Membrane curvature

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Membrane curvature is the geometrical measure or characterization of the curvature of membranes. The membranes can be naturally occurring or man-made (synthetic). An example of naturally occurring membrane is the lipid bilayer of cells, also known as cellular membranes. [1] Synthetic membranes can be obtained by preparing aqueous solutions of certain lipids. The lipids will then "aggregate" and form various phases and structures. According to the conditions (concentration, temperature, ionic strength of solution, etc.) and the chemical structures of the lipid, different phases will be observed. For instance, the lipid POPC (palmitoyl oleyl phosphatidyl choline) tends to form lamellar vesicles in solution, whereas smaller lipids (lipids with shorter acyl chains, up to 8 carbons in length), such as detergents, will form micelles if the CMC (critical micelle concentration) was reached. There are five commonly proposed mechanisms by which membrane curvature is created, maintained, or controlled: lipid composition, shaped transmembrane proteins, protein motif insertion/BAR domains, protein scaffolding, and cytoskeleton scaffolding. [2]

Contents

Geometry

A biological membrane is commonly described as a two-dimensional surface, which spans a three-dimensional space. So, to describe membrane shape, it is not sufficient to determine the membrane curling that is seen in a single cross-section of the object, because in general there are two curvatures that characterize the shape each point in space. Mathematically, these two curvatures are called the principal curvatures, and , and their meaning can be understood by the following thought experiment. If you cross-section the membrane surface at a point under consideration using two planes that are perpendicular to the surface and oriented in two special directions called the principal directions, the principal curvatures are the curvatures of the two lines of intercepts between the planes and the surface which have almost circular shapes in close proximity to the point under consideration. The radii of these two circular fragments, and , are called the principal radii of curvature, and their inverse values are referred to as the two principal curvatures. [3]

Curvature radii Curvature radii.JPG
Curvature radii

The principal curvatures and can vary arbitrarily and thereby give origin to different geometrical shapes, such as cylinder, plane, sphere and saddle. Analysis of the principal curvature is important, since a number of biological membranes possess shapes that are analogous to these common geometry staples. For instance, prokaryotic cells such as cocci, rods, and spirochette display the shape of a sphere, and the latter two the shape of a cylinder. Erythrocytes, commonly referred to as red blood cells, have the shape of a saddle, although these cells are capable of some shape deformation. The table below lists common geometric shapes and a qualitative analysis of their two principal curvatures.

Shape
Plane00
Cylinder+0
Sphere++
Saddle+-

Even though often membrane curvature is thought to be a completely spontaneous process, thermodynamically speaking there must be factors actuating as the driving force for curvature to exist. Currently, there are some postulated mechanisms for accepted theories on curvature; nonetheless, undoubtedly two of the major driving forces are lipid composition and proteins embedded and/or bound to membranes.

Induced by lipids

Dynamics

Perhaps the most simple and intuitive driving force in membrane curvature is the natural spontaneous curvature exhibited by some lipids. This is because, depending on their chemical structures, lipids tend to curve with a slight spontaneously negative or positive curvature. Lipids such as DOPC (dioleoyl phosphatidyl choline), diacyl glycerol, dioleyl phosphatidyl ethanolamine (DOPE) and cholesterol exhibit a negative spontaneous curvature. [4] On the other hand, lipids with smaller acyl chain area to polar head group area ratio tend to curve positively, in other words they exhibit positive spontaneous curvature. [5] The table below lists experimentally determined spontaneous curvatures for different lipids in DOPE.

LipidJs (nm−1) [6]
Lysophospholipids
L-lyso PC1/5.8
O-lyso PC1/3.8
P-lyso PC1/6.8
L-lyso PE<1/40
O-lyso PE<1/40
S-lyso PE<1/40
Other Lipids
DOPS 1/14.4
DOPC-1/20
PA-1/4.6
DOPE-1/3
Cholesterol-1/2.9
DCG-1/1.3

The energy requirements to generate a cylinder shaped cell from an originally flat membrane can be expressed as

where L is the length of the cylinder, JB is the difference between the spontaneous curvature, Js, for the lipids in the inner and outer leaflet divided by two, and Kb is the bending modulus of the bilayer.

