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Integral membrane protein

Integral membrane protein 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 Integral membrane protein rather than just read about it. In short: An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All transmembrane proteins can be classified as IMPs, but not all IMPs are transmembrane proteins.

Integral membrane protein — main illustration
Integral membrane protein — illustration

Key takeaways

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

Reference excerpt

An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All transmembrane proteins can be classified as IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a significant fraction of the proteins encoded in an organism's genome. Proteins that cross the membrane are surrounded by annular lipids, which are defined as lipids that are in direct contact with a membrane protein. Such proteins can only be separated from the membranes by using detergents, nonpolar solvents, or sometimes denaturing agents. Proteins that adhere only temporarily to cellular membranes are known as peripheral membrane proteins. These proteins can either associate with integral membrane proteins, or independently insert in the lipid bilayer in several ways.

Structure

Three-dimensional structures of ~160 different integral membrane proteins have been determined at atomic resolution by X-ray crystallography or nuclear magnetic resonance spectroscopy. They are challenging subjects for study owing to the difficulties associated with extraction and crystallization. In addition, structures of many water-soluble protein domains of IMPs are available in the Protein Data Bank. Their membrane-anchoring α-helices have been removed to facilitate the extraction and crystallization. Search integral membrane proteins in the PDB (based on gene ontology classification) IMPs can be divided into two groups:

Integral polytopic proteins (Transmembrane proteins) Integral monotopic proteins

Integral polytopic protein

The most common type of IMP is the transmembrane protein, which spans the entire biological membrane. Single-pass membrane proteins cross the membrane only once, while multi-pass membrane proteins weave in and out, crossing the membrane several times. Single pass membrane proteins can be categorized as Type I, which are positioned such that their carboxyl-terminus is towards the cytosol, or Type II, which have their amino-terminus towards the cytosol. Type III proteins have multiple transmembrane domains in a single polypeptide, while type IV consists of several different polypeptides assembled together in a channel through the membrane. Type V proteins are anchored to the lipid bilayer through covalently linked lipids. Finally Type VI proteins have both transmembrane domains and lipid anchors.

Integral monotopic proteins

Integral monotopic proteins are permanently attached to the cell membrane from one side. Three-dimensional structures of the following integral monotopic proteins have been determined:

prostaglandin H2 syntheses 1 and 2 (cyclooxygenases) lanosterol synthase and squalene-hopene cyclase microsomal prostaglandin E synthase carnitine O-palmitoyltransferase 2 Phosphoglycosyl transferase C There are also structures of integral monotopic domains of transmembrane proteins:

monoamine oxidases A and B fatty acid amide hydrolase mammalian cytochrome P450 oxidases corticosteroid 11-beta-dehydrogenases

Extraction Many challenges facing the study of integral membrane proteins are attributed to the extraction of those proteins from the phospholipid bilayer. Since integral proteins span the width of the phospholipid bilayer, their extraction involves disrupting the phospholipids surrounding them, without causing any damage that would interrupt the function or structure of the proteins. Several successful methods are available for performing the extraction including the uses of "detergents, low ionic salt (salting out), shearing force, and rapid pressure change".

Determination of protein structure The Protein Structure Initiative (PSI), funded by the U.S. National Institute of General Medical Sciences (NIGMS), part of the National Institutes of Health (NIH), has among its aim to determine three-dimensional protein structures and to develop techniques for use in structural biology, including for membrane proteins. Homology modeling can be used to construct an atomic-resolution model of the "target" integral protein from its amino acid sequence and an experimental three-dimensional structure of a related homologous protein. This procedure has been extensively used for ligand-G protein–coupled receptors (GPCR) and their complexes.

Function IMPs include transporters, linkers, channels, receptors, enzymes, structural membrane-anchoring domains, proteins involved in accumulation and transduction of energy, and proteins responsible for cell adhesion. Classification of transporters can be found in Transporter Classification Database. As an example of the relationship between the IMP (in this case the bacterial phototrapping pigment, bacteriorhodopsin) and the membrane formed by the phospholipid bilayer is illustrated below. In this case the integral membrane protein spans the phospholipid bilayer seven times. The part of the protein that is embedded in the hydrophobic regions of the bilayer are alpha helical and composed of predominantly hydrophobic amino acids. The C terminal end of the protein is in the cytosol while the N terminal region is in the outside of the cell. A membrane that contains this particular protein is able to function in photosynthesis.

Examples Examples of integral membrane proteins:

Insulin receptor Some types of cell adhesion proteins or cell adhesion molecules (CAMs) such as integrins, cadherins, NCAMs, or selectins Some types of receptor proteins Glycophorin Rhodopsin Band 3 CD36 Glucose Permease Ion channels and Gates Gap junction Proteins G protein coupled receptors (e.g., Beta-adrenergic receptor) Seipin Photosystem I

See also Membrane protein Transmembrane protein Peripheral membrane protein Annular lipid shell Hydrophilicity plot Inner nuclear membrane protein

References

Illustrations

Integral membrane protein: E, extracellular space; P, plasma membrane; I, intracellular space
E, extracellular space; P, plasma membrane; I, intracellular space
Integral membrane protein: Group I and II transmembrane proteins have opposite orientations. Group I proteins have the N terminus on the far side and C terminus on the cytosolic side. Group II proteins have the C terminus on the far side and N terminus in the cytosol.
Group I and II transmembrane proteins have opposite orientations. Group I proteins have the N terminus on the far side and C terminus on the cytosolic side. Group II proteins have the C terminus on the far side and N terminus in the cytosol.
Integral membrane protein: Schematic representation of the different types of interaction between monotopic membrane proteins and the cell membrane: 1. interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix) 2. interaction by a hydrophobic loop 3. interaction by a covalently bound membrane lipid (lipidation) 4. electrostatic or ionic interactions with membrane lipids (e.g. through a calcium ion)
Schematic representation of the different types of interaction between monotopic membrane proteins and the cell membrane: 1. interaction by an amphipathic α-helix parallel to the membrane plane (in-plane membrane helix) 2. interaction by a hydrophobic loop 3. interaction by a covalently bound membrane lipid (lipidation) 4. electrostatic or ionic interactions with membrane lipids (e.g. through a calcium ion)

Worked examples

Example 1 — a first encounter with Integral membrane protein

Start with the simplest possible case. Write down what Integral membrane protein 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 Integral membrane protein 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 Integral membrane protein 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 Integral membrane protein

In research
Integral membrane protein 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 Integral membrane protein 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
Integral membrane protein is common in secondary-school and first-year university syllabi. It links to neighbouring topics Integral membrane proteins, Protein structure, so understanding it makes those chapters shorter.
In everyday life
Look for Integral membrane protein 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 Integral membrane protein in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Integral membrane protein 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 Integral membrane protein out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Integral membrane protein in simple terms?

An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All transmembrane proteins can be classified as IMPs, but not all IMPs are transmembrane proteins.

Why does Integral membrane protein 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 Integral membrane protein?

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 Integral membrane protein.

Tags

  • Integral membrane proteins
  • Protein structure

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