SmartWork5 Chapter 10
Many viruses enter cells through receptor-mediated endocytosis. Which of the following strategies could be affective in blocking entry of this class of viruses into cells and could be used to treat viral infections?
-Block the function of adaptin -Block the receptor with an antibody Some viruses such as the human rhinovirus 2 gain entry into their host cells through receptor-mediated endocytosis by binding the LDL receptor on the surface of cells. Developing drugs that could block the entry of substrates into the cell by receptor-mediated endocytosis might be an effective treatment for things like colds caused by the rhinovirus or to treat other infections caused by viruses that enter the cell through receptor-mediated endocytosis. The receptor, adaptin, and clathrin are all required for viral entry so inhibition of any of these three would block receptor-mediated endocytosis. Increasing clathrin activity would not prevent viral entry. Actin filaments are not involved in receptor-mediated endocytosis, so blocking them would not affect viral entry.
Which statements are true of receptor-mediated endocytosis?
-It allows the internalization of extracellular substances in clathrin-coated vesicles. -It allows cholesterol-carrying low-density lipoproteins (LDLs) to be taken up by cells. -The process can be hijacked by viruses to gain entry into cells. In most animal cells, specific macromolecules can be taken up from the extracellular fluid via clathrin-coated vesicles. The macromolecules bind to complementary receptors on the cell surface and enter the cell as receptor-macromolecule complexes in clathrin-coated vesicles. This process, called receptor-mediated endocytosis, provides a selective concentrating mechanism that increases the efficiency of internalization of particular macromolecules more than 1000-fold compared with ordinary pinocytosis. Such is the case when animal cells import the cholesterol they need to make new membranes. Cholesterol is transported in the bloodstream bound to proteins in the form of particles called low-density lipoproteins, or LDL. Cholesterol-containing LDLs bound to receptors on the cell surface are ingested by receptor-mediated endocytosis and delivered to endosomes. In the acidic interior of endosomes, the LDL dissociates from its receptor. These empty receptors are returned, via transport vesicles, to the plasma membrane for reuse. The LDL is delivered to lysosomes, where it is broken down, allowing free cholesterol to enter the cytosol. Receptor-mediated endocytosis is also used to import many other essential metabolites: the vitamin B12 and iron required to make hemoglobin is taken up by immature red blood cells. At the same time, receptor-mediated endocytosis can also be exploited by viruses, such as the influenza virus, to gain entry into cells.
Match the following structures used in receptor-mediated endocytosis with their functions.
Adaptin- mediates the contact bw the receptor and another component Clathrin- shapes the forming vesicle Receptor-mediated endocytosis occurs when cargo molecules to be imported into the cell are bound by a specific receptor in the plasma membrane. An adaptin protein binds to the receptor and acts as an adaptor or bridge between the receptor bound to cargo and the clathrin molecule. The growing vesicle is shaped by the clathrin molecules, which form a closed cage around the vesicle. After formation, the vesicle is uncoated and clathrin leaves. The vesicle is then transported to the proper intracellular location.
Oligosaccharide chains added in the ER can undergo further modification in which organelle(s)?
Both the cis and trans Golgi networks- Many of the oligosaccharide chains that are added to proteins in the ER undergo further modifications in the Golgi apparatus. On some proteins, for example, more complex oligosaccharide chains are created by a highly ordered process in which sugars are added and removed by a series of enzymes that act in a rigidly determined sequence as the protein passes through the Golgi stack. As would be expected, the enzymes that act early in the chain of processing events are located in cisternae close to the cis face, while enzymes that act late are located in cisternae near the trans face.
How do clathrin-coated vesicles select their cargo molecules?
Cargo receptors bind specifically to cargo proteins and to clathrin. Vesicles destined for different compartments have different types of protein coats. The cargo for these vesicles is selected by specifically binding to cargo receptors that interact with a specific type of protein coat.
