Capsid

For the leaf bug, see Miridae

Miridae

The large and diverse insect family Miridae contains the plant bugs, leaf bugs, and grass bugs, and may also be known as capsid bugs. It is the largest family of true bugs belonging to the suborder Heteroptera, with over 10,000 known species and new ones constantly being described...

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A capsid is the protein shell of a virus. It consists of several oligomer

Oligomer

In chemistry, an oligomer consists of a less than five number of monomer units , in contrast to a polymer that, at least in principle, consists of an unlimited number of monomers. Many oils are oligomeric, such as liquid paraffin. Plasticizers are oligomeric esters widely used to soften...

ic structural subunits made of protein

Protein

Proteins are organic compounds made of amino acids arranged in a linear chain and folded into a globular form. The amino acids in a polymer chain are joined together by the peptide bonds between the carboxyl and amino groups of adjacent amino acid residues...

 called protomer

Protomer

In structural biology, a protomer is the structural unit of an oligomeric protein. A protomer can be a protein subunit or several different subunits, that assemble in a defined stoichiometry to form an oligomer. The protomer is the smallest subset of different subunits that form the oligomer. The...

s. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomere

Capsomere

Capsomeres make up the capsid, an outer covering of protein that protects the genetic material of a virus.Capsomere: protein subunits of the capsid. They are self-assembled to form the capsid....

s. 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

Sphere

A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...

, while the helical shape is cylindrical. The capsid faces may consist of one or more proteins. For example, the foot-and-mouth disease

Foot-and-mouth disease

Foot-and-mouth disease, FMD or hoof-and-mouth disease is a highly contagious and sometimes fatal viral disease of cloven-hoofed animals, including domestic animals such as cattle, water buffalo, sheep, goats and pigs, as well as antelope, bison and other wild bovids, and deer...

 virus capsid has faces consisting of three proteins named VP1-3.

Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope

Viral envelope

Many viruses have viral envelopes covering their protein capsids. The envelopes typically are derived from portions of the host cell membranes , but include some viral glycoproteins. Functionally, viral envelopes are used to help viruses enter host cells...

. The envelope is acquired by the capsid from an intracellular membrane in the virus' host; some examples would include the inner nuclear membrane, the golgi

Golgi

Golgi may refer to:*Camillo Golgi , Italian physician and scientist after which the following terms are named:**Golgi apparatus , an organelle in the eukaryotic cell...

 membrane, or the cell's outer membrane

Cell membrane

The cell membrane is the biological membrane separating the interior of a cell from the outside environment....

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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

Protein biosynthesis

Protein synthesis is the process in which cells build proteins. The term is sometimes used to refer only to protein translation but more often it refers to a multi-step process, beginning with amino acid synthesis and transcription of nuclear DNA into messenger RNA which is then used as input to...

 mechanism of the cell. During the assembly process, a portal subunit is assembled at one vertex of the capsid. Through this portal, viral DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information...

 or RNA

RNA

Ribonucleic acid is a biologically important type of molecule that consists of a long chain of nucleotide units. Each nucleotide consists of a nitrogenous base, a ribose sugar, and a phosphate...

 is transported into the capsid. The structure and assembly of the herpes virus capsid portal protein has been imaged via cryo-electron microscopy

Cryo-electron microscopy

Electron cryomicroscopy is a form of electron microscopy where the sample is studied at cryogenic temperatures...

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It is this capsid or protein shell which makes protective vaccines a possibility. 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.