biology

Glycosylphosphatidylinositol

Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol rather than just read about it. In short: Glycosylphosphatidylinositol () or glycophosphatidylinositol (GPI) is a phosphoglyceride that can be attached to the C-terminus of a protein during posttranslational modification. The resulting GPI-anchored proteins play key roles in a wide variety of biological processes.

Key takeaways

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

Reference excerpt

Glycosylphosphatidylinositol () or glycophosphatidylinositol (GPI) is a phosphoglyceride that can be attached to the C-terminus of a protein during posttranslational modification. The resulting GPI-anchored proteins play key roles in a wide variety of biological processes. GPI is composed of a phosphatidylinositol group linked through a carbohydrate-containing linker (glucosamine and mannose glycosidically bound to the inositol residue) and via an ethanolamine phosphate (EtNP) bridge to the C-terminal amino acid of a mature protein. The two fatty acids within the hydrophobic phosphatidyl-inositol group anchor the protein to the cell membrane.

Synthesis Glycosylated (GPI-anchored) proteins contain a signal sequence, thus directing them to the endoplasmic reticulum (ER). The protein is co-translationally inserted in the ER membrane via a translocon and is attached to the ER membrane by its hydrophobic C terminus; the majority of the protein extends into the ER lumen. The hydrophobic C-terminal sequence is then cleaved off and replaced by the GPI-anchor. As the protein processes through the secretory pathway, it is transferred via vesicles to the Golgi apparatus and finally to the plasma membrane where it remains attached to a leaflet of the cell membrane. Since the glypiation is the sole means of attachment of such proteins to the membrane, cleavage of the group by phospholipases will result in controlled release of the protein from the membrane. The latter mechanism is used in vitro; i.e. membrane proteins released from membranes in enzymatic assays are glypiated proteins. The inositol residue is modified with palmitate or myristate at position 2 prior to mannose and ethanolamine phosphate transfer. This is most often removed soon after addition to the C terminus of a protein in the endoplasmic reticulum; in nucleated cells, only 5 to 10 percent of mature GPI-anchored proteins retain the marker, however, in erythrocytes, the majority of GPI-anchored proteins are acylated with myristate on the anchor. In Trypanosoma brucei, by contrast, mannosyltransferase activity does not require acylation of the inositol residue and consequently unacylated GPI anchors are transferred to the parasite's variant surface glycoprotein.

A more up-to-date as well as more complete view is KEGG hsa00563. See also Reactome R-HSA-162710.

Cleavage Phospholipase C (PLC) is an enzyme known to cleave the phospho-glycerol bond found in GPI-anchored proteins. Treatment with PLC will cause release of GPI-linked proteins from the outer cell membrane, but acylation of the inositol residue can interfere with PLC cleavage. The T-cell marker Thy-1 and acetylcholinesterase, as well as both intestinal and placental alkaline phosphatases, are known to be GPI-linked and are released by treatment with PLC. GPI-linked proteins are thought to be preferentially located in lipid rafts, suggesting a high level of organization within plasma membrane microdomains.

GPI-anchor synthesis deficiencies

In humans Defects in the GPI-anchor synthesis occur in rare acquired diseases such as paroxysmal nocturnal hemoglobinuria (PNH) and congenital diseases such as hyperphosphatasia with mental retardation syndrome (HPMRS). In PNH a somatic defect in blood stem cells, which is required for GPI synthesis, results in faulty GPI linkage of decay-accelerating factor (DAF) and CD59 in red blood cells. The most common cause of PNH are somatic mutations in the X-chromosomal gene PIGA. However, a PNH case with a germline mutation in the autosomal gene PIGT and a second acquired somatic hit has also been reported. Without these proteins linked to the cell surface, the complement system can lyse the cell, and high numbers of RBCs are destroyed, leading to hemoglobinuria. For patients with HPMRS, disease-causing mutations have been reported in the genes PIGV, PIGO, PGAP2 and PGAP3.

In other species The variable surface glycoproteins from the sleeping sickness protozoan Trypanosoma brucei are attached to the plasma membrane via a GPI anchor.

References

External links

Glycosylphosphatidylinositols at the U.S. National Library of Medicine Medical Subject Headings (MeSH) "Gpi Anchor Structure". Sigma-Aldrich.

Illustrations

Worked examples

Example 1 — a first encounter with Glycosylphosphatidylinositol

Start with the simplest possible case. Write down what Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol

In research: Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol 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: Glycosylphosphatidylinositol is common in secondary-school and first-year university syllabi. It links to neighbouring topics Membrane biology, so understanding it makes those chapters shorter.
In everyday life: Look for Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Glycosylphosphatidylinositol means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
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Frequently asked questions

What is Glycosylphosphatidylinositol in simple terms?

Why does Glycosylphosphatidylinositol 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 Glycosylphosphatidylinositol?

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 Glycosylphosphatidylinositol.