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In virology, a spike protein or peplomer protein is a
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
that forms a large structure known as a spike or peplomer projecting from the surface of an enveloped
virus A virus is a submicroscopic infectious agent that replicates only inside the living Cell (biology), cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Viruses are ...
. as cited in The proteins are usually glycoproteins that form dimers or trimers.


History and etymology

The term "peplomer" refers to an individual spike from the viral surface; collectively the layer of material at the outer surface of the
virion A virion (plural, ''viria'' or ''virions'') is an inert virus particle capable of invading a Cell (biology), cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, t ...
has been referred to as the "peplos". The term is derived from the Greek
peplos A peplos () is a body-length garment established as typical attire for women in ancient Greece by , during the late Archaic Greece, Archaic and Classical Greece, Classical period. It was a long, rectangular cloth with the top edge folded down ab ...
, "a loose outer garment", "robe or cloak", or "woman smantle". Early systems of viral taxonomy, such as the LwoffHorneTournier system proposed in the 1960s, used the appearance and morphology of the "peplos" and peplomers as important characteristics for classification. More recently, the term "peplos" is considered a synonym for
viral envelope A viral envelope is the outermost layer of many types of viruses. It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes. A viral envelope protein or E protein is a protein in the en ...
.


Properties

Spikes or peplomers are usually rod- or club-shaped projections from the viral surface. Spike proteins are membrane proteins with typically large external
ectodomain An ectodomain is the domain of a membrane protein that extends into the extracellular space (the space outside a cell). Ectodomains are usually the parts of proteins that initiate contact with surfaces, which leads to signal transduction. A n ...
s, a single transmembrane domain that anchors the protein in the
viral envelope A viral envelope is the outermost layer of many types of viruses. It protects the genetic material in their life cycle when traveling between host cells. Not all viruses have envelopes. A viral envelope protein or E protein is a protein in the en ...
, and a short tail in the interior of the
virion A virion (plural, ''viria'' or ''virions'') is an inert virus particle capable of invading a Cell (biology), cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, t ...
. They may also form protein–protein interactions with other viral proteins, such as those forming the nucleocapsid. They are usually glycoproteins, more commonly via ''N''-linked than ''O''-linked
glycosylation Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not ...
.


Functions

Spikes typically have a role in viral entry. They may interact with cell-surface receptors located on the host cell and may have hemagglutinizing activity as a result, or in other cases they may be
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s. For example, influenza virus has two surface proteins with these two functions, hemagglutinin and
neuraminidase Exo-α-sialidase (, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkag ...
. The binding site for the cell-surface receptor is usually located at the tip of the spike. Many spike proteins are membrane fusion proteins. Being exposed on the surface of the virion, spike proteins can be
antigen In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune response. ...
s.


Examples

Spikes or peplomers can be visible in
electron micrograph A micrograph is an image, captured photographically or digitally, taken through a microscope or similar device to show a magnify, magnified image of an object. This is opposed to a macrograph or photomacrograph, an image which is also taken ...
images of enveloped viruses such as orthomyxoviruses, paramyxoviruses, rhabdoviruses, filoviruses, coronaviruses, bunyaviruses, arenaviruses, and retroviruses.


Coronaviruses

Coronavirus Coronaviruses are a group of related RNA viruses that cause diseases in mammals and birds. In humans and birds, they cause respiratory tract infections that can range from mild to lethal. Mild illnesses in humans include some cases of the comm ...
es exhibit coronavirus spike protein, also known as the S protein, on their surfaces; S is a class I fusion protein and is responsible for mediating viral entry as the first step in viral infection. It is highly antigenic and accounts for most
antibodies An antibody (Ab) or immunoglobulin (Ig) is a large, Y-shaped protein belonging to the immunoglobulin superfamily which is used by the immune system to identify and neutralize antigens such as bacteria and viruses, including those that caus ...
produced by the
immune system The immune system is a network of biological systems that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to bacteria, as well as Tumor immunology, cancer cells, Parasitic worm, parasitic ...
in response to infection. For this reason the spike protein has been the focus of development for COVID-19 vaccines in response to the COVID-19 pandemic caused by the virus
SARS-CoV-2 Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is a strain of coronavirus that causes COVID-19, the respiratory illness responsible for the COVID-19 pandemic. The virus previously had the Novel coronavirus, provisional nam ...
. A
subgenus In biology, a subgenus ( subgenera) is a taxonomic rank directly below genus. In the International Code of Zoological Nomenclature, a subgeneric name can be used independently or included in a species name, in parentheses, placed between the ge ...
of the betacoronaviruses, known as embecoviruses (not including SARS-like coronaviruses), have an additional shorter surface protein known as
hemagglutinin esterase Hemagglutinin esterase (HEs) is a glycoprotein that certain enveloped viruses possess and use as an invading mechanism. HEs helps in the attachment and destruction of certain sialic acid receptors that are found on the host cell surface. V ...
. The COVID-19 pandemic necessitated identification of viral particles in electron micrographs of patient tissue samples. A number of reports misidentified normal subcellular structures as coronaviruses due to their superficial resemblance to coronavirus morphology, and because the distinctive spikes of coronaviruses are apparent by negative stain but much less visible in thin section.


Influenza viruses

Most influenza virus subgroups have two surface proteins described as peplomers,
neuraminidase Exo-α-sialidase (, sialidase, neuraminidase; systematic name acetylneuraminyl hydrolase) is a glycoside hydrolase that cleaves the glycosidic linkages of neuraminic acids: : Hydrolysis of α-(2→3)-, α-(2→6)-, α-(2→8)- glycosidic linkag ...
(an
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
) and hemagglutinin (also a class I fusion protein). Some instead have a single
hemagglutinin esterase Hemagglutinin esterase (HEs) is a glycoprotein that certain enveloped viruses possess and use as an invading mechanism. HEs helps in the attachment and destruction of certain sialic acid receptors that are found on the host cell surface. V ...
protein with both functions.


Retroviruses

Retroviruses such as the
human immunodeficiency virus The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause AIDS, acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of th ...
(HIV) have surface peplomers. These are
protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chains. Protein complexes are distinct from multidomain enzymes, in which multiple active site, catalytic domains are found in a single polypeptide chain. ...
es formed by two proteins, gp41 and gp120, both expressed from the '' env'' gene, collectively forming a spike protein complex that mediates viral entry.


Gallery


See also

* Viral entry * Viral life cycle


References

{{Use dmy dates, date=December 2020 Virology