Protein transport across membranes

Protein transport across membranes

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Protein Transport in Gram-Negative Bacteria: The Sec-Dependent Pathway

Many newly synthesized proteins of the outer membrane and the periplasm of Gram-negative bacteria are transported in the unfolded state via the Sec pathway. All proteins that are transported Sec-dependent have a signal peptide of 18-26 amino acids at the N-terminus.

The membrane uptake complex

The translocase complex of the inner membrane consists of two trimeric complexes: SecDFyajCp and SecYEGp. This complex is bound with high affinity by SecAp. Electron microscopic studies showed that SecYEGp forms a ring-like structure and possibly represents a type of channel through which the protein is transported; the other components are responsible for transport through the channel. Interestingly, the two channel-forming complex components are homologous to the eukaryotic transport machinery through the ER membrane: SecYp is homologous to Sec61p from yeast and Sec61a from mammalian cells, SecEp shows homologies to Sss1p from yeast and to Sec61g from mammals.

SecBp is a chaperone of the cytoplasm. It binds approx. 150 amino acids of the preprotein either already translated or still on the ribosome, keeps it in a transport-competent (i.e. unfolded) state and directs it to the plasma membrane.

How does the translocation of the unfolded protein come about?

  • The complex of SecBp and growing polypeptide chain on the ribosome probably already binds the SecAp homodimer in the cytoplasm.
  • This binding then causes the binding of ATP to SecAp, which induces a conformational change of the SecAp and inserts this partly together with approx. 20-30 amino acids of the newly synthesized polypeptide into the membrane (presumably into the SecYEGp translocation channel).
  • The newly synthesized polypeptide also activates the ATPase activity of SecAp. ATP hydrolysis leads to the release of SecAp from the membrane, whereupon another SecAp dimer can bind to the membrane-bound preprotein and can channel a further 20-30 amino acids through the membrane.

When the protein has passed the SecYEGp channel, the signal peptidase cleaves the signal peptide (yellow) and the protein is released into the periplasm.


Agarraberes, F. A .; Dice, J. F. (2001):Protein translocation across membranes.. In: Biochim. Biophys. Acta. 1513, 1-24
Driessen, A. J .; Manting, E. H .; van der Does., C. (2001):The structural basis of protein targeting and translocation in bacteria.. In: Nature Struct. Bio.. 8, 492-498

Post-translational protein transport

Of the post-translational protein transport In biology, denotes a mechanism called proteins after their synthesis (see translation) transported through a membrane. Various forms of post-translational protein transport can be found in prokaryotic and eukaryotic cells.

Post-translational protein transport takes place primarily in organelles such as mitochondria, plastids (and also their thylakoids) and peroxisomes, as well as in the cell nucleus. Post-translational protein transport is also found on the cytoplasmic membrane of bacteria.

Another form of post-translational transport is found at the endoplasmic reticulum (ER), where proteins are transported from the cytosolic side through the ER membrane into the lumen of the ER.

During this process, the proteins are completely synthesized in the cytoplasm and only then transported through the ER membrane. Most of the knowledge available today about this form of ER transport comes from studies carried out on yeast Saccharomyces cerevisiae were carried out. The signal sequence of a protein determines whether a secretory protein is co- or post-translationally transported into the ER. In the case of more hydrophilic signal sequences, the affinity of the signal recognition particle (SRP) to be less pronounced in relation to the signal sequence, as a result of which there is no fixed binding and thus no translation pause that would initiate cotranslational transport. The protein is completely synthesized in the cytoplasm and detached from the ribosome.

In order to keep the transport substrate in a translocation-competent state, cytosolic chaperones of the Hsp70 family bind to the protein. Little is known about the subsequent process of targeting the protein to the ER membrane. The bound cytosolic factors do not seem to play an essential role for the actual transport through the ER membrane, since transport proteins are denatured by urea (and thus freed from HSP70) in vitro can be transported post-translationally just as efficiently as the native proteins.

Reconstitution experiments with purified components from yeast microsomes showed that, in addition to the luminal chaperone Kar2p and ATP, a special membrane complex is required to in vitro to transport post-translational prepro-a-factor. This complex is the hetero-heptameric Sec complex, which is composed of the trimeric Sec61 complex and the tetrameric Sec62 / Sec63 complex. Similar to the ribosome with the Sec61 complex, the Sec complex forms ring-shaped structures in the membrane.

