at the cytosolic face of the NPC, as the diameter of their capsids exceeds the size of the NPC. The DNA genome of HBV is derived from reverse transcription of an RNA pregenome. Genome maturation occurs in cytosolic capsids and progeny capsids can deliver the genome into the nucleus causing nuclear genome amplification. The karyophilic capsids are small enough to pass the NPC, but nuclear entry of capsids with an immature genome is halted in the nuclear basket on the nuclear side of the NPC, and the genome remains encapsidated. In contrast, capsids with a mature genome enter the basket and consequently liberate the genome. Investigating the difference between immature and mature capsids, we found that mature capsids had to disintegrate in order to leave the nuclear basket. The arrest of a karyophilic cargo at the nuclear pore is a rare phenomenon, which has been described for only very few cellular proteins participating in nuclear entry. We analyzed the interactions causing HBV capsid retention. By pull-down assays and partial siRNA depletion, we showed that HBV capsids directly interact with nucleoporin 153 (Nup153), an essential protein of the nuclear basket which participates in nuclear transport via importin β. The binding sites of importin β and capsids were shown to overlap but capsid binding was 150-fold stronger. In cellulo experiments using digitonin-permeabilized cells confirmed the interference between capsid binding and nuclear import by importin β. Collectively, our findings describe a unique nuclear import strategy not only for viruses but for all karyophilic cargos.

Expression of viral polymerase and phosphorylation of core protein determine core and capsid localization of the human hepatitis B virus

“…Similarly to other DNA viruses, the viral genome is transported within the viral capsid, which is composed of 240 copies of the core protein. Upon arrival on the nuclear side of the nuclear pore (Schmitz et al, 2010), the capsid dissociates to core protein dimers leading to release of the viral genome (Rabe et al, 2003). As core proteins assemble in the absence of any other viral or cellular factor at increased concentrations (Seifer & Standring, 1995), the core dimers reassemble in the nucleus to form capsids (Rabe et al, 2009).…”

Hepatite B e C: diagnóstico e tratamento

“…Após sua entrada, o nucleocapsídeo contendo DNA circular viral é levado do citoplasma para o núcleo, onde sofre conversão em DNA circular covalentemente fechado através de mecanismos de reparo de DNA do hospedeiro 32,33 . Este DNA é responsável pelo estabelecimento da doença crônica do HBV à medida que adquire componentes do hospedeiro para formar minicromossomo.…”

Untitled

“…Through a still poorly understood process, the polymerase-bound rcDNA is released into nucleoplasm. Evidences suggest that viral nucleocapsids are transported via microtubuli to the nuclear pores where mature capsids disintegrate and release both core capsid subunits and rcDNA-polymerase complexes into the nucleoplasm (13)(14)(15). After viral polymerase removal and the completion of the positive strand by the host replicative machinery, the conversion into covalently closed circular (ccc) DNA occurs (16).…”