The radii of membrane cylinders that form in intracellular membrane-transport pathways are typically ~25–30 nm. [7] So, the spontaneous curvature necessary to generate such cylinders equals ~(1/50) nm–1. As JB results from a difference in the spontaneous curvatures of the monolayers, an unusual membrane lipid composition would be required to produce such curvature. The lipids cholesterol, DOPE and diacylglycerol are characterized by strongly negative spontaneous curvatures (figure 1) and therefore have the potential to generate a large membrane curvature. However, even for these lipids, the required JB can be reached only if they are extensively concentrated in the internal monolayer.

Clustering

Different changes to lipid structure, such as tail saturation, affect the overall shape of the lipid. A change in shape such as the one shown, when disproportionately in higher concentration on one side of the membrane, allows the membrane to curve. Lipid unsaturation effect.svg
Different changes to lipid structure, such as tail saturation, affect the overall shape of the lipid. A change in shape such as the one shown, when disproportionately in higher concentration on one side of the membrane, allows the membrane to curve.

Multiple factors influence whether a lipid will exhibit positive or negative curvature. For example, the presence of double bonds in the tail of a lipid will increase the occupied space of the tail, and thus increase the lipid's propensity to induce negative curvature. [8] In the figure, the different shape of lipids with a double bond - also known as unsaturated - can be visualized. However, a single conically shaped lipid will not induce curvature across an entire region of the membrane. Instead, clustering of similarly shaped lipids in one leaflet compared to the other is required to induce curvature. [8] This difference in lipid composition between leaflets is actively formed and controlled within cells by proteins such as flippases, or removed to discourage curvature by proteins such as scramblases. [9] When asymmetric lipid compositions are present and the membrane is unable to curve due to other surrounding factors, the membrane is destabilized - further supporting the crucial role that lipid composition plays in membrane curvature. [10] When the membrane does curve, a higher number of lipids will be required to be present on the positive curvature side of the membrane to cover the increased surface area that is present compared to the negatively curved side. [2]

Induced by proteins

Some biologically occurring lipids do exhibit spontaneous curvature which could explain the shapes of biological membranes. Nevertheless, calculations show that spontaneous lipid curvature alone is either insufficient or would require conditions that are unrealistic to drive the degree of curvature observed in most cells. It is now known that lipid curvature is "aided" by protein structures in order to generate complete cellular curvature.

Clustering

Transmembrane proteins with inherent curvature inducing curvature in a membrane. Curved Transmembrane Proteins.jpg
Transmembrane proteins with inherent curvature inducing curvature in a membrane.

Transmembrane proteins with an inherently conical shape will be more stable in, and induce curvature in membranes. [2] Depending on the shape of the protein, this can induce either positive or negative curvature. An example is the voltage-gated potassium channel having a larger diameter on the outer leaflet than the inner leaflet of the membrane. [11] As seen in the figure, the larger amount of space taken up in the one leaflet causes the membrane to curve away from that side. [8]

Not only does the protein effect membrane curvature, but membrane curvature can affect membrane proteins as well. Conically shaped proteins will be less stable in membranes that are constrained to be planar, and cylindrically shaped proteins will be less stable in membranes that are constrained to have high curvature. Thus, as highly curved vesicles are formed from relatively planar membranes, proteins can be either included or excluded from the forming vesicles based on their shape. [8]

Motif insertion

Insertion of a piece of a protein into one leaflet of the membrane induces curvature. Protein Insertion Membrane Curvature.jpg
Insertion of a piece of a protein into one leaflet of the membrane induces curvature.

The hydrophobic portion of protein can act as "wedge" when inserting into lipid bilayer. Epsin is one example that utilizes this mechanism to drive membrane bending. Epsin has several amphipathic alpha helices that allows it to partition between the hydrophobic core of the membrane and surrounding aqueous, hydrophilic environment. Another interesting characteristic of epsin and other proteins that bind to membranes is the fact that it shows high binding affinity for a fairly common membrane lipid, phosphatidylinositol 4,5-bisphosphate (PI-4,5-P2). [12] Unlike other proteins that simply bend the membrane through sheer rigidity, epsin is a globular soluble protein and thus not rigid. The insertion of its helices into the membrane force the neighboring lipids of the leaflet that has been bound to expand laterally. This displacement of lipids on only one of the leaflets increases the bilayer's curvature. This figure shows membrane bending by insertion of a hydrophobic protein motif into a lipid bilayer. The figure illustrates a slightly different mechanism. In this case, the membrane-bending protein does not exhibit intrinsic rigidity. Instead they are often globular and soluble. The protein epsin is an example. Epsin has an ENTH (epsin N-terminal homology) domain which inserts its amphipathic alpha helix into the membrane. Epsin has high binding affinity for the membrane if PI-4,5-P2 is present. [12]

