Name: membrane-spanning helices and β-barrels
Uploaded: 2023-04-30T10:41:44.000Z
Duration: 1 min 9 s
Channel: JoVE

K12 Standards

Alabama Course of Study Science

9-12

Biology

from-molecules-to-organisms-structures-and-processes

From Molecules to Organisms: Structures and Processes

9-12.B.1.4.a

multi-pass-transmembrane-proteins-and-barrels

Multi-pass Transmembrane Proteins and &#946;-barrels

Dans les protéines membranaires multi-passes, l’interaction entre plusieurs domaines transmembranaires détermine leur structure et leur fonction.
Les récepteurs couplés aux protéines G sont la plus grande famille de protéines membranaires. Ils contiennent sept hélices alpha transmembranaires qui agissent pour transmettre des signaux entre les environnements extracellulaires et intracellulaires de la cellule.
En revanche, de nombreuses protéines membranaires formant des canaux, telles que les porines, contiennent plusieurs brins bêta de la protéine. Ceux-ci forment des liaisons hydrogène pour former une feuille bêta cylindrique continue, créant une structure rigide en forme d’anneau appelée bêta-baril.
Les acides aminés contenus dans les brins alternent entre les résidus polaires et non polaires. Les groupes non polaires pointent vers l’extérieur du canon et interagissent avec la membrane hydrophobe.
Les chaînes latérales polaires s’orientent vers l’ouverture hydrophile interne, qui forme un canal de l’espace extracellulaire à l’espace intracellulaire permettant le passage de petits solutés polaires.

protéines transmembranaires à passage multiple et tonneau β

Dans les protéines transmembranaires à passages multiples, la chaîne polypeptidique traverse la membrane plus d'une fois. La chaîne polypeptidique transmembranaire forme une structure en hélice α ou en brin β. Les protéines transmembranaires à passages multiples contenant l'hélice α sont omniprésentes, tandis que celles contenant le brin β se trouvent principalement dans les bactéries à Gram négatif, les mitochondries et les chloroplastes.

α-Helix contenant des protéines transmembranaires à passage multiple

Les protéines transmembranaires à passages multiples telles que les récepteurs liés aux protéines G (GPCR) et la bactériorhodopsine contiennent plusieurs domaines transmembranaires. Tous les GPCR possèdent sept hélices α transmembranaires, mais chaque récepteur possède son domaine extracellulaire spécifique et son site de liaison à la protéine G. Les GPCR se lient à un ligand et activent une protéine membranaire appelée protéine G. La protéine G activée interagit alors soit avec un canal ionique, soit avec une enzyme de la membrane.

La bactériorhodopsine présente dans certaines bactéries photosynthétiques possède sept hélices α transmembranaires. Il s’agit d’une pompe à protons alimentée par la lumière qui contribue à la production d’ATP en générant un gradient de protons à travers la membrane.

Brins β contenant des protéines transmembranaires multi-passes

Certaines protéines membranaires bactériennes contiennent des brins β transmembranaires, qui sont disposés en une structure en forme d'anneau appelée baril β. Un minimum de huit brins β sont nécessaires pour former la structure en forme de tonneau où les bords des deux brins β étroitement espacés forment ensemble des liaisons hydrogène. La protéine porine observée dans la membrane de certaines bactéries à Gram négatif contient un baril β transmembranaire. Outre les bactéries, les protéines porines se trouvent également dans les membranes mitochondriales et chloroplastiques, ce qui conforte l'hypothèse selon laquelle les mitochondries et les chloroplastes sont issus de procaryotes.

Une partie de ce texte est adaptée d'Openstax, Biologie 2e,​ <a href="https://openstax.org/books/biology-2e/pages/9-1-signaling-molecules-and-cellular-receptors">Section 9.1: Signaling Molecules and Cellular Receptors</a>.

Protéines Transmembranaires à Passage Multiple et Tonneau β

Dans les protéines transmembranaires à passages multiples, la chaîne polypeptidique traverse la membrane plus d’une fois. La chaîne polypeptidique transmembranaire forme soit un ...

Education

Enseignement

JoVE Core

Cell Biology

Biologie cellulaire

Unit 1

Unité 1

Membrane Structure and Components

Structure et composants de la membrane

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

membrane-spanning helices and β-barrels

Membrane-spanning proteins can cross the cell membrane more than once. Their transmembrane chain usually folds into either an alpha-helix or a beta-strand. Alpha-helix multi-pass proteins are common, while beta-strand proteins are found mainly in gram-negative bacteria, mitochondria, and chloroplasts.

Some multi-pass alpha-helix proteins are G-protein-linked receptors, or GPCRs, and bacteriorhodopsin. GPCRs have seven transmembrane alpha-helices. Each receptor also has its own extracellular domain and G-protein-binding site. They bind a ligand and activate a membrane protein called a G-protein, which then interacts with either an ion channel or an enzyme in the membrane.