The actual post-translational transport of the proteins through the ER membrane takes place in two steps. First, the protein to be transported binds to the Sec complex independently of Kar2p and ATP, with the cytosolic components of the Sec62 / Sec63 subcomplex probably being a species Signal sequence antenna form in the membrane. Cross-linking studies have shown that the signal sequence of the protein is recognized and bound by the large subunit of the Sec61 complex during this phase. In the second step of the transport process, the bound polypeptide chain is moved through the channel formed by the Sec61 complex, as in cotranslational transport, whereby Kar2p and ATP are also required for efficient transport. Kar2p is an Hsp70 homologous protein that is required for post-translational transport both in vivo and in vitro. The ATP-bound form of Kar2p binds to the DnaJ homologous domain of the Sec63p protein from the Sec62 / 63 subcomplex located in the lumen of the ER and is transferred to the polypeptide chain of the protein to be translocated under ATP hydrolysis with low sequence specificity.

For the model protein prepro-a-factor it could be shown in vitro that the transport through the ER membrane takes place according to the principle of a molecular ratchet. The transport substrate can diffuse freely in both directions as a result of the Brownian molecular movement in the translocon channel. Only when the luminal Kar2p attaches to the polypeptide chain does a directed transport process arise, since areas of the polypeptide chain located in the lumen can no longer slide back into the channel due to the bound Kar2p. Through the successive binding of further Kar2p proteins to the polypeptide chain, the protein is transported into the lumen of the ER.

Model of the post-translational protein transport across the yeast ER membrane Saccharomyces cerevisiae.

  1. The protein to be transported is completely synthesized in the cytoplasm and kept in a translocation-competent state by cytosolic chaperones. The signal sequence of the protein is recognized and bound by the Sec complex in a step that is independent of ATP and Kar2p.
  2. The N-terminus of the polypeptide chain probably inserts in the form of a hairpin into the translocation channel formed by the Sec61 subcomplex.
  3. The actual translocation of the polypeptide chain through the ER membrane takes place in an ATP and Kar2p-dependent process. The luminal Kar2p is activated by the DnaJ domain of Sec63p to bind peptides. As soon as the polypeptide chain to be transported emerges on the luminal side of the ER membrane, Kar2p is transferred to these areas with ATP cleavage, which prevents the chain from sliding back (molecular ratchet). Through the successive binding of further Kar2p proteins, the undirected Brownian molecular movement of the polypeptide chain in the translocon is converted into a directed transport process.

In Gram-negative bacteria, bacterial protein secretion also has to cross the outer membrane of the bacteria. To this end, the organisms have developed at least five different systems (type I to type V).

Nanoporous membranes for medicine and biotechnology

Nanoporous aluminum oxide membranes, produced by means of anodic oxidation, are characterized by parallel pores open on both sides with a regular pore structure, high accessibility and narrow pore size distribution. Furthermore, these membranes are characterized by high optical transparency and low intrinsic fluorescence. Potential fields of application for nanoporous aluminum oxide membranes are in the bio and food industry (sterility of products) and in medical technology (sterile filtration). Furthermore, the nanoporous membranes can be used in tissue engineering for cell cultivation. The current research focus is the application of nanoporous membranes as a three-dimensional biochip array.

Manufacturing / membrane preparation
By anodic oxidation of pure aluminum (4N) in acidic electrolytes, a nanoporous oxide layer with parallel nanopores arranged perpendicular to the surface is created on the surface of the aluminum. The pore size can be set between 20 and 450 nm using the manufacturing parameters anodizing voltage, temperature and electrolyte concentration. The layer thickness (30-200 µm) is determined by the anodizing time. For the production of self-supporting aluminum oxide membranes, the porous layers produced by anodic oxidation are detached from the base metal using a special process. The still closed pores of the membrane obtained are opened by a further etching step. This gives a high open porosity of 40-50% [1 - 3].
The membranes can currently be produced as self-supporting membranes in areas of up to 100 cm² and can be given any external shape by laser cutting. Using a process patented by Fraunhofer IWM, the aspect ratio of pore diameter / pore length can be optimized while increasing mechanical stability at the same time. The base material "aluminum" is pre-structured by means of thermomechanical embossing or laser-based structuring. After the anodic oxidation, very thin areas (up to 1.5 µm) are obtained, which are stabilized by the aluminum bars (Fig. 1) Pore ​​inner walls can also be surface-functionalized for various applications (Fig. 1 right) [3-5]. In contrast to the conventional product Anopore, membranes are available that can be adapted to customer requirements and applications in terms of pore diameter, membrane size and surface properties.