What protein can assemble into a basketlike network that gives budding vesicles their shape?
Clathrin- Vesicles that bud from membranes usually have a distinctive protein coat on their cytosolic surface and are therefore called coated vesicles. The coat serves at least two functions: it helps shape the membrane into a bud and it captures molecules for onward transport. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse. Cells produce several kinds of coated vesicles, each with a distinctive protein coat. The best-studied vesicles are those that have an outer coat made of the protein clathrin. These clathrin-coated vesicles bud from both the Golgi apparatus on the outward secretory pathway and from the plasma membrane on the inward endocytic pathway. At the plasma membrane, for example, each vesicle starts off as a clathrin-coated pit. Clathrin molecules assemble into a basketlike network on the cytosolic surface of the membrane, and it is this assembly process that starts shaping the membrane into a vesicle.
Which best describes a pathway that a protein might follow from synthesis to secretion?
Cytosol → ER → transport vesicle → Golgi apparatus → transport vesicle → plasma membrane The synthesis of all proteins—except for those made in mitochondria or chloroplasts—begins in the cytosol. From there, they enter the ER for folding, assembly, and initial glycosylation. Transport vesicles ferry proteins that have been properly folded and assembled to the Golgi apparatus for further modification and sorting. Proteins destined for secretion do not pass through endosomes. Instead, they leave the trans Golgi network either in transport vesicles, which fuse with the plasma membrane immediately, or in secretory vesicles, which await a signal to stimulate membrane fusion.
Where in the cell are some proteins initially decorated with an oligosaccharide tree on asparagine residues?
ER- Many of the proteins that enter the ER lumen or ER membrane are converted to glycoproteins in the ER by the covalent attachment of short, branched oligosaccharide side chains composed of multiple sugars. This process of glycosylation is carried out by glycosylating enzymes present in the ER but not in the cytosol. In the ER, individual sugars are not added one by one to the protein to create an oligosaccharide side chain. Instead, a preformed, branched oligosaccharide containing a total of 14 sugars is attached en bloc to all proteins that carry the appropriate site for glycosylation. The oligosaccharide is originally attached to a specialized lipid, called dolichol, in the ER membrane; it is then transferred to the amino (NH2) group of an asparagine side chain on the protein, immediately after a target asparagine emerges in the ER lumen during protein translocation. The addition takes place in a single enzymatic step that is catalyzed by a membrane-bound enzyme (an oligosaccharyl transferase) that has its active site exposed on the lumenal side of the ER membrane—which explains why cytosolic proteins are not glycosylated in this way. The addition of the 14-sugar oligosaccharide in the ER is only the first step in a series of further modifications before the mature glycoprotein reaches the cell surface. This oligosaccharide processing begins in the ER and continues in the Golgi apparatus.
Which organelle sorts ingested molecules and recycles some of them back to the plasma membrane?
Endosomes- Eukaryotic cells are continually taking up fluid, along with large and small molecules, by the process of endocytosis. Certain specialized cells are also able to internalize large particles and even other cells. The material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first buds inward and then pinches off to form an intracellular endocytic vesicle. The ingested materials, including the membrane components, are delivered to endosomes, from which they can be recycled to the plasma membrane or sent to lysosomes for digestion.
Which cellular compartment acts as the main sorting station for extracellular cargo molecules taken up by endocytosis?
Endosomes- Just as the Golgi network acts as the main sorting station in the outward secretory pathway, the endosomal compartment serves this function in the inward endocytic pathway. The acidic environment of the endosome (pH 5 to 6) plays a crucial part in the sorting process by causing many (but not all) receptors to release their bound cargo. The routes taken by receptors once they have entered an endosome differ according to the type of receptor: (1) most are returned to the same plasma membrane domain from which they came, as is the case for the LDL receptor discussed earlier; (2) some travel to lysosomes, where they are degraded; and (3) some proceed to a different domain of the plasma membrane, thereby transferring their bound cargo molecules across the cell from one extracellular space to another, a process called transcytosis.