Bacteriorhodopsin is found in certain photosynthetic bacteria. It also has seven transmembrane alpha-helices. This protein acts as a light-driven proton pump. By building a proton gradient across the membrane, it helps drive ATP production.

Some bacterial membrane proteins use transmembrane beta-strands instead of helices. These beta-strands arrange into a ring-shaped structure called a beta-barrel. At least eight beta-strands are needed to form this barrel-like shape. The edges of two closely spaced beta-strands form hydrogen bonds with each other.

Porin is a well-known beta-barrel protein found in the membranes of some gram-negative bacteria. Porin proteins are also found in mitochondrial and chloroplast membranes. This distribution supports the idea that mitochondria and chloroplasts evolved from prokaryotes.

Membrane-Spanning Helices and β-Barrels

membrane-spanning-helices-and-beta-barrels

what are membranes?

Membrane Receptors and Cell Signals

The plasma membrane sets the outer boundary of a cell and helps control how the cell interacts with its environment. It lets some substances enter, keeps others out, and releases some materials in controlled amounts. The membrane also needs to stay flexible so cells such as red and white blood cells can change shape as they move through narrow capillaries. The membrane surface also carries markers that help cells recognize one another. This recognition is important during early development,...

membrane fluidity

How Cell Membranes Stay Flexible

Cell membranes stay flexible because of the fluid mosaic model. The plasma membrane is a mosaic of phospholipids, cholesterol, proteins, and carbohydrates. These parts are separate but loosely attached, so the membrane keeps a fluid character. The membrane is still fairly rigid, but it can flow and self-seal after a very fine needle passes through it. Phospholipid tails also affect membrane fluidity. Saturated fatty acids in phospholipids have straight tails because they are packed with...

fluid mosaic model

Fluid Mosaic Model of the Membrane

The fluid mosaic model explains the structure and function of the plasma membrane. Scientists first identified the plasma membrane in the 1890s, and by 1915 they had identified its main chemical parts: lipids and proteins. Early electron micrographs showed a railroad track pattern, which helped lead to the first widely accepted membrane model in 1935.

membrane lipids

Membrane Lipids in Cells and Tissues

Membrane lipids give biological membranes their structure and help set their properties. In mammalian cells, lipids make up about 50% of the membrane on average. The amount can be much lower in the inner mitochondrial membrane or much higher in the myelin sheath around nerve cells. Several phospholipids are common in mammalian membranes. These include phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and sphingomyelin. At physiological pH, phosphatidylserine carries a negative...

asymmetric lipid bilayer

Membrane Lipid Unevenness

Biological membranes have an uneven mix of lipids between the inner and outer layers. This membrane asymmetry gives each side a different chemical makeup and supports different cell functions. Evidence for this uneven layout came from treating an erythrocyte membrane with phospholipase. Phospholipase is an enzyme that breaks lipids into fatty acids and hydrophilic groups. It acted only on the outer layer, while the inner layer stayed intact. The treatment degraded about 80% of total membrane...

membrane asymmetry regulating transporters

membrane-asymmetry-regulating-transporters

Membrane Lipid Flipping and Transport

Membrane lipid flipping and transport help cells keep an uneven, or asymmetric, distribution of phospholipids across the membrane. This balance is controlled by flippase, floppase, and scramblase enzymes, which move lipids between the two membrane leaflets. Flippases move specific phospholipids from the outer leaflet to the inner leaflet. In eukaryotes, these enzymes are type-IV P-type ATPases, also called P4-ATPases. They are membrane pumps that use ATP and contain one transmembrane domain...

membrane carbohydrates

Membrane Sugars and Cell Identity

Membrane carbohydrates help give cells their surface identity and support plasma membrane function. The plasma membrane is a dynamic barrier made of lipids, proteins, and carbohydrates. These sugar chains have unique structural and chemical traits that help the membrane work properly for cell growth and survival. Membrane carbohydrates do not have a hydrophobic region, so they are found only on the outer surface of the cell. Sugar addition, or glycosylation, of proteins occurs in the lumens of...

membrane proteins

Facilitated Transport Across Cell Membranes

Facilitated transport uses membrane proteins to help materials cross the plasma membrane. These integral transmembrane proteins act as channels or carriers. They move selected substances through the membrane more easily than the lipid layer alone would allow. Channel proteins provide a hydrophilic, or water-friendly, path through the membrane. Their core forms a hydrated opening that lets solutes pass between the inside and outside fluids of the cell. This helps polar compounds avoid the...