A BAR domain of a protein inducing and stabilizing the curvature of a membrane. BAR Domain on Membrane.jpg
A BAR domain of a protein inducing and stabilizing the curvature of a membrane.

Another example of protein interactions that directly affect membrane curvature is that of the BAR (Bin, amphiphysin, Rvs’) domain. The BAR domain is present in a large family of proteins. Relative to the cellular lipid bilayer, this domain is rigid and exhibits a "banana" shape. It has been postulated that the positively charged amino acid residues in the concave region of the BAR domain would come into contact with the negatively charged polar head groups of lipids in the bilayer, thus allows the binding process. [4] Upon binding, the membrane's curvature is increased by the rigid domain. [12] This figure shows the bending of a membrane by a banana-shape like BAR domain.

In the figure, an illustration of a BAR domain present in a number of proteins. The membrane curvature is induced by the very shape of this proteic region. This domain attaches to the lipid bilayer through strong coulombic interactions. This idea is supported by the existence of positively charged amino acid residues in the concave region of the BAR domain. [13] These amino acids would come into contact with the negatively charged polar head groups of lipids in the bilayer. This form phenomenon is also referred to as the "scaffold mechanism".

Scaffolding

Cage like structure of clathrin. When this structure forms around a membrane, it pulls the membrane into a tight curvature until eventual vesicle budding. Clathrin cage viewed by croelectron microscopy.jpg
Cage like structure of clathrin. When this structure forms around a membrane, it pulls the membrane into a tight curvature until eventual vesicle budding.

A classical example of membrane bending by rigid protein scaffold is clathrin. Clathrin is involved in cellular endocytosis and is sequestrated by specific signaling molecules. Clathrin can attach to adaptor protein complexes on the cellular membrane, and it polymerizes into lattices to drive greater curvature, resulting in endocytosis of a vesicular unit. Coat protein complex I (COP1) and coat protein complex II (COPII) follow similar mechanism in driving membrane curvature. [14] This figure shows a protein coating that induces curvature. As mentioned above, proteins such as clathrin are recruited to the membrane through signaling molecules and assemble into larger polymeric structures that form a rigid structure which serves as a frame for the membrane. Clathrin binds to its receptors that are present in the membrane.

The figure shows a protein coating that induces curvature. As mentioned above, proteins such as clathrin are recruited to the membrane through signaling molecules and assemble into larger polymeric structures that form a rigid structure which serves as a frame for the membrane. Clathrin binds to its receptors that are present in the membrane.


Cytoskeleton

Cytoskeletal structure inducing membrane curvature. The inherent shape of a cell - as controlled by its cytoskeleton - requires that the bilayer membrane curve around it. Cytoskeleton Membrane Curvature.jpg
Cytoskeletal structure inducing membrane curvature. The inherent shape of a cell - as controlled by its cytoskeleton - requires that the bilayer membrane curve around it.

The overall shape of a cell is mostly determined by its cytoskeletal structure. This shape will vary widely depending on the location and function of the cell. The cell membrane must be able to curve around and fit the shape determined by these functions. [2] This requires the membrane to be fluid enough to do so in a stable manner, and is often stabilized by the other mechanisms listed in this article, in particular lipid composition.

Mammalian cells will usually remain the roughly the same shape, with a common exception being locomotive cells. In order to move, these cells will often modify their structure via lamellipodia and filopodia. The membrane must be able to actively adapt to these changing curvature restraints in order for the cell to move effectively and without damaging the cell membrane. [8]

Crowding

This figure illustrates membrane bending caused by protein crowding. When a high local concentration of proteins (shown in green) are present on the membrane surface (shown in black), membrane curvature can be induced. This hypothesis reasoned that the high protein concentration increases the likelihood of repulsions between proteins, therefore generates steric pressure between proteins. To relieve such pressure, lipid membrane has to bend in order to decrease protein repulsions. Protein crowding membrane bending.png
This figure illustrates membrane bending caused by protein crowding. When a high local concentration of proteins (shown in green) are present on the membrane surface (shown in black), membrane curvature can be induced. This hypothesis reasoned that the high protein concentration increases the likelihood of repulsions between proteins, therefore generates steric pressure between proteins. To relieve such pressure, lipid membrane has to bend in order to decrease protein repulsions.