Which of the following is a difference between exocytic and endocytic pathways?
Exocytic pathways often start with synthesis of proteins, whereas endocytic pathways involve breaking down macromolecules like proteins. Endocytic pathways and exocytic pathways both use transport vesicles to move lipids, membrane components, proteins, and soluble molecules from the outside of the cell to inside of the cell or vice versa. Endocytic pathways bring molecules in from the outside of the cell into an endosome, which can then mature into a lysosome. Endocytosed molecules do not travel to the Golgi in vesicles.
Which is true of the constitutive exocytosis pathway?
It operates continually in all eukaryotic cells. In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network and fuses with the plasma membrane in the process of exocytosis. This constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins, enabling the plasma membrane to expand prior to cell division and refreshing old lipids and proteins in nonproliferating cells. The constitutive pathway also carries soluble proteins to the cell surface for secretion. Some of these proteins remain attached to the cell surface, some are incorporated into the extracellular matrix, and still others diffuse into the extracellular fluid to nourish or signal other cells. Entry into the constitutive pathway does not require a particular signal sequence like those that direct proteins to endosomes or back to the ER. And proteins secreted by the constitutive pathway do not aggregate and are therefore carried automatically to the plasma membrane.
Which statement about receptor-mediated endocytosis of LDL particles is true?
LDL receptors in the plasma membrane associate with clathrin-coated pits. Cholesterol-containing LDLs, which are secreted by the liver, bind to receptors located on the surface of cells. These receptors are recognized by adaptin proteins and then internalized by clathrin-coated vesicles. The vesicles lose their coat and fuse with endosomes, whose acidic internal environment promotes dissociation of LDL from its receptor. The LDL particles are then delivered to lysosomes, where they are degraded to release their cholesterol. Cholesterol then escapes into the cytosol, where it can be used to synthesize new membrane. Meanwhile, back in the endosome, the remaining LDL receptors are removed by transport vesicles that return them to the plasma membrane, where they can be reused to capture and internalize more LDL cholesterol. Some individuals inherit a defective version of the gene encoding the LDL receptor protein; in some cases, the receptors are missing, while in others, they are present but nonfunctional. In either case, because the cells are deficient in taking up LDL, cholesterol accumulates in the blood and predisposes the individuals to develop atherosclerosis.
What is true of protein glycosylation in the ER?
Oligosaccharides are added by an enzyme that has its active site on the lumenal side of the ER membrane. In the ER, individual sugars are not added one by one to the protein to create an oligosaccharide side chain. Instead, a preformed, branched oligosaccharide containing a total of 14 sugars is attached en bloc to all proteins that carry the appropriate site for glycosylation. The oligosaccharide is originally attached to a specialized lipid, called dolichol, in the ER membrane; it is then transferred to the amino (NH2) group of an asparagine side chain on the protein, immediately after a target asparagine emerges in the ER lumen during protein translocation. The addition takes place in a single enzymatic step that is catalyzed by a membrane-bound enzyme (an oligosaccharyl transferase) that has its active site exposed on the lumenal side of the ER membrane—which explains why cytosolic proteins are not glycosylated in this way. A simple sequence of three amino acids, of which the target asparagine is one, defines which sites in a protein receive the oligosaccharide. Oligosaccharide side chains linked to an asparagine NH2 group in a protein are said to be N-linked, and this is by far the most common type of linkage found on glycoproteins.
What is one of the main differences in the behavior of the proteins in a vesicle destined for constitutive secretion, and the proteins in the vesicle destined for regulated secretion?
Proteins in the regulated secretion vesicle tend to aggregate and become highly concentrated in the ionic conditions in the vesicle. Constitutive secretion vesicles contain lipids and proteins that are continuously supplying the plasma membrane with new components. Proteins in regulated secretion vesicles form concentrated aggregates so that when they are released in response to a signal, the levels of the protein can rapidly increase.
