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Capsid

Schematic of a cytomegalovirus
Illustration of geometric model changing between two possible capsids. A similar change of size has been observed as the result of a single amino-acid mutation[1]

A capsid is the protein shell of a virus, enclosing its genetic material. It consists of several oligomeric (repeating) structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The proteins making up the capsid are called capsid proteins or viral coat proteins (VCP). The capsid and inner genome is called the nucleocapsid.

Capsids are broadly classified according to their structure. The majority of the viruses have capsids with either helical or icosahedral[2][3] structure. Some viruses, such as bacteriophages, have developed more complicated structures due to constraints of elasticity and electrostatics.[4] The icosahedral shape, which has 20 equilateral triangular faces, approximates a sphere, while the helical shape resembles the shape of a spring, taking the space of a cylinder but not being a cylinder itself.[5] 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.[6]

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 in the virus' host; examples include the inner nuclear membrane, the Golgi membrane, and the cell's outer membrane.[7]

Once the virus has infected a cell and begins replicating itself, new capsid subunits are synthesized using the protein biosynthesis mechanism of the cell. In some viruses, including those with helical capsids and especially those with RNA genomes, the capsid proteins co-assemble with their genomes. In other viruses, especially more complex viruses with double-stranded DNA genomes, the capsid proteins assemble into empty precursor procapsids that include a specialized portal structure at one vertex. Through this portal, viral DNA is translocated into the capsid.[8]

Structural analyses of major capsid protein (MCP) architectures have been used to categorise viruses into lineages. For example, the bacteriophage PRD1, the algal virus Paramecium bursaria Chlorella virus-1 (PBCV-1), mimivirus and the mammalian adenovirus have been placed in the same lineage, whereas tailed, double-stranded DNA bacteriophages (Caudovirales) and herpesvirus belong to a second lineage.[9][10][11][12]

  1. ^ Asensio MA, Morella NM, Jakobson CM, Hartman EC, Glasgow JE, Sankaran B, et al. (September 2016). "A Selection for Assembly Reveals That a Single Amino Acid Mutant of the Bacteriophage MS2 Coat Protein Forms a Smaller Virus-like Particle". Nano Letters. 16 (9): 5944–50. Bibcode:2016NanoL..16.5944A. doi:10.1021/acs.nanolett.6b02948. OSTI 1532201. PMID 27549001. S2CID 16706951.
  2. ^ Lidmar J, Mirny L, Nelson DR (November 2003). "Virus shapes and buckling transitions in spherical shells". Physical Review E. 68 (5 Pt 1): 051910. arXiv:cond-mat/0306741. Bibcode:2003PhRvE..68e1910L. doi:10.1103/PhysRevE.68.051910. PMID 14682823. S2CID 6023873.
  3. ^ Vernizzi G, Olvera de la Cruz M (November 2007). "Faceting ionic shells into icosahedra via electrostatics". Proceedings of the National Academy of Sciences of the United States of America. 104 (47): 18382–6. Bibcode:2007PNAS..10418382V. doi:10.1073/pnas.0703431104. PMC 2141786. PMID 18003933.
  4. ^ Vernizzi G, Sknepnek R, Olvera de la Cruz M (March 2011). "Platonic and Archimedean geometries in multicomponent elastic membranes". Proceedings of the National Academy of Sciences of the United States of America. 108 (11): 4292–6. Bibcode:2011PNAS..108.4292V. doi:10.1073/pnas.1012872108. PMC 3060260. PMID 21368184.
  5. ^ Branden C, Tooze J (1991). Introduction to Protein Structure. New York: Garland. pp. 161–162. ISBN 978-0-8153-0270-4.
  6. ^ "Virus Structure (web-books.com)". Archived from the original on 2021-02-07. Retrieved 2007-07-10.
  7. ^ Alberts B, Bray D, Lewis J, Raff M, Roberts K, Watson JD (1994). Molecular Biology of the Cell (4th ed.). p. 280.
  8. ^ Newcomb WW, Homa FL, Brown JC (August 2005). "Involvement of the portal at an early step in herpes simplex virus capsid assembly". Journal of Virology. 79 (16): 10540–6. doi:10.1128/JVI.79.16.10540-10546.2005. PMC 1182615. PMID 16051846.
  9. ^ Krupovic M, Bamford DH (December 2008). "Virus evolution: how far does the double beta-barrel viral lineage extend?". Nature Reviews. Microbiology. 6 (12): 941–8. doi:10.1038/nrmicro2033. PMID 19008892. S2CID 31542714.
  10. ^ Forterre P (March 2006). "Three RNA cells for ribosomal lineages and three DNA viruses to replicate their genomes: a hypothesis for the origin of cellular domain". Proceedings of the National Academy of Sciences of the United States of America. 103 (10): 3669–74. Bibcode:2006PNAS..103.3669F. doi:10.1073/pnas.0510333103. PMC 1450140. PMID 16505372.
  11. ^ Khayat R, Tang L, Larson ET, Lawrence CM, Young M, Johnson JE (December 2005). "Structure of an archaeal virus capsid protein reveals a common ancestry to eukaryotic and bacterial viruses". Proceedings of the National Academy of Sciences of the United States of America. 102 (52): 18944–9. doi:10.1073/pnas.0506383102. PMC 1323162. PMID 16357204.
  12. ^ Laurinmäki PA, Huiskonen JT, Bamford DH, Butcher SJ (December 2005). "Membrane proteins modulate the bilayer curvature in the bacterial virus Bam35". Structure. 13 (12): 1819–28. doi:10.1016/j.str.2005.08.020. PMID 16338410.

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