Sterile filtration
Reliable membranes are required for sterile filtration in biotechnology and medical technology. In the case of ceramic filtration materials made of anodically oxidized aluminum, the possibility of significantly larger pores occurring by chance can be excluded due to the manufacturing process. This cannot be guaranteed with membrane filters made of polymers. Filtration membranes made of nanoporous aluminum oxide can therefore provide excellent particle retention as well as complete protection e.g. B. ensure against infectious germs, bacteriophages or viruses. The specific flow rate is determined by the pore size, porosity and membrane thickness and can be set specifically for use by the user over two size ranges. So far, application-oriented filter modules with adjustable pore sizes in the range of 12-200 nm have been developed for sterile filtration. Biosafety was tested using non-pathogenic plant viruses (tobacco mosaic viruses). Figure 2 shows the virus retention of different membranes. From a pore size of less than 18 nm, complete retention could be guaranteed for the tobacco mosaic viruses.

Cell cultivation
Hepatocytes are bipolar epithelial cells of the liver with selective substance exchange on the basolateral and apical cell side. In order to correspond to the microenvironment in vivo, a carrier material is to be favored for the cultivation of the primary cells, which enables them a basolateral exchange of substances. A co-culture with non-parenchymal cells should optimize the culture of primary hepatocytes. Therefore, nanoporous aluminum oxide membranes were used for cell co-culture [6 - 9]. Primary hepatocytes and non-parenchymal cells were each cultivated on one side of the membrane and used in a two-chamber flow reactor. The hepatocytic synthesis of urea served as a marker to determine the functional degree of differentiation in the course of the culture.
The culture of primary hepatocytes on aluminum oxide membranes was successful with different pore sizes (25-240 nm). Electron microscopic examinations showed that the cells were firmly anchored to the carrier material. The membranes overgrown with non-parenchymal cells by hepatocytes in coculture were placed as a partition in a flow-through chamber which was continuously perfused with medium. In contrast to the conventional monolayer culture, the urea synthesis rate remained at the same high level during a culture period of 7 days (see Fig. 3). Primary hepatocytes could be cultivated in co-culture with non-parenchymal cells of the liver on nanoporous aluminum oxide membranes with a sustained high urea synthesis output. The use of the cell-covered membranes in a flow-through reactor offers optimal culture conditions for primary hepatocytes while maintaining specific hepatocyte properties. Differentiated hepatocytes can be propagated in this reactor for transplantation purposes.

Biochip array
Microarray biochip technology is an important tool for diagnostics in medicine, pharmacy, biochemistry, genetics and microbiology. Classic biochips (microarrays) consist of a large number of systematically arranged small spots on a planar 2D carrier made of glass, plastic or metal. The transition from the planar 2D matrix to a porous 3D matrix based on nanoporous aluminum oxide results in a significant increase in the usable surface. The significant increase in the number and density of probes leads to a considerable increase in the measurement sensitivity and speed with a simultaneous miniaturization of the chip size and a reduction in the sample volume. The development takes place in cooperation with the company Smart Membranes, Halle.
The developed biochip arrays have a total size of 26 x 76 mm 2 with spot diameters of 300 µm (thermomechanical embossing) or 30 µm (laser-structured). The spots contain the 3D pore matrix with a layer thickness of 50 µm and a pore diameter of 225 nm with a porosity of 42%.
In order to ensure a firm coupling of oligonucleotides or nucleic acids (DNA) to the surface of the developed biochip array, the inner walls of the pores must be activated beforehand. The organosilane linker 3-aminopropyltrimethoxysilane was used for activation. This gives the surface of the biochip array a positive charge with reactive primary amines (-NH 3+). Due to the electrostatic interaction of the positively charged aminosilane groups of the biochip array with the negatively charged groups of the DNA phosphate backbone, a solid electrostatic bond between the amines (-NH 3+) and the DNA. The detection of the silanization with 3-minopropyltrimethoxysilane was carried out by means of transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX nanospot analysis). By detecting silicon and nitrogen on the basis of the EDX spectra obtained, activation of the pore inner wall after silanization could be demonstrated for the biochip arrays produced. Successful activation is followed by biofunctionalization with the DNA probes. Hybridizable DNA sequences with different base pairs and fluorescence-labeled with cyanine dyes (Cy3 and Cy5) were used as probes. The DNA probes were immobilized by spotting followed by UV crosslinking. After various washing steps, the fluorescence intensity of the bound DNA probes is measured using a plate reader. It was found that the coupling of the DNA probes takes place primarily via the amino groups of the silane layer and that unspecific binding on the pure substrate is negligible. The immobilization efficiency and stability of the DNA probes of the new 3D biochip arrays is shown in Figure 4 compared to conventional amino groups on functionalized HTA slides.The concentration of the DNA probe molecules determined by the fluorescence intensity is 5.3 to 19.4 times higher than with conventional 2D biochip arrays.

