Virion

A virion (plural, ''viria'' or ''virions'') is an inert virus particle capable of invading a cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, thus enabling the virus to replicate. The genetic material (''core'', either DNA or RNA, along with occasionally present virus core protein) inside the virion is usually enclosed in a protection shell, known as the capsid.

While the terms "virus" and "virion" are occasionally confused, recently "virion" is used solely to describe the virus structure outside of cells, while the terms "virus/viral" are broader and also include biological properties such as the infectivity of a virion.

Components

A virion consists of one or more nucleic acid genome molecules (single-stranded or double-stranded RNA or DNA) and coatings (a capsid and possibly a viral envelope). The virion may contain other proteins (for example with enzymatic activities) and/or nucleoproteins.

Capsid

In the vast majority of viruses, the DNA and RNA components are packed into a protein shell, the capsid. The capsid proteins are often differentiated into major and minor capsid proteins (MCP and mCP). In exceptional cases, there are also viruses without a capsid (i.e., true virions), such as the RNA viruses of the '' Narnaviridae'' and the viroids of the '' Pospiviroidae'' (with the Citrus Exocortis Viroid and the Citrus Bark Crack Viroid).

If the genome consists of several segments, these are usually packaged together in a capsid (e.g., influenza viruses), and in some viruses, the segments can also be individually packaged in their own capsids (e.g., in '' Nanoviridae'').

Since the genome of viruses is relatively simple, the capsid architecture relies on repetition of simple structures, similar to the faces of a polyhedron. Each face in turn is formed by a repetition of simpler sub-units, with the amount of repetitions called a triangulation number (T). Similar capsid structures can be used by many different types of viruses.

In many viruses, the virions have icosahedral symmetry, which can be ideally isometric or elongated. Many virions also have other shapes:
* '' Inoviridae'' and ''Filoviridae'': thread-like/filamentous/helical
* '' Ampullaviridae'': bottle-shaped
* '' Bicaudaviridae'', ''Fuselloviridae'', ''Halspiviridae'' and '' Thaspiviridae'': spindle- to lemon-shaped
* ''Poxviridae'' and '' Ovaliviridae'': ovoid to ellipsoid
* ''Gammaretrovirus'' and other ''Retroviridae'', such as HIV, roughly round to complexly multiform ( pleomorphic).
From observations using microscopy, there are indications of many more distinct shapes.

Tail

In some groups of viruses—such as the class '' Caudoviricetes'' ("tail viruses") and the genus '' Tupanvirus''—the capsid carries an appendage called the "tail".

The tail of the '' Caudoviricetes'' is usually divided into:
* a neck, possibly with collar a long, possibly contractile tail sheath
* base plate
* possibly tail fibers/tail spikes
The latter are used to establish contact with the host cell. The tail of these viruses serves as an injection device to introduce their own genome into the host cell. The ''Caudoviricetes'' tail material is also differentiated into major and minor tail proteins (MTP and mTP), as seen in the Enterobacteria phage lambda. In addition, there may be a tail spike protein (TSP) or tail fiber protein (TFP).

Even in viruses with helical morphology (such as the '' Rudiviridae'' and '' Ahmunviridae''), the terminal fiber proteins responsible for the receptor binding are called tail fiber proteins.

Spikes

Spikes (peplomers) can protrude from the capsid, as in the ''Coronaviridae'', the '' Tectiviridae'', and others. These are used to establish contact with the host cell.

In viruses of the genus '' Chlorovirus'', the virions have a single spike that serves as an injection device; an extendable injection apparatus are found in virions of the family '' Tectiviridae''.

Viral envelope

In many virus species, the virion also has an outer membrane, the viral envelope. The envelope includes a lipid bilayer and surface proteins, similar to the cell membranes, that are usually used for the envelope construction when the virus is exiting the cell. This structure helps with attachment to the cell and also assists evading the immune system of the host organism while the virion is searching for a cell to infect.

References

Protein Data Bank in Europe
NMR structure of the gpu tail protein from lambda bacteriophage
On: ''ebi.ac.uk''


N. J. Dimmock, Andrew J. Easton, Keith Leppard: ''Introduction to Modern Virology.'' 6th edition, Wiley & Blackwell, Malden 2007, ISBN 978-1-4051-3645-7, p. 49, Chapter 4: ''Classification of Viruses.''.

Matthew Dunne, Nikolai S. Prokhorov, Martin J. Loessner, Petr G. Leiman: ''Reprogramming bacteriophage host range: design principles and strategies for engineering receptor binding proteins.'' In: ''Current Opinion in Biotechnology.'' Volume 68, April 2021, pp. 272–281, doi:10.1016/j.copbio.2021.02.006.

Audrey Leprince, Jacques Mahillon: ''Phage Adsorption to Gram-Positive Bacteria.'' In: ''MDPI'': ''Viruses.'' Volume 15, No. 1, October 29, 2022, p. 196, doi:10.3390/v15010196.


Yu Zhang, Zhongjie Zhu, Yuchan Ma, Zhifeng Fu: ''Paper-based analytical device integrated with bacteriophage tail fiber protein for bacteria detection and antimicrobial susceptibility test.'' In: ''Biosensors and Bioelectronics'', volume 239, November 1, 2023, p. 115629; doi:10.1016/j.bios.2023.115629.

Sources

* {{cite book, last1=Reynolds, first1=M.M., last2=Theodore, first2=L., title=A Guide to Virology for Engineers and Applied Scientists: Epidemiology, Emergency Management, and Optimization, publisher=Wiley, year=2023, isbn=978-1-119-85313-8, chapter-url=https://books.google.com/books?id=ILukEAAAQBAJ&pg=PA24, chapter=Basics of Virology, access-date=2024-11-30, pages=19–32

Virology