lipids as anchors

Lipid Anchors in Membrane Proteins

Lipid anchors help attach proteins to the plasma membrane. In eukaryotic cells, the main lipid anchors are prenyl groups, fatty acyl groups, and glycosylphosphatidylinositol, or GPI, groups. Prenyl and fatty acyl groups anchor proteins on the cytosolic side of the membrane. GPI anchors proteins on the extracellular side. Prenylated proteins often have a CaaX motif at the carboxy terminus. In this sequence, C means cysteine, a means any aliphatic amino acid, and X means any other amino acid...

single-pass transmembrane proteins

How Membrane Proteins Span Cell Membranes

Membrane proteins help cells communicate, send signals, stick to other cells, and move molecules across the membrane. Integral membrane proteins are tightly linked to the cell membrane and carry out these jobs. Some integral membrane proteins sit only in one membrane layer. Fatty acid amide hydrolase is one example, and it is found on the cytoplasmic side of the membrane monolayer. Other integral membrane proteins are transmembrane proteins, which span across the membrane. These proteins have...

multi-pass transmembrane proteins and &#946;-barrels

Membrane-spanning proteins can cross the cell membrane more than once. Their transmembrane chain usually folds into either an alpha-helix or a beta-strand. Alpha-helix multi-pass proteins are common, while beta-strand proteins are found mainly in gram-negative bacteria, mitochondria, and chloroplasts. Some multi-pass alpha-helix proteins are G-protein-linked receptors, or GPCRs, and bacteriorhodopsin. GPCRs have seven transmembrane alpha-helices. Each receptor also has its own extracellular...

detergent purification of membrane proteins

detergent-purification-of-membrane-proteins

How Detergents Solubilize Membrane Proteins

Detergents help solubilize membrane proteins so they can be purified from the lipid bilayer. Their hydrophobic region can replace membrane phospholipids and keep integral membrane proteins in solution. A detergent works well only when its monomers reach the critical micelle concentration, or CMC. At that point, the molecules assemble into micelles, which are small clusters of detergent. Above the CMC, free detergent monomers stay in equilibrium with the micelles. The CMC varies with the...

protein diffusion in the membrane

How Membrane Proteins Move Side to Side

Membrane proteins can move across the cell surface by lateral diffusion, and they can also rotate in place. Scientists confirmed lateral movement with a cell fusion experiment. Mouse and human cells were fused to make hybrid cells. The membrane proteins on the two cells were tagged with red and green fluorescent markers. At first, each color stayed on its own side of the fused cell. After 40 minutes, the colors mixed across the whole cell surface, showing that the proteins had moved laterally. <br...

membrane domains

Lipid Rafts and Sperm Membrane Regions

Membrane domains organize specific lipids and proteins in defined parts of a cell membrane. This arrangement supports cell signaling, adhesion, and other important cellular processes. These domains can vary in size, composition, function, and how long they last. Some membrane domains are built from proteins. The membrane can contain distinct protein groups that carry out a cell’s specific functions. A clear example is the plasma membrane of a human sperm cell, which has different proteins in...

mechanisms of membrane domain formation

How Cell Membranes Form Domains

Cell membranes form domains when lipids and proteins gather into distinct regions. These regions, or membrane domains, are shaped by different physical properties that help some molecules stay together while others move apart. Some domains form through protein-protein interactions. Others depend on specific lipids, such as sphingolipids and sterols. Large proteins, including bacteriorhodopsin, can also cluster and create visible membrane domains. Membrane proteins can be held in place by...

mechanisms of membrane-bending

How Proteins Shape Cell Membranes

Cell membranes can bend into many shapes because they are flexible and made of a fluid mosaic. That bending can be temporary, as in vesicles, or long-lasting, as in microvilli. Cells control the size, place, and time of membrane curvature. Membrane bending can start inside the membrane itself or from proteins attaching to it. Changes in lipid makeup can create curvature, and proteins can help by making the membrane asymmetric or by clustering certain lipids. The shape of integral proteins also...

key terms and concepts

Explore how multi-pass transmembrane proteins form alpha-helices or beta-barrels, with examples such as GPCRs, bacteriorhodopsin, and porins.

Membrane-Spanning Helices and β-Barrels | JoVE K12

cell biology

JoVE Core Biologie cellulaire illustre les processus cellulaires complexes dans des leçons vidéos complètes mais faciles à comprendre. Cette collection se concentre sur les composants et fonctions essentiels de la cellule ; cela inclut les organelles et les membranes ainsi que le transport et le métabolisme énergétique cellulaire. Elle est complétée par nos vidéos de scientifiques à l'action qui démontrent l'application de la biologie cellulaire dans des expériences concrètes.

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9-12.B.1.2

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9-12.B.1.2.a

Membrane-Spanning Helices and β-Barrels

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