Which proteins play a central role in the fusion of a vesicle with a target membrane?
SNAREs- Rab proteins, tethering proteins, and SNAREs help direct transport vesicles to their target membranes. Interaction between Rab proteins on the vesicle and tethering proteins on the target membrane provide the initial recognition. A v-SNARE on the vesicle then binds to a complementary t-SNARE on the target membrane, ensuring that the transport vesicle docks at an appropriate target membrane. Following vesicle docking, SNARE proteins catalyze the fusion of the vesicle and target membranes. Once appropriately triggered, the tight pairing of v-SNAREs and t-SNAREs draws the two lipid bilayers into close apposition. The force of the SNAREs winding together squeezes out any water molecules that remain trapped between the two membranes, allowing their lipids to flow together to form a continuous bilayer.
When a vesicle fuses with the plasma membrane, which way will the monolayer that was exposed to the interior of the vesicle face?
The cell exterior- Most cell membranes are asymmetric and have distinct "inside" and "outside" faces: the cytosolic monolayer always faces the cytosol, while the noncytosolic monolayer is exposed to either the cell exterior—in the case of the plasma membrane—or the interior space (lumen) of an organelle. This asymmetry is preserved as membranes, in the form of vesicles, which bud from one organelle and fuse with another or with the plasma membrane.
Ricin is one of the most powerful toxins known. The protein consists of two subunits: the A chain is an enzyme that inhibits translation and the B chain is a lectin that binds to carbohydrates on the cell surface. What is the most likely mechanism by which ricin enters the cell?
The protein is internalized by endocytosis. Ricin is a powerful toxin produced by the castor bean plant. Less than 2 mg injected into the bloodstream will kill an adult human. The protein is a heterodimer composed of an A chain, which inhibits protein translation, and a B chain, which binds to carbohydrates and glycoproteins on the cell surface. Because ricin is a large protein, it must be taken into the cell via endocytosis. The toxin is internalized by both receptor-mediated endocytosis and pinocytosis. It is then transported via endosomes to the Golgi apparatus and from there into the ER. It escapes from the ER to the cytosol by partially unfolding in the ER lumen, triggering its release to the cytosol for degradation. Once in the cytosol, the A chain refolds and exerts its toxic influence on protein synthesis.
You have attached green fluorescent protein (GFP) to the carboxy terminal end of a secreted yeast protein. You express this protein in normal yeast cells, secretory mutant A cells, and secretory mutant B cells (see image). Using fluorescent microscopy, you observe the expected results, with protein secretion in normal cells, ER accumulation in mutant A, and Golgi apparatus accumulation in mutant B. You also express the GFP-fusion protein in double-mutant yeast cells containing mutations in both the gene underlying mutant A and the gene underlying mutant B. What is the correct location and explanation for where the GFP-fusion protein will accumulate in these A and B double-mutant yeast cells?
The protein will accumulate in the ER because that is an earlier step in the secretory pathway. Yeast cells containing mutations in the genes underlying both secretory mutant A and secretory mutant B will look like secretory A mutants, with secreted proteins accumulating in the ER. This is because secreted proteins pass through the ER before moving on to the Golgi apparatus. If a mutation traps proteins in the ER, those proteins cannot proceed to the next step.
What distinguishes proteins destined for regulated secretion?
Their surface properties allow them to form aggregates that are packaged into secretory vesicles. Proteins destined for regulated secretion have special surface properties that cause them to aggregate with one another under the ionic conditions (acidic pH and high Ca2+) that prevail in the trans Golgi network. The aggregated proteins are packaged into secretory vesicles, which pinch off from the network and await a signal instructing them to fuse with the plasma membrane. Proteins secreted by the constitutive pathway, on the other hand, do not aggregate and are therefore carried automatically to the plasma membrane. Selective aggregation has another function: it allows secretory proteins to be packaged into secretory vesicles at concentrations much higher than the concentration of the unaggregated protein in the Golgi lumen. This increase in concentration can reach 200-fold, enabling secretory cells to release large amounts of the protein promptly when triggered to do so.