The protein crowding mechanism hypothesizes that proteins can bend membrane without directly perturbing membrane structures like the above mechanisms. [15] [16] When a high enough local concentration of protein is present on membrane surface, repulsion between protein molecules on the membrane surface can induce membrane curvature. [17] Although contribution of this mechanism remains unclear, multiple experimental and computation evidences have shown its potential in bending membrane. A recent study even shows that protein crowding can cause membrane bending and leads to membrane fission. [18] [19] These studies suggest that high local protein concentration can overcome the energy barrier to bend lipid membrane, and thus can contribute to membrane bending.

Related Research Articles

<span class="mw-page-title-main">Biological membrane</span> Enclosing or separating membrane in organisms acting as selective semi-permeable barrier

A biological membrane, biomembrane or cell membrane is a selectively permeable membrane that separates the interior of a cell from the external environment or creates intracellular compartments by serving as a boundary between one part of the cell and another. Biological membranes, in the form of eukaryotic cell membranes, consist of a phospholipid bilayer with embedded, integral and peripheral proteins used in communication and transportation of chemicals and ions. The bulk of lipid in a cell membrane provides a fluid matrix for proteins to rotate and laterally diffuse for physiological functioning. Proteins are adapted to high membrane fluidity environment of lipid bilayer with the presence of an annular lipid shell, consisting of lipid molecules bound tightly to surface of integral membrane proteins. The cell membranes are different from the isolating tissues formed by layers of cells, such as mucous membranes, basement membranes, and serous membranes.

<span class="mw-page-title-main">Endocytosis</span> Cellular process

Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. Endocytosis includes pinocytosis and phagocytosis. It is a form of active transport.

<span class="mw-page-title-main">Lipid bilayer</span> Membrane of two layers of lipid molecules

The lipid bilayer is a thin polar membrane made of two layers of lipid molecules. These membranes are flat sheets that form a continuous barrier around all cells. The cell membranes of almost all organisms and many viruses are made of a lipid bilayer, as are the nuclear membrane surrounding the cell nucleus, and membranes of the membrane-bound organelles in the cell. The lipid bilayer is the barrier that keeps ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. Lipid bilayers are ideally suited to this role, even though they are only a few nanometers in width, because they are impermeable to most water-soluble (hydrophilic) molecules. Bilayers are particularly impermeable to ions, which allows cells to regulate salt concentrations and pH by transporting ions across their membranes using proteins called ion pumps.

<span class="mw-page-title-main">Peripheral membrane protein</span> Membrane proteins that adhere temporarily to membranes with which they are associated

Peripheral membrane proteins, or extrinsic membrane proteins, are membrane proteins that adhere only temporarily to the biological membrane with which they are associated. These proteins attach to integral membrane proteins, or penetrate the peripheral regions of the lipid bilayer. The regulatory protein subunits of many ion channels and transmembrane receptors, for example, may be defined as peripheral membrane proteins. In contrast to integral membrane proteins, peripheral membrane proteins tend to collect in the water-soluble component, or fraction, of all the proteins extracted during a protein purification procedure. Proteins with GPI anchors are an exception to this rule and can have purification properties similar to those of integral membrane proteins.

<span class="mw-page-title-main">Fluid mosaic model</span> Describe the fluid mosaic model of plasma membrane

The fluid mosaic model explains various observations regarding the structure of functional cell membranes. According to this biological model, there is a lipid bilayer in which protein molecules are embedded. The phospholipid bilayer gives fluidity and elasticity to the membrane. Small amounts of carbohydrates are also found in the cell membrane. The biological model, which was devised by Seymour Jonathan Singer and Garth L. Nicolson in 1972, describes the cell membrane as a two-dimensional liquid that restricts the lateral diffusion of membrane components. Such domains are defined by the existence of regions within the membrane with special lipid and protein cocoon that promote the formation of lipid rafts or protein and glycoprotein complexes. Another way to define membrane domains is the association of the lipid membrane with the cytoskeleton filaments and the extracellular matrix through membrane proteins. The current model describes important features relevant to many cellular processes, including: cell-cell signaling, apoptosis, cell division, membrane budding, and cell fusion. The fluid mosaic model is the most acceptable model of the plasma membrane. Its main function is to separate the contents of the cell from the exterior.

