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

In multi-pass membrane proteins, the interaction between multiple transmembrane domains determines their structure and function.
G protein-coupled receptors are the largest family of membrane proteins. They contain seven transmembrane alpha-helices which act to transmit signals between the cell&#39;s extracellular and intracellular environments.
In contrast, many channel-forming membrane proteins, such as porins, contain multiple beta-strands of the protein. These form hydrogen bonds to form a continuous cylindrical beta-sheet, creating a rigid ring-like structure called a beta-barrel.
The amino acids in the strands alternate between polar and non-polar residues. Non-polar groups point towards the outside of the barrel and interact with the hydrophobic membrane.
The polar side chains orient towards the inner hydrophilic opening, which forms a channel from the extracellular to intracellular space allowing the passage of small polar solutes.

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

In multi-pass transmembrane proteins, the polypeptide chain crosses the membrane more than once. The transmembrane polypeptide chain either forms an &#945;-helix or &#946;-strand structure. &#945;-Helix containing multi-pass transmembrane proteins are ubiquitous, whereas &#946;-strand&#160;containing ones are mainly found in gram-negative bacteria, mitochondria, and chloroplasts.
&#945;-Helix containing multi-pass transmembrane proteins 
Multi-pass transmembrane proteins such as G-protein-linked receptors (GPCRs) and bacteriorhodopsin contain multiple transmembrane domains. All GPCRs have seven transmembrane &#945;-helices, but each receptor has its specific extracellular domain and G-protein-binding site. GPCRs bind a ligand and activate membrane protein called G-protein. The activated G-protein then interacts with either an ion channel or an enzyme in the membrane.
Bacteriorhodopsin found in certain photosynthetic bacteria has seven transmembrane &#945;-helices. It is a light-driven proton pump that helps in ATP production by generating a proton gradient across the membrane.
&#946;-Strands containing multi-pass transmembrane proteins 
Some bacterial membrane proteins contain transmembrane &#946;-strands, which are arranged into a ring-like structure called the &#946;-barrel. A minimum of eight &#946;-strands are required to form the barrel-like structure where the edges of the two closely spaced &#946;-strands form hydrogen bonds together. The porin protein observed in the membrane of some gram-negative bacteria contains a transmembrane &#946;-barrel. Apart from bacteria, porin proteins are also found in mitochondrial and chloroplast membranes, which support the hypothesis that both mitochondria and chloroplast are evolved from prokaryotes.
Some part of this text is adapted from Openstax, Biology 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>.

In multi-pass transmembrane proteins, the polypeptide chain crosses the membrane more than once. The transmembrane polypeptide chain either forms an ...

Education

JoVE Core

Cell Biology

Unit 1

Membrane Structure and Components

KEY TERMS AND CONCEPTS

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...

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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...

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 Cell Biology illustrates complex cellular processes in easy-to-understand yet comprehensive animated video lessons. This collection focuses on the essential components and functions of cells, including organelles and membranes, cellular transport, and cell energetics. It is complemented by scientist-in-action videos that demonstrate the application of cell biology in real-world research experiments.

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9-12.B.1.1

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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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