A viroplasm, sometimes called "virus factory" or "virus inclusion", is an inclusion body in a cell where

viral replication Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur. Through the generation of abundant copies of its genome ...

and assembly occurs. They may be thought of as viral factories in the cell. There are many viroplasms in one infected cell, where they appear dense to

electron microscopy An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing i ...

. Very little is understood about the mechanism of viroplasm formation.

Definition

A viroplasm is a perinuclear or a

cytoplasm The cytoplasm describes all the material within a eukaryotic or prokaryotic cell, enclosed by the cell membrane, including the organelles and excluding the nucleus in eukaryotic cells. The material inside the nucleus of a eukaryotic cell a ...

ic large compartment where

and assembly occurs. The viroplasm formation is caused by the interactions between the virus and the infected cell, where viral products and cell elements are confined.

Groups of viruses that form viroplasms

Viroplasms have been reported in many unrelated groups of

Eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

viruses that replicate in

, however, viroplasms from plant viruses have not been as studied as viroplasms from animal viruses. Viroplasms have been found in the cauliflower mosaic virus,

rotavirus Rotaviruses are the most common cause of diarrhea, diarrhoeal disease among infants and young children. Nearly every child in the world is infected with a rotavirus at least once by the age of five. Immunity (medical), Immunity develops with ...

, vaccinia virus and the rice dwarf virus. These appear electron-dense under an

electron microscope An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...

and are insoluble.

Structure and formation

Viroplasms are localized in the perinuclear area or in the

of infected cells and are formed early in the infection cycle. The number and the size of viroplasms depend on the virus, the virus isolate, hosts species, and the stage of the infection. For example, viroplasms of

mimivirus ''Mimivirus'' is a genus of giant viruses, in the family ''Mimiviridae''. It is believed that Amoeba serve as their natural hosts. It also refers to a group of phylogenetically related large viruses. In colloquial speech, APMV is more commonly ...

have a similar size to the nucleus of its host, the

amoeba An amoeba (; less commonly spelled ameba or amœba; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of Cell (biology), cell or unicellular organism with the ability to alter its shape, primarily by ...

'' Acanthamoeba polyphaga''. A virus can induce changes in composition and organization of host cell cytoskeletal and membrane compartments, depending on the step of the

cycle. This process involves a number of complex interactions and signaling events between viral and host cell factors. Viroplasms are formed early during the infection; in many cases, the cellular rearrangements caused during virus infection lead to the construction of sophisticated inclusions —viroplasms— in the cell where the factory will be assembled. The viroplasm is where components such as replicase enzymes, virus genetic material, and host proteins required for replication concentrate, and thereby increase the efficiency of replication. At the same time, large amounts of

ribosomes Ribosomes () are macromolecular machines, found within all cells, that perform biological protein synthesis (messenger RNA translation). Ribosomes link amino acids together in the order specified by the codons of messenger RNA molecules to fo ...

, protein-synthesis components, protein folding chaperones, and

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

are recruited. Some of the membrane components are used for viral replication while some others will be modified to produce viral envelopes, when the viruses are enveloped. The viral replication,

protein synthesis Protein biosynthesis, or protein synthesis, is a core biological process, occurring inside cells, balancing the loss of cellular proteins (via degradation or export) through the production of new proteins. Proteins perform a number of critica ...

and assembly require a considerable amount of energy, provided by large clusters of mitochondria at the periphery of viroplasms. The virus factory is often enclosed by a membrane derived from the

rough endoplasmic reticulum The endoplasmic reticulum (ER) is a part of a transportation system of the eukaryotic cell, and has many other important functions such as protein folding. The word endoplasmic means "within the cytoplasm", and reticulum is Latin for "little n ...

or by cytoskeletal elements. In animal cells, virus particles are gathered by the

microtubule Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nanometer, nm and have an inner diameter bet ...

-dependent aggregation of toxic or misfolded protein near the

microtubule organizing center The microtubule-organizing center (MTOC) is a structure found in eukaryotic cells from which microtubules emerge. MTOCs have two main functions: the organization of eukaryotic flagella and cilia and the organization of the mitotic and meiotic spi ...

(MTOC), so the viroplasms of animal viruses are generally localized near the MTOC. MTOCs are not found in plant cells. Plant viruses induce the rearrangement of membranes structures to form the viroplasm. This is mostly shown for plant

RNA virus An RNA virus is a virus characterized by a ribonucleic acid (RNA) based genome. The genome can be single-stranded RNA (ssRNA) or double-stranded (Double-stranded RNA, dsRNA). Notable human diseases caused by RNA viruses include influenza, SARS, ...

es.

Functions

Viroplasm is the location within the infected cell where

and assembly take place. Wrapping the viroplasm with a membrane, concentrates the viral components required for the

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

replication and the morphogenesis of new virus particles, so it increases the efficiency of the processes. The recruitment of cellular membranes and cytoskeleton to generate virus replication sites can also benefit viruses in other ways. Disruption of cellular membranes can, for example, slow the transport of immunomodulatory proteins to the surface of infected cells and protect against innate and acquired immune responses, and rearrangements to cytoskeleton can facilitate virus release. The viroplasm could also prevent virus degradation by

protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalysis, catalyzes proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the formation of new protein products ...

s and

nuclease In biochemistry, a nuclease (also archaically known as nucleodepolymerase or polynucleotidase) is an enzyme capable of cleaving the phosphodiester bonds that link nucleotides together to form nucleic acids. Nucleases variously affect single and ...

s. In the case of the Cauliflower mosaic virus (CaMV), viroplasms improve the virus transmission by the

aphid Aphids are small sap-sucking insects in the Taxonomic rank, family Aphididae. Common names include greenfly and blackfly, although individuals within a species can vary widely in color. The group includes the fluffy white Eriosomatinae, woolly ...

vector Vector most often refers to: * Euclidean vector, a quantity with a magnitude and a direction * Disease vector, an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematics a ...

. Viroplasms also control release of virions when the insect stings an infected plant cell or a cell near the infected cells.

Possible co-evolution with the host

Aggregated structures may protect viral functional complexes from the cellular degradation systems. For example, formation of viral factories of the ASFV viroplasm is very similar to the aggresome formation. An aggresome is a perinuclear site where misfolded

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

are transported and stored by the cell components for their destruction. It has been proposed that the viroplasm could be the product of a co-evolution between the virus and its host. It is possible that a cellular response originally designed to reduce the toxicity of misfolded proteins is exploited by cytoplasmic viruses to improve their replication, the virus

capsid 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 m ...

synthesis, and assembly. Alternatively, the activation of host defense mechanisms may involve sequestration of virus components in aggregates to prevent their dissemination, followed by their neutralisation. For example, viroplasms of mammalian viruses contain certain elements of the cellular degradation machinery which might enable cellular protective mechanisms against viral components. Given the co-evolution of viruses with their host cells, changes in cell structure induced during infection are likely to involve a combination of the two strategies.

Use in diagnostics

Presence of viroplasms is used to diagnose certain viral infections. Understanding the phenomena of virus aggregation and of the cell response to the presence of virus, and whether viroplasms facilitate or inhibit viral replication, may help to develop new therapeutic approaches against virus infections in animal and plant cells.

References

{{Portal bar, Viruses Virology