出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/02/10 21:09:07」(JST)
Variable surface glycoproteins or Variant surface glycoproteins (VSG) are a type of proteins coating the surface of some infectious microorganisms (e.g. Trypanosoma brucei) and helping them to evade the host's immune system by mean of antigenic variation. The VSG protein is a key molecule for immune escape and parasitic success.
Combinatorial processes increase the diversity of variable surface glycoproteins.[1] The parasite is expressing a series of antigenically distinct VSGs from an estimated 1000 VSG genes. The genes are located in subtelomeric region and are often activated by the duplicative transposition of a silent basic copy gene into an unlinked telomerically located expression site, producing an active expression-linked copy of that gene.[2]
| Variable surface glycoprotein | |
|---|---|
| Identifiers | |
| Organism | Trypanosoma brucei |
| Symbol | Tb927.5.4730 |
| Alt. symbols | Tb05.26C7.380 |
| Entrez | 3657576 |
| Other data | |
| Chromosome | 5: 1.41 - 1.41 Mb |
| Variant surface glycoprotein MITAT 1.2 | |
|---|---|
| Identifiers | |
| Organism | Trypanosoma brucei |
| Symbol | ? |
| Alt. symbols | VSG 221 |
| UniProt | P26332 |
| Other data | |
In Trypanosoma brucei (see Trypanosoma_brucei#VSG_coat), the variable surface glycoprotein is a major surface antigen.[3]
The cell surface of the bloodstream form features a dense coat of variable surface glycoproteins which is replaced by an equally dense coat of procyclins when the parasite differentiates into the procylic form in the tsetse fly midgut. There is a very fast inhibition of VSG gene transcription which occurs as soon as the temperature is lowered.[4]
The VSGs from T. brucei are attached to the plasma membrane via a glycosyl-phosphatidylinositol (GPI) anchor.[5]
The coat is composed of VSG dimers and forms a macromolecular diffusion barrier. Glycoproteins vary in primary amino acid sequence, the number of N-glycosylation sites, and the types of N-linked oligosaccharides and glycosylphosphatidylinositol membrane anchors they contain. VSG MITat.1.5 is glycosylated at all three potential N-glycosylation sites.[6]
The Bloodstream Expression Site (BES), or Telomeric Expression Site, is used for changing different variable surface glycoproteins when in host's blood stream to escape the complement system. BESs are polymorphic in size and structure but reveal a surprisingly conserved architecture in the context of extensive recombination. Very small BESs do exist and many functioning BESs do not contain the full complement of expression site associated genes (ESAGs).[7] There is a collection of an estimated 20-30 sites, each being active at a time.[8] Active VSG expression sites are depleted of nucleosomes.[9]
The gene repertoires in T brucei have diverged to become strain-specific.[10]
The variable surface glycoprotein genes of T. brucei have been classified into two groups depending upon whether or not duplication of the genes is observed when they are expressed.[11]
Variable surface glycoproteins are also found in other Trypanosoma species,
In Trypanosoma equiperdum, a parasaite causing the covering sickness in horses, These proteins allow the parasite to efficiently evade the host animal's immune system.[12] These VSGs allow the organism to constantly manipulate and change the surface structure of its proteins, which means it is constantly being presented to the immune system as a new foreign organism and this prevents the body from mounting a large enough immune response to eradicate the disease.[12] In this sense, Trypanosoma equiperdum is a very efficient organism; it may infect less species than other diseases, but it infects and survives very efficiently within its specified hosts. The VSG proteins in T. equiperdum are also phosphorylated.[13]
A VSG gene from Trypanosoma evansi, a parasite that causes a form of surra in animals, has been cloned in Escherichia coli. The expressed protein is immunoreactive with all the sera combinations. The animals immunized with whole cell lysate or recombinant protein show similar antibody reactions in ELISA (Enzyme-linked immunosorbent assay) and CATT (Card Agglutination Test for Trypanosomiasis).[14] The variable surface glycoprotein RoTat 1.2 PCR can be used as a specific diagnostic tool for the detection of T. evansi infections.[15]
The smallest VSG protein (40 kDa in size) to date (1996) has been found in Trypanosoma vivax, which bears little carbohydrate.[16]
INn Trypanosoma congolense, in vitro analyses of the incorporated sugars after hydrolysis of the glycoprotein showed that glucosamine and mannose are utilized in the biosynthesis of the carbohydrate moiety directly whereas galactose was converted possibly to other intermediates before being incorporated into the antigen. The unglycosylated VSG with a molecular weight of 47 kDa had completely lost its size heterogeneity.[17]
全文を閲覧するには購読必要です。 To read the full text you will need to subscribe.
| リンク元 | 「VSG」「変異表面糖タンパク質」「異型表面糖タンパク質」 |
| 拡張検索 | 「Trypanosoma variant surface glycoprotein」 |
| 関連記事 | 「variant」「surface」「surface glycoprotein」 |