8/11/2019 Viral Description

1/4

Viral Description & Life Cycle

Hepatitis B virus (HBV) is a DNA virus of the hepadnaviridae family. The HBV

virion, also called a Dane particle, is approximately 42 nm in diameter and consists of

an inner protein core and an outer protein envelope. The outer envelope is composed

of several proteins, known as hepatitis B surface proteins (HBs), which encase thenucleocapsid, or inner protein shell, made of hepatitis B core proteins (HBc) (Bruss

and Ganem, 1991). The nucleocapsid is arranged into an icosahedral formation and

contains the hepatitis B genome and at least one hepatitis B polymerase protein. The

figure below shows a diagram of the DNA virus.

Figure 3. Image fromhttp://www.hon.ch/Library/Theme/HepB/hbvirus.GIF

The viral genome is circular and is characterized by regions that are composed of

single, double, and triple-stranded DNA. Within the genome are four partially

overlapping open reading frames (ORFs) that encode seven different HBV genes,

including but not limited to the hepatitis B surface antigen gene, also called the S

gene, the hepatitis B core antigen gene, known as the C gene, the X gene, and the

polymerase (P) gene (Ocama et. al., 2005). The figure below shows an image of the

viral genome.

http://www.hon.ch/Library/Theme/HepB/hbvirus.GIFhttp://www.hon.ch/Library/Theme/HepB/hbvirus.GIFhttp://www.hon.ch/Library/Theme/HepB/hbvirus.GIFhttp://www.hon.ch/Library/Theme/HepB/hbvirus.GIF

8/11/2019 Viral Description

2/4

Figure 4. Image fromhttp://www.scielo.br/img/revistas/rhc/v59n4/21391f1.gif

Unlike other envelope proteins, HBs span the lipid bilayer multiple times. There are

three different size proteins based on the reading frame, yielding small, medium, and

large HBs. The large hepatitis B surface antigen is believed to be most responsible for

conducting viral-host cell attachment. A specific receptor, however, has yet to be

identified. Another unique feature is that HBs can be secreted as subviral particles,

lacking all other parts of the virion. Such particles are termed hepatitis B filamentsand hepatitis B spheres, each of which has a diameter of 22nm. Increased levels can

be found during acute infection. However, because these particles lack core proteins,

polymerase, and viral DNA, they are non-infectious and do not appear to be

advantageous to the virus (Bruss and Ganem, 1991).

Once the virus enters the body of a host, it infects liver cells. In doing so, the viral

surface proteins fuse with the host cell membrane, allowing the core particle to enter

the liver cell. Although early steps in viral entry are not clearly defined, the uncoated

particle (or nucleocapsid) is transported into the nuclear membrane of the host cell.The viral DNA is then brought into the nucleus and is repaired to form a covalently

closed-circular form, known as cccDNA. Viral DNA is not integrated into the host

DNA like some viruses. Instead, once the DNA is recircularized, transcription of viral

DNA and proteins begins and is controlled by four promoter elements and two

enhancer elements. DNA polymerase then begins copying the DNA. The polymerase

protein uses its unique shape to initiate reverse transcription and copy the DNA.

http://www.scielo.br/img/revistas/rhc/v59n4/21391f1.gifhttp://www.scielo.br/img/revistas/rhc/v59n4/21391f1.gifhttp://www.scielo.br/img/revistas/rhc/v59n4/21391f1.gifhttp://www.scielo.br/img/revistas/rhc/v59n4/21391f1.gif

8/11/2019 Viral Description

3/4

Unlike retroviruses, hepadnaviruses bind polymerase proteins into a stem-loop

formation, which are subsequently packaged by core proteins in the golgi and secreted

via exocytosis into the blood stream, where it can contact other liver cells and

continue replication (Rehermann and Michelina, 2005).

Figure 5. Image fromhttp://mgl.snu.ac.kr/images/HBV%20life%20cycle.jpg

Bruss, V. & Ganem, D. 1991. The role of envelope proteins in hepatitis B virus

assembly.Proceedings of the National Academy of Science USA88:1059-1063.

http://mgl.snu.ac.kr/images/HBV%20life%20cycle.jpghttp://mgl.snu.ac.kr/images/HBV%20life%20cycle.jpghttp://mgl.snu.ac.kr/images/HBV%20life%20cycle.jpghttp://mgl.snu.ac.kr/images/HBV%20life%20cycle.jpg

8/11/2019 Viral Description

4/4

Ocama, P., Opio, C., & Lee, W. 2005. Hepatitis B virus infection: Current status. The

American Journal of Medicine. 118(12):1413-1420.

Rehermann, B. and Nascimbeni, M. 2005. Immunology ofhepatitis B virus and

hepatitis C virus infection.Immunology 5: 215-229.

Return toChristie's Immunology Home Page

Christie Brough. Biology 307: Immunology. Dr. S. Sarafova. Davidson College. May

4, 2007.

http://www.bio.davidson.edu/people/sosarafova/assets/bio307/chbrough/index.htmlhttp://www.bio.davidson.edu/people/sosarafova/assets/bio307/chbrough/index.htmlhttp://www.bio.davidson.edu/people/sosarafova/assets/bio307/chbrough/index.htmlhttp://www.bio.davidson.edu/people/sosarafova/assets/bio307/chbrough/index.html