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Variant surface glycoprotein

Variant surface glycoprotein is a biology topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Variant surface glycoprotein rather than just read about it. In short: Variant surface glycoprotein (VSG) is a ~60kDa protein which densely packs the cell surface of protozoan parasites belonging to the genus Trypanosoma. This genus is notable for their cell surface proteins.

Variant surface glycoprotein — main illustration
Variant surface glycoprotein — illustration

Key takeaways

  • Variant surface glycoprotein belongs to biology; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Variant surface glycoprotein to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Variant surface glycoprotein from memory before moving on to harder problems.

Reference excerpt

Variant surface glycoprotein (VSG) is a ~60kDa protein which densely packs the cell surface of protozoan parasites belonging to the genus Trypanosoma. This genus is notable for their cell surface proteins. They were first isolated from Trypanosoma brucei in 1975 by George Cross. VSG allows the trypanosomatid parasites to evade the mammalian host's immune system by extensive antigenic variation. They form a 12–15 nm surface coat. VSG dimers make up ~90% of all cell surface protein and ~10% of total cell protein. For this reason, these proteins are highly immunogenic and an immune response raised against a specific VSG coat will rapidly kill trypanosomes expressing this variant. However, with each cell division there is a possibility that the progeny will switch expression to change the VSG that is being expressed. VSG has no prescribed biochemical activity.

The parasite has a large cellular repertoire of antigenically distinct VSGs (~1500/2000 complete and partial (pseudogenes)) located in telomeric and subtelomeric arrays (on megabase chromosomes or minichromosomes). VSGs are expressed from a bloodstream expression site (BES, ES) in a polycistron by RNA polymerase I (recruited to a ribosomal-type promoter) with other ES-associated genes (ESAGs), of which transferrin receptor (Tfr: ESAG6, ESAG7) is one. Only one VSG gene is expressed at a time, as only one of the ~15 ES are active in a cell. VSG expression is 'switched' by homologous recombination of a silent basic copy gene from an array (directed by homology) into the active telomerically located expression site. During this transition, trypanosomes simultaneously display both pre- and post-switch VSGs on their surface. This coat replacement process is critical for the survival of recently switched cells because initial VSGs remain targets for the escalating host Ab response. Mosaic VSG genes can be created by homologous recombination of a partial VSG gene from an array. This partial gene may replace any portion of the residing VSG gene, creating a new mosaic VSG. VSG half-life measurements suggest that initial VSGs may persist on the surface of genetically switched trypanosomes for several days. It remains unclear whether the regulation of VSG switching is purely stochastic or whether environmental stimuli affect switching frequency. The fact that switching occurs in vitro suggests that there is at least some host-independent, stochastic element to the process. The antigenic variation causes cyclical waves of parasitemia, which is one of the characteristics of human African trypanosomiasis. The cyclical process take 5–8 days. This occurs because a diverse range of coats expressed by the trypanosome population means that the immune system is always one step behind: it takes several days for an immune response against a given VSG to develop, giving the population time to diversify as individuals undergo further switching events. The repetition of this process prevents the extinction of the infecting trypanosome population, allowing chronic persistence of parasites in the host and enhancing opportunities for transmission.

In Trypanosoma brucei In Trypanosoma brucei, the cell surface is covered by a dense coat of ~5 million VSG dimers, ~90% of all cell surface protein and ~10% of total cell protein. The properties of the VSG coat that enable immune evasion are:

Shielding – the dense nature of the VSG coat (VSG proteins pack shoulder-to-shoulder) prevents the immune system of the mammalian host from accessing the plasma membrane or any other parasitic invariant surface epitopes (such as ion channels, transporters, receptors etc.). The coat is uniform, made up of millions of copies of the same molecule; therefore, VSG is the only part of the trypanosome that the immune system can recognize. Periodic antigenic variation – the VSG coat undergoes frequent stochastic genetic modification—'switching'—allowing variants expressing a new VSG coat to escape the specific immune response raised against the previous coat. This antigenic variation creates cyclical waves of parasitemia characteristic of Human African Trypanosomiasis. Antigen 'cleaning' and VSG recycling—VSG is efficiently recycled through the trypanosome flagellar pocket, allowing antibodies to be 'cleaned' from VSG before re-incorporation back into the cellular membrane. Importantly, VSGs recognized and bound by antibodies are selectively pushed toward the flagellar pocket at a quicker rate than unidentified VSG; in this scenario, the antibody acts as a 'sail', which quickens the process of VSG being brought to the area of recycling. The VSGs from T. brucei are attached to the plasma membrane via a covalent attachment to two glycosyl-phosphatidylinositol (GPI) anchors (one per monomer), which directs its forward-trafficking from the ER to the flagellar pocket for incorporation into the membrane, as predicted by the GPI valence hypothesis. VSGs are replaced by an equally dense coat of procyclins when the parasite differentiates into the procyclic 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.

