Viral Capsids and Envelopes: Structure and Function


Virus particles contain the viral genome packaged in a protein coat called the capsid. For some viruses, the capsid is surrounded by lipid bilayer that contains viral proteins, usually including the proteins that enable the virus to bind to the host cells. This lipid and protein structure is called the virus envelope, and is derived from the host cell membranes. The capsid and envelope play many roles in viral infection, including virus attachment to cells, entry into cells, release of the capsid contents into the cells, and packaging of newly formed viral particles. The capsid and envelope are also responsible for transfer of the viral genetic material from one cell to another. These structures also determine the stability characteristics of the virus particle, such as resistance to chemical or physical inactivation.

Key concepts:

  • Viral genetic material is packaged inside protein structures called capsids.

  • Viruses are divided into two groups: enveloped viruses are surrounded by an outer lipid membrane; nonenveloped viruses lack this membrane.

  • Where present, the envelope contains the viral proteins, which mediate binding to host cells. Where no envelope is present, this function is carried out by the outer capsid proteins. In general, nonenveloped viruses are more stable and can survive much longer in the environment.

  • Capsids and envelopes determine the method of viral entry into and exit from host cells.

Keywords: icosahedral symmetry; helical symmetry; membrane protein; receptor; virus entry; virus assembly

Figure 1.

Helical and icosahedral symmetry. (a) The structure of rabies virus, an example of a virus containing a helical nucleocapsid, labelled here as ribonucleoprotein. From (b) Icosahedral symmetry, with axes of 2‐, 3‐ and 5‐fold symmetry indicated. From http://www‐ with permission of Dr Shaun Heaphy.

Figure 2.

The replication cycle of influenza virus, showing entry (left) and budding (right). Redrawn after with permission of Dr Robert Webster.

Figure 3.

The stages of entry of human immunodeficiency virus (HIV). gp, glycoprotein. Redrawn after with the permission of Dr Joseph Sodroski.



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Further Reading

Flint J, Enquist LW, Krug RM, Racaniello VR and Skalka AM (2000) Principles of Virology. Washington DC: ASM Press.

Harrison SC (2007) Principles of virus structure. In: Knipe DM, Howley PM, Griffin DE, Martin MA, Lamb RA, Roizman B and Straus SE (eds) Field's Virology, 5th edn, pp. 59–98. Philadelphia, PA: Lippincott Williams and Wilkins.

Helenius A (2007) Virus entry and uncoating. In: Knipe DM, Howley PM, Griffin DE, Martin MA, Lamb RA, Roizman B and Straus SE (eds) Field's Virology, 5th edn, pp. 99–118. Philadelphia, PA: Lippincott Williams and Wilkins.

Hunter E (2007) Virus assembly. In: Knipe DM, Howley PM, Griffin DE, Martin MA, Lamb RA, Roizman B and Straus SE (eds) Field's Virology, 5th edn, pp. 141–168. Philadelphia, PA: Lippincott Williams and Wilkins.

Laver WG, Bischofberger N and Webster RG (1999) Disarming flu viruses. Scientific American 280: 78–87.

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How to Cite close
Lucas, William(Apr 2010) Viral Capsids and Envelopes: Structure and Function. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001091.pub2]