A viral protein is both a component and a product of a
virus
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
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. Viral proteins are grouped according to their functions, and groups of viral proteins include
structural protein
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, respond ...
s,
nonstructural protein In virology, a nonstructural protein is a protein encoded by a virus but that is not part of the viral particle. They typically include the various enzymes and transcription factors the virus uses to replicate itself, such as a viral protease ( 3CL ...
s,
regulatory proteins, and accessory proteins.
Viruses are non-living and do not have the means to reproduce on their own, instead depending on their host cell's resources in order to reproduce. Thus, viruses do not code for many of their own viral proteins, and instead use the host cell's machinery to produce the viral proteins they require for replication.
Viral structural proteins
Most viral structural proteins are components for the capsid and the envelope of the virus.
Capsid
The genetic material of a virus is stored within a viral protein structure called the capsid. The capsid is a "shield" that protects the viral nucleic acids from getting degraded by host enzymes or other types of pesticides or pestilences. It also functions to attach the
virion
A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.
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to its host, and enable the virion to penetrate the host cell membrane. Many copies of a single viral protein or a number of different viral proteins make up the capsid, and each of these viral proteins are coded for by one
gene
In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
from the viral
genome
In the fields of molecular biology and genetics, 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 g ...
. The structure of the capsid allows the virus to use a small number of viral genes to make a large capsid.
Several
protomer
In structural biology, a protomer is the structural unit of an oligomeric protein. It is the smallest unit composed of at least two different protein chains that form a larger hetero-oligomer by association of two or more copies of this unit.
The ...
s, oligomeric (viral) protein subunits, combine to form
capsomere
The capsomere is a subunit of the capsid, an outer covering of protein that protects the genetic material of a virus. Capsomeres self-assemble to form the capsid.
Subunits called protomers aggregate to form capsomeres. Various arrangements of ca ...
s, and capsomeres come together to form the capsid.
Capsomeres can arrange into an
icosahedral
In geometry, an icosahedron ( or ) is a polyhedron with 20 faces. The name comes and . The plural can be either "icosahedra" () or "icosahedrons".
There are infinitely many non- similar shapes of icosahedra, some of them being more symmetrica ...
, helical, or complex capsid, but in many viruses, such as the herpes simplex virus, an icosahedral capsid is assembled.
Three asymmetric and nonidentical viral protein units make up each of the twenty identical triangular faces in the icosahedral capsid.
Viral envelope
The
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 ma ...
of some viruses are enclosed in a membrane called the viral envelope. In most cases, the viral envelope is obtained by the capsid from the host cell's plasma membrane when a virus leaves its host cell through a process called budding. The viral envelope is made up of a
lipid bilayer
The lipid bilayer (or phospholipid bilayer) is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many vir ...
embedded with viral proteins, including viral
glycoproteins.
These viral glycoproteins bind to specific receptors and coreceptors on the membrane of host cells, and they allow viruses to attach onto their target host cells.
Some of these glycoproteins include:
*
Hemagglutinin
In molecular biology, hemagglutinins (or ''haemagglutinin'' in British English) (from the Greek , 'blood' + Latin , 'glue') are receptor-binding membrane fusion glycoproteins produced by viruses in the '' Paramyxoviridae'' family. Hemagglutinins a ...
,
neuraminidase
Exo-α-sialidase (EC 3.2.1.18, 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)- glyc ...
, and
M2 protein
The Matrix-2 (M2) protein is a proton-selective viroporin, integral in the viral envelope of the influenza A virus. The channel itself is a homotetramer (consists of four identical M2 units), where the units are helices stabilized by two disulfi ...
in the
influenza virus
''Orthomyxoviridae'' (from Greek ὀρθός, ''orthós'' 'straight' + μύξα, ''mýxa'' 'mucus') is a family of negative-sense RNA viruses. It includes seven genera: ''Alphainfluenzavirus'', ''Betainfluenzavirus'', '' Gammainfluenzavirus'', ' ...
*
gp160
''Env'' is a viral gene that encodes the protein forming the viral envelope. The expression of the ''env'' gene enables retroviruses to target and attach to specific cell types, and to infiltrate the target cell membrane.
Analysis of the structur ...
, composed of subunits gp120 and gp41, in 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 acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immun ...
(HIV).
Viral glycoproteins play a critical role in virus-to-cell fusion. Virus-to-cell fusion is initiated when viral glycoproteins bind to cellular receptors.
Viral membrane fusion proteins
The fusion of the viral envelope with the
cellular membrane
The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment (th ...
requires high energy to occur. Viral membrane fusion proteins act as catalysts to overcome this high
energy barrier
In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joules per mole (J/mol), kilojoules pe ...
.
Following viral glycoprotein binding to
cellular receptor
In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a recepto ...
s, viral membrane fusion proteins undergo a change in structure conformation.
This change in conformation then facilitates the destabilization and fusion of the viral envelope with the cellular membrane by allowing fusion loops (FLs) or hydrophobic
fusion peptide
Membrane fusion proteins (not to be confused with chimeric or fusion proteins) are proteins that cause fusion of biological membranes. Membrane fusion is critical for many biological processes, especially in eukaryotic development and viral entry ...
s (FPs) on the viral envelope to interact with the cell membrane.
Most viral membrane fusion proteins would end up in a hairpin-like conformation after fusion, in which FLs/FPs and the transmembrane domain are all on the same side of the protein.
Viral glycoproteins and their three-dimensional structures, before and after fusion, have allowed a wide range of structural conformations to be discovered.