[1] Hanaoka T.-A. et al .: Appl. Organometallic. Chem. 12, 367-373 (1998)
[2] Heilmann A. et al .: Applied Surface Science 144-145, 682-685 (1999)
[3] Heilmann A. et al .: Journal of Nanoscience and Nanotechnology 3, 375-379 (2003)
[4] Müller S. et al .: J. Biomedical Nanotechnology 2, 16-22 (2006)
[5] Thormann A. et al .: Small 3, 1032-1040 (2007)
[6] Hoess A. et al .: Journal of Biomedical Materials Research A (2012) doi. 10.1002 / jbm.a.34158
[7] Hoess A. et al .: Acta biomaterialica 3, 43-50 (2007)
[8] Hoess A. et al .: Advanced Engineering Materials 12, B269-B275 (2010)
[9] Ferraz N. et al .: Journal of Nanoscience and Nanotechnology 11, 6698-6704 (2011)

Dipl.-Ing. (FH) Annika Thormann, Researcher Employee polymer films and membranes, Fraunhofer Institute for Mechanics of Materials Prof. Dr. Andreas Heilmann, Head of Biological Materials and Interfaces, Fraunhofer Institute for Mechanics of Materials IWM


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Hydrogen permeability through palladium membranes

Palladium has excellent permeability only to hydrogen.

This property makes it possible to use a palladium membrane to filter only hydrogen from mixed gases, which means that high-purity hydrogen can be generated. This takes place z. B. Application as the smallest possible "atomic sieve" in the production of hydrogen for fuel cells as well as the production of ultra-pure hydrogen for the manufacture of semiconductors and LEDs.

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Five variations

So far, five types (type I to type V) of secretion systems are known in Gram-negative bacteria, two of which use the Sec system.

Type I prototype is that Escherichia coli-Hemolysin transport system. Three membrane proteins allow Sec-independent secretion in one step. Type II prototype is that Klebsiella-Pullulanase secretion system. A large number of proteins form a transport complex with a pore in the outer membrane. The transport takes place in two steps, with the first step being Sec-dependent through the cytoplasmic membrane. Type III prototype is that Yersinia-Yop system. A multitude of proteins form a needle structure that protrudes from the cytoplasm through both membranes. Proteins are transported Sec-independently in one step. Type III systems are partially activated by contact with target cells and also allow proteins to be injected into the target cell. Type IV prototype is the Vir system of Agrobacterium tumefaciens. A multitude of proteins form a complex that protrudes through both membranes. Proteins are transported Sec-independently in one step. Some type IV systems allow injection of proteins into the target cell after contact with target cells. Some type IV systems carry proteins and DNA. Type V (car transporter) prototype is the IgA protease from Neisseria. The proteins are secreted into the periplasm as a function of Sec via the cytoplasmic membrane. A domain at the C-terminal end of the protein is integrated into the outer membrane and transports the protein to the outside, where it is proteolytically cut off.


