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Envelope glycoprotein GP120 (or gp120) is a
glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glyco ...
exposed on the surface of the
HIV envelope ''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 structure ...
. It was discovered by Professors Tun-Hou Lee and Myron "Max" Essex of the
Harvard School of Public Health The Harvard T.H. Chan School of Public Health is the public health school of Harvard University, located in the Longwood Medical Area of Boston, Massachusetts. The school grew out of the Harvard- MIT School for Health Officers, the nation's firs ...
in 1988. The 120 in its name comes from its
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
of 120
kDa The dalton or unified atomic mass unit (symbols: Da or u) is a non-SI unit of mass widely used in physics and chemistry. It is defined as of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at re ...
. Gp120 is essential for virus entry into cells as it plays a vital role in attachment to specific
cell surface receptor Cell surface receptors (membrane receptors, transmembrane receptors) are receptors that are embedded in the plasma membrane of cells. They act in cell signaling by receiving (binding to) extracellular molecules. They are specialized integral m ...
s. These receptors are DC-SIGN,
Heparan Sulfate Proteoglycan Heparan sulfate (HS) is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan (HSPG, i.e. Heparan Sulfate ProteoGlycan) in which two or three HS chains are attached in close proximity to cell surface or extracellular ...
and a specific interaction with the
CD4 In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). CD4 is found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic ce ...
receptor Receptor may refer to: *Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse *Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
, particularly on
helper T-cell The T helper cells (Th cells), also known as CD4+ cells or CD4-positive cells, are a type of T cell that play an important role in the adaptive immune system. They aid the activity of other immune cells by releasing cytokines. They are considere ...
s. Binding to CD4 induces the start of a cascade of
conformational change In biochemistry, a conformational change is a change in the shape of a macromolecule, often induced by environmental factors. A macromolecule is usually flexible and dynamic. Its shape can change in response to changes in its environment or oth ...
s in gp120 and gp41 that lead to the fusion of the viral membrane with the host
cell 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 (t ...
. Binding to CD4 is mainly
electrostatic Electrostatics is a branch of physics that studies electric charges at rest ( static electricity). Since classical times, it has been known that some materials, such as amber, attract lightweight particles after rubbing. The Greek word for ...
although there are van der Waals interactions and
hydrogen bonds In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
. Gp120 is coded by the HIV ''env''
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 ...
, which is around 2.5 kb long and codes for around 850 amino acids.Kuiken, C., Leitner, T., Foley, B., ''et al.'' (2008)
"HIV Sequence Compendium"
Los Alamos National Laboratory.
The primary ''env'' product is the protein gp160, which gets cleaved to gp120 (~480 amino acids) and gp41 (~345 amino acids) in the endoplasmatic reticulum by the cellular
protease A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the ...
furin Furin is a protease, a proteolytic enzyme that in humans and other animals is encoded by the ''FURIN'' gene. Some proteins are inactive when they are first synthesized, and must have sections removed in order to become active. Furin cleaves these s ...
. The crystal structure of core gp120 shows an organization with an outer domain, an inner domain with respect to its termini and a bridging sheet. Gp120 is anchored to the
viral membrane 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. Numerous human pathogenic viruses in circulation are encase ...
, or envelope, via non-covalent bonds with the
transmembrane A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequent ...
glycoprotein Glycoproteins are proteins which contain oligosaccharide chains covalently attached to amino acid side-chains. The carbohydrate is attached to the protein in a cotranslational or posttranslational modification. This process is known as glyco ...
, gp41. Three gp120s and gp41s combine in a trimer of
heterodimer In biochemistry, a protein dimer is a macromolecular complex formed by two protein monomers, or single proteins, which are usually non-covalently bound. Many macromolecules, such as proteins or nucleic acids, form dimers. The word ''dimer'' ha ...
s to form the envelope spike, which mediates attachment to and entry into the host cell.