… excerpt ends here. Continue reading the full article.

Illustrations

Variant surface glycoprotein: The Trypanosoma brucei cell membrane is densely packed with VSG dimers, which make up ~90% of its cell surface protein, and which allows for the parasite to evade the immune system and establish chronic infection.
The Trypanosoma brucei cell membrane is densely packed with VSG dimers, which make up ~90% of its cell surface protein, and which allows for the parasite to evade the immune system and establish chronic infection.
Variant surface glycoprotein: Mechanisms of VSG switching in T. brucei: A. Structure of the expression site including the expression site associated genes (ESAG), 70 base pair repeat up-stream sequence, expressed VSG gene, and the telomere B. Mechanism of array VSG conversion: A silent VSG is copied from a subtelomeric VSG array into an ES, where it replaces the active VSG. C. Telomeric VSG conversion: A telomeric VSG (including 70 bp repeat sequence upstream and telomere downstream) replaces the active VSG in the ES D. Segmental VSG conversion: Sequence is copied from multiple inactive VSG genes and combined into a novel mosaic VSG that occupies the ES E. Transcriptional VSG switching: A non-recombination based mechanism that activates a new (previously silent) ES, while inactivating the previously active ES.
Mechanisms of VSG switching in T. brucei: A. Structure of the expression site including the expression site associated genes (ESAG), 70 base pair repeat up-stream sequence, expressed VSG gene, and the telomere B. Mechanism of array VSG conversion: A silent VSG is copied from a subtelomeric VSG array into an ES, where it replaces the active VSG. C. Telomeric VSG conversion: A telomeric VSG (including 70 bp repeat sequence upstream and telomere downstream) replaces the active VSG in the ES D. Segmental VSG conversion: Sequence is copied from multiple inactive VSG genes and combined into a novel mosaic VSG that occupies the ES E. Transcriptional VSG switching: A non-recombination based mechanism that activates a new (previously silent) ES, while inactivating the previously active ES.
Variant surface glycoprotein: Structure of one of the N-terminal VSG variants. VSG genes have a largely conserved N-terminal secondary and tertiary structure (composed of two alpha-helices which form the dimerization interface, and which couple into a four helix bundle), while still allowing for variable primary sequence. This variable primary sequence allows for VSG to be antigenically distinct from one another, the crux to antigenic variation.
Structure of one of the N-terminal VSG variants. VSG genes have a largely conserved N-terminal secondary and tertiary structure (composed of two alpha-helices which form the dimerization interface, and which couple into a four helix bundle), while still allowing for variable primary sequence. This variable primary sequence allows for VSG to be antigenically distinct from one another, the crux to antigenic variation.

Worked examples

Example 1 — a first encounter with Variant surface glycoprotein

Start with the simplest possible case. Write down what Variant surface glycoprotein claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In biology, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Variant surface glycoprotein before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Variant surface glycoprotein ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Variant surface glycoprotein

In research
Variant surface glycoprotein appears in biology research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Variant surface glycoprotein in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Variant surface glycoprotein is common in secondary-school and first-year university syllabi. It links to neighbouring topics Glycoproteins, Kinetoplastid proteins, Parasitic excavates, so understanding it makes those chapters shorter.
In everyday life
Look for Variant surface glycoprotein outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Variant surface glycoprotein in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Variant surface glycoprotein means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Variant surface glycoprotein out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Variant surface glycoprotein in simple terms?

Variant surface glycoprotein (VSG) is a ~60kDa protein which densely packs the cell surface of protozoan parasites belonging to the genus Trypanosoma. This genus is notable for their cell surface proteins.

Why does Variant surface glycoprotein matter?

Because it connects several biology ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Variant surface glycoprotein?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Variant surface glycoprotein.

Tags

  • Glycoproteins
  • Kinetoplastid proteins
  • Parasitic excavates

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