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A simple model explains the transport of molecules through membranes

There is new knowledge about the transport of molecules across cell membranes. They come from Wolfgang Bauer from the Medical Clinic I at the University of Würzburg and from Walter Nadler, who works at the Michigan Technological University in the USA. The new knowledge has been published online since July 21 in PNAS, the journal of the US National Academy of Sciences.

Transport of molecules through channels (hatched): On the left the molecules in a higher concentration, on the right in a lower concentration. The transport thus takes place from left to right, whereby the molecules have to squeeze through the channels. The forces between the channel and the molecule determine how fast the transport takes place. Graphics: Bauer / Nadler

In all organisms it is of fundamental importance that molecules are transported through cell membranes. This ensures, for example, the absorption of nutrients from the intestine into the body, the elimination of waste products via the kidneys, but also the communication between the cells.

Most of the time, the molecules are passed through the membranes via special channels or pores. This type of transport depends entirely on the way in which the molecules in the passages are influenced by forces. Such forces arise, for example, from binding sites to which the molecules in the channels can dock.

"So far, it has not been clear whether these binding sites tend to increase or decrease transport," explains Bauer. Together with his colleague Nadler, he used a simple theoretical model to show that the attractive forces of a binding site increase the transport of molecules - but only up to a certain threshold. If the forces increase, the same binding site suddenly acts as an obstacle.

"In this way, the transport of substances through the membranes can be controlled," says Bauer, "and our model explains the mechanism of this control quantitatively." This is not only of fundamental importance for understanding processes in the cell, but is also, according to the Würzburg scientists, for nanotechnology play a role: "It is conceivable that the 'speed' of molecular motors can be controlled via the strength of binding sites."

Further information: Prof. Dr. Dr. Wolfgang R. Bauer, T (0931) 201-36198 or 201-36327, email: [email protected]

Wolfgang R. Bauer and Walter Nadler: "Molecular transport through channels and pores: Effects of in-channel interactions and blocking", PNAS, July 21, 2006, 10.1073 / pnas.0601769103

Facilitated diffusion and active mass transport

We now know that some substances can diffuse the biomembranes unhindered. Passive transport does not require any energy. But what happens to the large molecules?

With the facilitated diffusion, polar substances such as amino acids or sugar as well as charged ions can cross the semipermeable membrane without consuming metabolic energy. So-called membrane proteins make this possible. There are channel proteins and carrier proteins (transport proteins, Fig. 6).

Fig. 6: Membrane proteins

Channel proteins are also known as integral membrane proteins because they are embedded in the hydrophobic lipid layer and are thus integrated into the membrane. Inside they form hydrophilic ion channels that allow ions or other larger molecules to pass through. The opening and closing of these channels is regulated by signal proteins or an electrical voltage.

Transport proteins, on the other hand, bind to the substrates to be transported and channel them through the double layer of the semipermeable membrane.

The cell membrane also has what are known as aquaporins for water transport. Although water is a very small molecule and can usually pass through the membrane without problems, water transport is very important. The cell can regulate its uptake or release of water in a more targeted manner via the aquaporins.

In addition to passive material transport, active material transport is also possible. As the name suggests, this requires energy supplied from the outside. With the help of integral membrane proteins, substances are transported through a membrane against their concentration gradient. There is the primary active transport and the secondary active transport.

During primarily active transport, energy is obtained by hydrolyzing ATP with the help of enzymes. This energy drives the transport. This plays a major role in the neurobiology of Na + / K + ion pumps.

Secondary active transport uses the concentration gradient of a substance to transport another substance through a membrane. However, this gradient was previously built up via primary transport, i.e. the hydrolysis of ATP molecules.

Understanding membranes better

The new class of membranes could be used successfully in the separation of substances.

But the theoretical understanding of these polymer membranes is still patchy. In the specialist magazine Chemical Reviews, two researchers from the Helmholtz Center Hereon and the University of Göttingen are now presenting a study that identifies the knowledge gaps and shows promising solutions.

Whether in desalination plants, wastewater treatment or the separation of CO2 membranes play a central role in technology. The Helmholtz Center Hereon has been working on a new variant for several years: It consists of special polymers that form pores of the same size on a nanometer scale. The substances to be separated, such as certain proteins, can literally slip through these pores. Since these separating layers are very thin and therefore relatively fragile, they are connected to a sponge-like structure with much coarser pores - it gives the structure the necessary mechanical stability.