<span class="mw-page-title-main">Clathrin</span> Protein playing a major role in the formation of coated vesicles

Clathrin is a protein that plays a major role in the formation of coated vesicles. Clathrin was first isolated and named by Barbara Pearse in 1976. It forms a triskelion shape composed of three clathrin heavy chains and three light chains. When the triskelia interact they form a polyhedral lattice that surrounds the vesicle, hence the protein's name, which is derived from the Latin clathrum meaning lattice. Coat-proteins, like clathrin, are used to build small vesicles in order to transport molecules within cells. The endocytosis and exocytosis of vesicles allows cells to communicate, to transfer nutrients, to import signaling receptors, to mediate an immune response after sampling the extracellular world, and to clean up the cell debris left by tissue inflammation. The endocytic pathway can be hijacked by viruses and other pathogens in order to gain entry to the cell during infection.

<span class="mw-page-title-main">SNARE (protein)</span> Protein family

SNARE proteins – "SNAPREceptor" – are a large protein family consisting of at least 24 members in yeasts, more than 60 members in mammalian cells, and some numbers in plants. The primary role of SNARE proteins is to mediate vesicle fusion – the fusion of vesicles with the target membrane; this notably mediates exocytosis, but can also mediate the fusion of vesicles with membrane-bound compartments. The best studied SNAREs are those that mediate the neurotransmitter release of synaptic vesicles in neurons. These neuronal SNAREs are the targets of the neurotoxins responsible for botulism and tetanus produced by certain bacteria.

<span class="mw-page-title-main">Vesicular transport adaptor protein</span>

Vesicular transport adaptor proteins are proteins involved in forming complexes that function in the trafficking of molecules from one subcellular location to another. These complexes concentrate the correct cargo molecules in vesicles that bud or extrude off of one organelle and travel to another location, where the cargo is delivered. While some of the details of how these adaptor proteins achieve their trafficking specificity has been worked out, there is still much to be learned.

<span class="mw-page-title-main">BAR domain</span> Group of highly conserved protein dimerisation domains

In molecular biology, BAR domains are highly conserved protein dimerisation domains that occur in many proteins involved in membrane dynamics in a cell. The BAR domain is banana-shaped and binds to membrane via its concave face. It is capable of sensing membrane curvature by binding preferentially to curved membranes. BAR domains are named after three proteins that they are found in: Bin, Amphiphysin and Rvs.

<span class="mw-page-title-main">ENTH domain</span> InterPro Domain

The epsin N-terminal homology (ENTH) domain is a structural domain that is found in proteins involved in endocytosis and cytoskeletal machinery.

<span class="mw-page-title-main">Epsin</span>

Epsins are a family of highly conserved membrane proteins that are important in creating membrane curvature. Epsins contribute to membrane deformations like endocytosis, and block vesicle formation during mitosis.

<span class="mw-page-title-main">AP2 adaptor complex</span>

The AP2 adaptor complex is a multimeric protein that works on the cell membrane to internalize cargo in clathrin-mediated endocytosis. It is a stable complex of four adaptins which give rise to a structure that has a core domain and two appendage domains attached to the core domain by polypeptide linkers. These appendage domains are sometimes called 'ears'. The core domain binds to the membrane and to cargo destined for internalisation. The alpha and beta appendage domains bind to accessory proteins and to clathrin. Their interactions allow the temporal and spatial regulation of the assembly of clathrin-coated vesicles and their endocytosis.

A cell membrane defines a boundary between a cell and its environment. The primary constituent of a membrane is a phospholipid bilayer that forms in a water-based environment due to the hydrophilic nature of the lipid head and the hydrophobic nature of the two tails. In addition there are other lipids and proteins in the membrane, the latter typically in the form of isolated rafts.

