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Schematic of a Cytomegalovirus
Icosahedral capsid of an Adenovirus
For the leaf bug, see Miridae.

A capsid is the protein shell of a virus. It consists of several oligomeric subunits made of protein. The capsid encloses the genetic material of the virus.

Capsids are broadly classified according to their structure. The majority of viruses have capsids with either helical or icosahedral structure. Some viruses, such as bacteriophages, have developed more complicated structures. The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere, while the helical shape is cylindrical.[1] The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease virus capsid has faces consisting of three proteins named VP1-3.[2]

Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope. The envelope is acquired by the capsid from an intracellular membrane; some examples would include the inner nuclear membrane, the golgi membrane, or the cell's outer membrane.[3]

Once the virus has infected the cell, it will start replicating itself, using the mechanisms of the infected host cell. During this process, new capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell. During the assembly process, a portal subunit is assembled at one vertex of the capsid. Through this portal, viral DNA or RNA is transported into the capsid.[4] The structure and assembly of the Herpes virus Capsid Portal Protein has been imaged via cryo-electron microscopy.[5]

Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into families. For example, the bacteriophage PRD1, Paramecium bursaria Chlorella algal virus, and mammalian adenovirus have been placed in the same family.[6]


  1. Branden, Carl and Tooze, John (1991). Introduction to Protein Structure. pp. 161–162. ISBN 0-8153-0270-3. 
  2. "Virus Structure (". 
  3. Alberts, Bruce; Bray, Dennis; Lewis, Julian; Raff, Martin; Roberts, Keith; Watson, James D. (1994). Molecular Biology of the Cell (4 ed.). p. 280. 
  4. Newcomb WW, Homa FL, Brown JC (2005 Aug). "Involvement of the portal at an early step in herpes simplex virus capsid assembly". Journal of Virology. 79 (16): 10540-6. PMID 16051846. 
  5. Cardone G, Winkler DC, Trus BL, Cheng N, Heuser JE, Newcomb WW, Brown JC, Steven AC (2007 May 10). "Visualization of the herpes simplex virus portal in situ by cryo-electron tomography". Virology. 361 (2): 426-34. PMID 17188319. 
  6. Khayat et al. classified Sulfolobus turreted icosahedral virus (STIV) and Laurinmäki et al. classified bacteriophage Bam35 - Proc. Natl. Acad. Sci. U.S.A. 103, 3669 (2006); 102, 18944 (2005); Structure 13, 1819 (2005)

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