Viral membrane fusion proteins have been grouped into four different classes, and each class is identified by characteristic structural conformations:
* Class I: Post-fusion conformation has a distinct central coiled-coil structure composed of signature trimer of α-helical hairpins. An example of a Class I viral fusion protein is the HIV glycoprotein, gp41.
* Class II: Protein lacks the central coiled-coil structure. Contains a characteristic elongated β- sheet ectodomain structure that refolds to give a trimer of hairpins. Examples of class II viral fusion proteins include the dengue virus E protein, and the west nile virus E protein.
* Class III: Structural conformation is a combination of features from Class I and Class II viral membrane fusion proteins. An example of a Class III viral fusion protein is the rabies virus glycoprotein, G.
* Class IV: Class IV viral fusion proteins are fusion-associated small transmembrane (FAST) proteins. They do not form trimers of hairpins or hairpin structures themselves, and they are the smallest known viral fusion proteins. FAST proteins are coded for by members of the nonenveloped
reoviridae
''Reoviridae'' is a family of double-stranded RNA viruses. Member viruses have a wide host range, including vertebrates, invertebrates, plants, protists and fungi. They lack lipid envelopes and package their segmented genome within multi-layere ...
family of viruses.
Viral nonstructural proteins
Viral nonstructural proteins are proteins coded for by the genome of the virus and are expressed in infected cells.
However, these proteins are not assembled in the virion.
During the replication of viruses, some viral nonstructural proteins carry out important functions that affect the replication process itself.
Similarly, during the assembly of viruses, some of these proteins also carry out important functions that affect the assembly process.
Some of these viral nonstructural protein functions are replicon formation, immunomodulation, and transactivation of viral structural protein encoding genes.
Replicon formation
Viral nonstructural proteins interact with host cell proteins to form the replicon, otherwise known as the replication complex.
In the
hepatitis C virus
The hepatitis C virus (HCV) is a small (55–65 nm in size), enveloped, positive-sense single-stranded RNA virus of the family '' Flaviviridae''. The hepatitis C virus is the cause of hepatitis C and some cancers such as liver cancer (hepatoc ...
, viral nonstructural proteins interact with cellular
vesicle
Vesicle may refer to:
; In cellular biology or chemistry
* Vesicle (biology and chemistry)
In cell biology, a vesicle is a structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer. Vesicles form nat ...
membrane transport protein
A membrane transport protein (or simply transporter) is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral transmembra ...
,
hVAP-33, to assemble the replicon.
Viral nonstructural 4b (
NS4B
Nonstructural protein 4B (NS4B) is a viral protein found in the hepatitis C virus. It has mass of 27 kDa and probably involved in process of intracellular membrane
The endomembrane system is composed of the different membranes (endomembranes) ...
) protein alters the host cell's membrane and starts the formation process of the replication complex.
Other viral nonstructural proteins such as
NS5A
Nonstructural protein 5A (NS5A) is a zinc-binding and proline-rich hydrophilic phosphoprotein that plays a key role in Hepatitis C virus RNA replication. It appears to be a dimeric form without ''trans''-membrane helices.
Structure
NS5A is deri ...
,
NS5B
Nonstructural protein 5B (NS5B) is a viral protein found in the hepatitis C virus (HCV). It is an RNA-dependent RNA polymerase, having the key function of replicating HCV's viral RNA by using the viral positive RNA strand as a template to catalyz ...
, and
NS3, are also recruited to the complex, and NS4B interacts with them and binds to viral
RNA.
Immunomodulation
The
immune response
An immune response is a reaction which occurs within an organism for the purpose of defending against foreign invaders. These invaders include a wide variety of different microorganisms including viruses, bacteria, parasites, and fungi which could ...
of a host to an infected cell can be adjusted through the immunomodulatory properties of viral nonstructural proteins.
Many species of large
DNA virus
A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and ...
es encode proteins which subvert host immune response, allowing proliferation of the virus. Such proteins hold potential in developing new bio-pharmaceutical treatments for
inflammatory disease
Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...
in humans, as the proteins have been proven to subvert inflammatory
immune mediators. Viral nonstructural protein NS1 in the
West Nile virus
West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family '' Flaviviridae'', from the genus '' Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The v ...
prevents complement activation through its binding to a complement control protein, factor H.
As a result, complement recognition of infected cells is reduced, and infected cells remain unharmed by the host's immune system.
Viral regulatory and accessory proteins
Viral regulatory and accessory proteins have many functions. These viral proteins control and influence viral gene expressions in the viral genome, including viral structural gene transcription rates.
Viral regulatory and accessory proteins also influence and adjust cellular functions of the host cell, such as the regulation of genes, and apoptosis.
In DNA viruses and retroviruses, viral regulatory proteins can enhance viral gene transcription, likewise, these proteins can also enhance host cellular gene transcription too.
Viral accessory proteins, also known as auxiliary proteins, are coded for by the genome of retroviruses.
Most viral accessory proteins only carry out their functions in specific types of cells.
Also, they do not have much influence on the replication of the virus.
However, in some instances, maintaining the replication of viruses would require the help (and function) of viral accessory proteins.
Endogenous retroviral proteins
Syncytin
Syncytin-1 also known as enverin is a protein found in humans and other primates that is encoded by the ERVW-1 gene ( endogenous retrovirus group W envelope member 1). Syncytin-1 is a cell-cell fusion protein whose function is best characterized ...
is an
endogenous retrovirus protein that has been captured in the mammalian genome to allow membrane fusion in placental morphogenesis.
References
External links
List of all known Viral Proteins in UniProtKBVirusMint
{{DEFAULTSORT:Viral Protein
Virology