Lipid bilayer mechanics is the study of the physical material properties of lipid bilayers, classifying bilayer behavior with stress and strain rather than biochemical interactions. Local point deformations such as membrane protein interactions are typically modelled with the complex theory of biological liquid crystals but the mechanical properties of a homogeneous bilayer are often characterized in terms of only three mechanical elastic moduli: the area expansion modulus Ka, a bending modulus Kb and an edge energy . For fluid bilayers the shear modulus is by definition zero, as the free rearrangement of molecules within plane means that the structure will not support shear stresses. These mechanical properties affect several membrane-mediated biological processes. In particular, the values of Ka and Kb affect the ability of proteins and small molecules to insert into the bilayer. Bilayer mechanical properties have also been shown to alter the function of mechanically activated ion channels.

<span class="mw-page-title-main">Lipid bilayer fusion</span>

In membrane biology, fusion is the process by which two initially distinct lipid bilayers merge their hydrophobic cores, resulting in one interconnected structure. If this fusion proceeds completely through both leaflets of both bilayers, an aqueous bridge is formed and the internal contents of the two structures can mix. Alternatively, if only one leaflet from each bilayer is involved in the fusion process, the bilayers are said to be hemifused. In hemifusion, the lipid constituents of the outer leaflet of the two bilayers can mix, but the inner leaflets remain distinct. The aqueous contents enclosed by each bilayer also remain separated.

A model lipid bilayer is any bilayer assembled in vitro, as opposed to the bilayer of natural cell membranes or covering various sub-cellular structures like the nucleus. They are used to study the fundamental properties of biological membranes in a simplified and well-controlled environment, and increasingly in bottom-up synthetic biology for the construction of artificial cells. A model bilayer can be made with either synthetic or natural lipids. The simplest model systems contain only a single pure synthetic lipid. More physiologically relevant model bilayers can be made with mixtures of several synthetic or natural lipids.

Vesicle fusion is the merging of a vesicle with other vesicles or a part of a cell membrane. In the latter case, it is the end stage of secretion from secretory vesicles, where their contents are expelled from the cell through exocytosis. Vesicles can also fuse with other target cell compartments, such as a lysosome. Exocytosis occurs when secretory vesicles transiently dock and fuse at the base of cup-shaped structures at the cell plasma membrane called porosome, the universal secretory machinery in cells. Vesicle fusion may depend on SNARE proteins in the presence of increased intracellular calcium (Ca2+) concentration.

<span class="mw-page-title-main">Cell membrane</span> Biological membrane that separates the interior of a cell from its outside environment

The cell membrane is a biological membrane that separates and protects the interior of all cells from the outside environment. The cell membrane consists of a lipid bilayer, made up of two layers of phospholipids with cholesterols interspersed between them, maintaining appropriate membrane fluidity at various temperatures. The membrane also contains membrane proteins, including integral proteins that span the membrane and serve as membrane transporters, and peripheral proteins that loosely attach to the outer (peripheral) side of the cell membrane, acting as enzymes to facilitate interaction with the cell's environment. Glycolipids embedded in the outer lipid layer serve a similar purpose. The cell membrane controls the movement of substances in and out of cells and organelles, being selectively permeable to ions and organic molecules. In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity, and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall and the carbohydrate layer called the glycocalyx, as well as the intracellular network of protein fibers called the cytoskeleton. In the field of synthetic biology, cell membranes can be artificially reassembled.

DP1/Yop1p is an integral membrane protein family that, along with the reticulons, is responsible for the shape of the tubular endoplasmic reticulum (ER) in yeast and mammalian cells. Furthermore, it is also believed that they might be involved in sheet ER formation.

Lysenin is a pore-forming toxin (PFT) present in the coelomic fluid of the earthworm Eisenia fetida. Pore-forming toxins are a group of proteins that act as virulence factors of several pathogenic bacteria. Lysenin proteins are chiefly involved in the defense against eukaryotic and prokaryotic pathogens. Following the general mechanism of action of PFTs lysenin is segregated as a soluble monomer that binds specifically to a membrane receptor, sphingomyelin in the case of lysenin. After attaching to the membrane, the oligomerization begins, resulting in a nonamer on top of membrane, known as a prepore. After a conformational change, which could be triggered by a decrease of pH, the oligomer is inserted into the membrane in the so-called pore state.

References

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