“A special feature is that these structures are formed in an act of self-organization,” describes Prof. Volker Abetz, head of the Hereon Institute for Membrane Research and Professor of Physical Chemistry at the University of Hamburg. "Compared to comparable membranes, some of which are laboriously produced with the help of particle accelerators, this promises relatively inexpensive manufacture." Wastewater treatment can be used, for example, to filter out unwanted dyes.

Progress through computer simulations

It is true that experts in the field have made considerable progress in the development of these new membranes in recent years. However, in order to be able to tailor them for specific applications, there is still a lack of comprehensive theoretical understanding. "So far there has been a lot of trial and error involved, and also a lot of gut instinct," says Abetz. “Now it should be about understanding these systems as fundamentally as possible.” For this reason, Marcus Müller, Professor of Theoretical Physics at the University of Göttingen, and Volker Abetz have published a review article in the journal Chemical Reviews. The work summarizes the current state of knowledge in the field of polymer membranes and identifies the most promising research approaches with which existing knowledge gaps can be closed.

Computer simulations play an important role here - they can be used to digitally reproduce what happens in detail during the manufacturing process. But: “The problem is that these processes are extremely complex and we are dealing with completely different length and time scales,” explains Müller. “And so far we have not been able to cover all of this with a description.” There are computer models that can be used to simulate individual aspects. While some of these models describe the behavior of individual polymer molecules, others simulate the membrane in a much coarser grid. So far, these different approaches have been rather vaguely linked with one another and the description of the chronological sequence of the various processes is also a challenge. For a deeper understanding, it would be advantageous if the models interlocked better than before.

Polymer membranes from the drawing board

"The production of polymer membranes can be compared to the production of a soufflé", describes Marcus Müller. "With both, the point is to stabilize the tiny pores that matter in good time before the whole thing collapses again." Among other things, it is unclear how and whether the simultaneous formation of separating layer and carrier layer influence each other and how this can be controlled in a targeted manner. Eine weitere Frage: Wie lassen sich die Poren so anordnen, dass sie einen möglichst hohen Durchfluss durch die Membran erlauben – ein entscheidendes Kriterium für die Wirtschaftlichkeit einer Membran. „Zum Glück werden sowohl die Computer als auch die Modelle immer besser, und das sollte deutliche Fortschritte erlauben“, sagt Müller. „So können wir auf den Supercomputer JUWELS in Jülich zugreifen, einem der schnellsten der Welt.“ Womöglich helfen künftig auch die Algorithmen des maschinellen Lernens – hier könnte unentdecktes Potenzial schlummern.

Doch nicht nur die Theorie ist gefordert, auch bei den Experimenten gibt es Arbeit. „Eine große Unbekannte ist zum Beispiel die Luftfeuchtigkeit“, erzählt Volker Abetz. „Wir wissen, dass sie die Bildung einer Polymermembran entscheidend beeinflussen kann. Aber um diesen Einfluss besser zu verstehen, wird es systematische Versuchsreihen brauchen.“ Lassen sich Hürden wie diese meistern, würde das Fernziel der Forschung ein Stückchen näher rücken: „Unser Traum ist, eine Polymermembran für eine bestimmte Anwendung erst als „digitalen Zwilling“ im Computer zu konstruieren und zu optimieren, um sie dann später im Labor gezielt realisieren zu können“, sagt Abetz. „Und vielleicht können wir im Rechner sogar ganz neue Strukturen entdecken, auf die wir im Experiment niemals stoßen würden.“

Seit wann gibt es den Nobelpreis eigentlich?

Alfred Nobel wurde reich, weil er 1866 das Dynamit erfunden hat. Er gründete die Stiftung, die seit 1901 jährlich den Nobelpreis vergibt. In seinem Testament sind fünf Preise vorgesehen: Physik, Chemie, Medizin/Physiologie, Literatur und Frieden. Jeder Preisträger bekommt eine Medaille, eine Urkunde und ein Preisgeld. Der Friedensnobelpreis wird in Oslo vergeben, die anderen in Stockholm vom schwedischen König.

Original: das Verleihungsprogramm 2014 in der Vitrine. Foto: Keindorf

Video: Protein transport Animation (August 2022).