Variability

Since gp120 plays a vital role in the ability of HIV-1 to enter CD4+ cells, its evolution is of particular interest. Many neutralizing antibodies bind to sites located in variable regions of gp120, so mutations in these regions will be selected for strongly. The diversity of ''env'' has been shown to increase by 1-2% per year in HIV-1 group M and the variable units are notable for rapid changes in amino acid sequence length. Increases in gp120 variability result in significantly elevated levels of viral replication, indicating an increase in viral fitness in individuals infected by diverse HIV-1 variants. Further studies have shown that variability in potential N-linked glycosylation sites (PNGSs) also result in increased viral fitness. PNGSs allow for the binding of long-chain carbohydrates to the high variability regions of gp120, so the authors hypothesize that the number of PNGSs in ''env'' might affect the fitness of the virus by providing more or less sensitivity to neutralizing antibodies. The presence of large carbohydrate chains extending from gp120 might obscure possible antibody binding sites. The boundaries of the potential to add and eliminate PNGSs are naively explored by growing viral populations following each new infection. While the transmitting host has developed a neutralizing antibody response to gp120, the newly infected host lacks immune recognition of the virus. Sequence data shows that initial viral variants in an immunologically naïve host have few glycosylation sites and shorter exposed variable loops. This may facilitate viral ability to bind host cell receptors. As the host immune system develops antibodies against gp120, immune pressures seem to select for increased glycosylation, particularly on the exposed variable loops of gp120. Consequently, insertions in ''env'', which confer more PNGSs on gp120 may be more tolerated by the virus as higher glycan density promotes the viral ability to evade antibodies and thus promotes higher viral fitness. In considering how much PNGS density could theoretically change, there may be an upper bound to PNGS number due to its inhibition of gp120 folding, but if the PNGS number decreases substantially, then the virus is too easily detected by neutralizing antibodies. Therefore, a stabilizing selection balance between low and high glycan densities is likely established. A lower number of bulky glycans improves viral replication efficiency and higher number on the exposed loops aids host immune evasion via disguise. The relationship between gp120 and neutralizing antibodies is an example of Red Queen evolutionary dynamics. Continuing evolutionary adaptation is required for the viral envelope protein to maintain fitness relative to the continuing evolutionary adaptations of the host immune neutralizing antibodies, and vice versa, forming a coevolving system.


Vaccine target

Since CD4 receptor binding is the most obvious step in HIV infection, gp120 was among the first targets of
HIV vaccine An HIV vaccine is a potential vaccine that could be either a preventive vaccine or a therapeutic vaccine, which means it would either protect individuals from being infected with HIV or treat HIV-infected individuals. It is thought that an HIV ...
research. Efforts to develop HIV vaccines targeting gp120, however, have been hampered by the chemical and structural properties of gp120, which make it difficult for
antibodies An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of ...
to bind to it. gp120 can also easily be shed from the surface of the virus and captured by
T cell A T cell is a type of lymphocyte. T cells are one of the important white blood cells of the immune system and play a central role in the adaptive immune response. T cells can be distinguished from other lymphocytes by the presence of a T-cell r ...
s due to its loose binding with gp41. A conserved region in the gp120 glycoprotein that is involved in the metastable attachment of gp120 to CD4 has been identified and targeting of invariant region has been achieved with a broadly neutralising antibody, IgG1-b12.
NIH The National Institutes of Health, commonly referred to as NIH (with each letter pronounced individually), is the primary agency of the United States government responsible for biomedical and public health research. It was founded in the late ...
research published in
Science Science is a systematic endeavor that builds and organizes knowledge in the form of testable explanations and predictions about the universe. Science may be as old as the human species, and some of the earliest archeological evidence ...
reports the isolation of 3 antibodies that neutralize 90% of
HIV-1 The subtypes of HIV include two major types, HIV type 1 (HIV-1) and HIV type 2 (HIV-2). HIV-1 is related to viruses found in chimpanzees and gorillas living in western Africa, while HIV-2 viruses are related to viruses found in the sooty mangabey ...
strains at the CD4bs region of gp120, potentially offering a therapeutic and vaccine strategy

However, most antibodies that bind the CDbs region of gp120 do not neutralize HIV, and rare ones that do such as IgG1-b12 have unusual properties such as asymmetry of the Fab arms or in their positioning. Unless a gp120-based vaccine can be designed to elicit antibodies with strongly neutralizing antiviral properties, there is concern that breakthrough infection leading to humoral production of high levels of non-neutralizing antibodies targeting the CD4 binding site of gp120 is associated with faster disease progression to AIDS.


