Name: how proteins shape cell membranes
Uploaded: 2023-04-30T09:16:36.000Z
Duration: 1 min 15 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

Mechanisms of Membrane-bending

Eine Membran hat eine positive Krümmung, wenn sie sich zur inneren oder zytoplasmatischen Seite hin biegt. Im Gegensatz dazu hat eine Membran eine negative Krümmung, wenn sie sich zur äußeren oder extrazellulären Seite hin biegt.
Es gibt mehrere Mechanismen, die eine Membranbiegung verursachen können.
Die lipidinduzierte Membranbiegung ist auf die asymmetrische Aggregation spezifischer Lipide in der Membran zurückzuführen.
Lipide mit großen Kopfgruppen induzieren eine positive Membrankrümmung, während Lipide mit kleinen Kopfgruppen eine negative Membrankrümmung erzeugen.
Ein weiterer Mechanismus, bei dem Proteine entweder die Membran verformen oder eine bereits deformierte Membran stabilisieren.
In einem anderen Mechanismus werden hydrophobe Proteindomänen reversibel in eine Schicht der Membran eingefügt, um ihre Fläche im Vergleich zur anderen Schicht zu vergrößern. Diese Asymmetrie zwingt die Membran, sich zu biegen.
Schließlich können die Druck- und Zugkräfte, die durch den Auf- und Abbau des Zytoskeletts und der zugehörigen Motorproteine erzeugt werden, die Membran auch so biegen, dass sie Vorsprünge bildet, wie z. B. Filopodien in Epithelzellen.

mechanismen der membranbiegung

Die Membranen lebender Zellen sind aufgrund ihrer Beschaffenheit eines “flüssigen Mosaiks”  flexibel. Ihre Krümmung in verschiedene Formen ist jedoch ein aktiver Prozess, der durch spezifische Lipide und Proteine reguliert wird. Die Membrankrümmung kann vorübergehend wie bei Vesikeln oder über einen langen Zeitraum stabil sein, wie es bei Mikrovilli der Fall ist. Zellen regulieren die Größe, Lage und Dauer der Membrankrümmung.

Eine Membrankrümmung kann aufgrund intrinsischer Veränderungen in der Lipidzusammensetzung oder extrinsischer Assoziation mit verschiedenen Proteinen auftreten. Die Proteine, die an der Bildung einer Membranasymmetrie oder der Clusterbildung spezifischer Lipide beteiligt sind, spielen eine wesentliche Rolle bei der lipidinduzierten Membrankrümmung. Auch die Form integraler Proteine kann die Krümmung der Membran beeinflussen. Beispielsweise bewirken konische oder umgekehrt konisch geformte Transmembranproteine eine Krümmung der Membran. Auch der Einbau amphipathischer Proteine wie Epsin in eine der Schichten führt zu einer Biegung der Membran.

Die gekrümmten Proteine wie Bin/Amphiphysin/Rvs oder BAR-Dimerproteine erzeugen eine positive Membrankrümmung, während das inverse BAR- oder I-BAR-Protein eine negative Krümmung der Membran induziert. BAR-Proteine fügen darüber hinaus ihre hydrophobe Domäne in die Membran ein, was die Krümmung der Membran weiter verstärkt. Neben den bekannten Mechanismen ist das Protein-Crowding ein kürzlich entdeckter Mechanismus für die Membrankrümmung, bei dem der durch Protein-Protein-Wechselwirkungen erzeugte seitliche Druck die Membrankrümmung antreibt. Das Zytoskelett spielt auch eine wichtige Rolle bei der Membrankrümmung, und die von ihm erzeugten Kräfte sind für die Morphologie einiger Organellen wie die des endoplasmatischen Retikulums und des Golgi-Apparats verantwortlich.

Zellorganellen wie das endoplasmatische Retikulum, der Golgi-Apparat und die Mitochondrien haben kompliziert gefaltete Membranen, und diese Faltungen sind für ihre Funktion unerlässlich. Die Krümmung der Membran ist auch für das Abschnüren der Membran, die Membranfusion und die Aktivierung von Enzymen, die die Membrankrümmung erkennen,   erforderlich. Mutationen in Genen, die membrankrümmende Proteine wie Myotubularin, Amphiphysin-2 und Dynamin-2 produzieren, können zu einer Gruppe neuromuskulärer Erkrankungen führen, die als zentronukleäre Myopathien bekannt sind.

Mechanismen der Membranbiegung

Die lebenden Membranen sind aufgrund ihrer fließenden Mosaiknatur flexibel; Ihr Biegen in verschiedene Formen ist jedoch ein aktiver Prozess, der durch ...

Education

Lehre

JoVE Core

Cell Biology

Zellbiologie

Unit 1

Einheit 1

Membrane Structure and Components

Membranstruktur und Komponenten

KEY TERMS AND CONCEPTS

KERNBEGRIFFE UND KONZEPTE

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 matters. Conical or inverse-conical transmembrane proteins can push the membrane to bend.

Some proteins bend membranes by inserting part of themselves into one layer of the membrane. Amphipathic proteins, such as epsin, do this and promote curvature. BAR proteins, short for Bin/amphiphysin/Rvs proteins, create positive curvature, while I-BAR proteins create negative curvature. BAR proteins can also add to bending by placing a hydrophobic domain into the membrane.

Protein crowding is another way membranes bend. In this case, proteins packed close together create lateral pressure, and that pressure drives curvature. The cytoskeleton also helps shape membranes. Its forces are important for the form of organelles such as the endoplasmic reticulum and Golgi apparatus.

The folded membranes of the endoplasmic reticulum, Golgi apparatus, and mitochondria are important for their function. Membrane bending also supports membrane scission, membrane fusion, and the activation of enzymes that sense curvature. Changes in genes for membrane-bending proteins, including myotubularin, amphiphysin-2, and dynamin-2, can cause centronuclear myopathies, a group of neuromuscular disorders.

How Proteins Shape Cell Membranes

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

multi-pass-transmembrane-proteins-and-barrels

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...

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

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

Learn how lipids, BAR proteins, protein crowding, and the cytoskeleton bend cell membranes. See how curvature helps organelle shape, fusion, and scission.

How Proteins Shape Cell Membranes | JoVE K12

cell biology

JoVE Core Zellbiologie veranschaulicht komplexe zelluläre Prozesse in leicht verständlichen und dennoch umfassenden animierten Videolektionen. Diese Sammlung konzentriert sich auf die wesentlichen Bestandteile und Funktionen von Zellen, einschließlich Organellen und Membranen, Zelltransport und Zellenergie. Ergänzt wird sie durch Videos von Wissenschaftlern in Aktion, die die Anwendung der Zellbiologie in realen Forschungsexperimenten demonstrieren.

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9-12.B.1.2.a

How Proteins Shape Cell Membranes

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