Competition

The protein gp120 is necessary during the initial binding of HIV to its target cell. Consequently, anything which binds to gp120 or its targets can physically block gp120 from binding to a cell. Only one such agent,
Maraviroc Maraviroc, sold under the brand names Selzentry (US) and Celsentri (EU), is an antiretroviral medication used to treat HIV infection. It is taken by mouth. It is in the CCR5 receptor antagonist class. It was approved for medical use in the Un ...
, which binds the
co-receptor A co-receptor is a cell surface receptor that binds a signalling molecule in addition to a primary receptor in order to facilitate ligand recognition and initiate biological processes, such as entry of a pathogen into a host cell. Properties The ...
CCR5 C-C chemokine receptor type 5, also known as CCR5 or CD195, is a protein on the surface of white blood cells that is involved in the immune system as it acts as a receptor for chemokines. In humans, the ''CCR5'' gene that encodes the CCR5 p ...
is currently licensed and in clinical use. No agent targeting gp120's main first cellular interaction partner,
CD4 In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). CD4 is found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic ce ...
, is currently licensed since interfering with such a central molecule of the immune system can cause toxic side effects, such as the anti-CD4 monoclonal antibody OKT4. Targeting gp120 itself has proven extremely difficult due to its high degree of variability and shielding. Fostemsavir (BMS-663068) is a methyl phosphate prodrug of the small molecule inhibitor BMS-626529, which prevents viral entry by binding to the viral envelope gp120 and interfering with virus attachment to the host CD4 receptor.aidsinfo.nih.gov/drugs/508/bms-663068/0/professional


HIV dementia

The HIV viral protein gp120 induces apoptosis of neuronal cells by inhibiting levels of furin and tissue plasminogen activator, enzymes responsible for converting pBDNF to mBDNF. gp120 induces mitochondrial-death proteins like caspases which may influence the upregulation of the death receptor Fas leading to apoptosis of neuronal cells, gp120 induces oxidative stress in the neuronal cells, and it is also known to activate STAT1 and induce interleukins IL-6 and IL-8 secretion in neuronal cells.


See also

* HIV envelope gene * HIV entry to the cell * '' gp41'' *
CD4 In molecular biology, CD4 (cluster of differentiation 4) is a glycoprotein that serves as a co-receptor for the T-cell receptor (TCR). CD4 is found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic ce ...
*
CCR5 C-C chemokine receptor type 5, also known as CCR5 or CD195, is a protein on the surface of white blood cells that is involved in the immune system as it acts as a receptor for chemokines. In humans, the ''CCR5'' gene that encodes the CCR5 p ...
*
Entry inhibitor Entry inhibitors, also known as fusion inhibitors, are a class of antiviral drugs that prevent a virus from entering a cell, for example, by blocking a receptor. Entry inhibitors are used to treat conditions such as HIV and hepatitis D. HIV entr ...
* Structure and genome of HIV


References


Further reading


Human Immunodeficiency Virus Glycoprotein 120
*


External links

* https://web.archive.org/web/20060219135317/http://www.aidsmap.com/en/docs/4406022B-85D7-4A9B-B700-91336CBB6B18.asp * http://www.mcld.co.uk/hiv/?q=gp120 * http://www.ebi.ac.uk/interpro/IEntry?ac=IPR000777 * {{DEFAULTSORT:Envelope Glycoprotein Gp120 Glycoproteins HIV/AIDS Viral structural proteins