The drug vinblastine disrupts microtubule polymerization. How would adding vinblastine to a cell affect the constitutive secretory pathway?
Transport vesicles will not be brought to either the Golgi apparatus or the plasma membrane. Microtubules are used as tracks for the transport of organelles and vesicles around the cell. Transport vesicles that bud off the endoplasmic reticulum are transported along microtubules to the Golgi apparatus. From the Golgi, vesicles are transported along microtubules to the plasma membrane, where the vesicle contents are released out of the cell.
In which process do Rab proteins function?
Vesicle tethering- Rab proteins are a family of small GTPases that are specific for each type of organelle and vesicle. Rab proteins on vesicles are recognized by tethering proteins on the target membrane and help capture and tether the vesicle for later docking and fusion.
Scientists have modified a clathrin molecule so that it still assembles but forms an open-ended lattice instead of a closed spherical cage. How would this clathrin molecule affect endocytosis in cells?
Vesicles cannot form properly without a clathrin cage, thus inhibiting endocytosis. Clathrin proteins are responsible for forming the closed spherical cage around the forming vesicle. This cage helps shape and build the vesicle. When clathrin is mutated, it will not form the proper cage and instead will make a flat sheet as an open-ended lattice. This flat sheet will not form the spherical vesicle and so endocytosis will not occur. Any processes that require clathrin to form the vesicle in the cell will be blocked. Other transport mechanisms that do not rely upon clathrin will still function normally.
Lysosomal enzymes are directed to lysosomes by which of the following?
a phosphorylated sugar group- The specialized digestive enzymes and membrane proteins of the lysosome are synthesized in the ER and transported through the Golgi apparatus to the trans Golgi network. While in the ER and the cis Golgi network, the enzymes are tagged with a specific phosphorylated sugar group (mannose 6-phosphate) so that when they arrive in the trans Golgi network they can be recognized by an appropriate receptor, the mannose 6-phosphate receptor. This tagging and recognition permits the lysosomal enzymes to be sorted and packaged into transport vesicles, which bud off and deliver their contents to lysosomes via endosomes.
Vesicle budding is driven by which of the following?
assembly of a protein coat- Vesicle budding is driven by the assembly of a protein coat on the cytosolic surface of a membrane. The protein coat serves at least two functions: it helps shape the membrane into a bud and it captures molecules for onward transport. After budding from its parent organelle, the vesicle sheds its coat, allowing its membrane to interact directly with the membrane to which it will fuse.
How do the interiors of the ER, Golgi apparatus, endosomes, and lysosomes communicate with each other?
by small vesicles that bud off of one organelle and fuse with another- Transport from the ER to the Golgi apparatus—and from the Golgi apparatus to other compartments of the endomembrane system—is carried out by the continual budding and fusion of transport vesicles. This vesicular transport extends outward from the ER to the plasma membrane, where it allows proteins and other molecules to be secreted by exocytosis, and it reaches inward from the plasma membrane to lysosomes, allowing extracellular molecules to be imported by endocytosis. Together, these pathways thus provide routes of communication between the individual organelles within the endomembrane system and between the interior of the cell and its surroundings.
The low pH inside endosomes leads to what outcome?
causing many internalized receptors to release their cargo- Low-density lipoproteins (LDLs) bind to LDL receptors on the cell surface and are internalized in clathrin-coated vesicles. The vesicles lose their coat and then fuse with endosomes. In the acidic environment of the endosome, LDL dissociates from its receptors. The LDL ends up in lysosomes, where it is degraded to release free cholesterol; the LDL receptors—usually empty—are returned to the plasma membrane via transport vesicles to be used again. An LDL receptor typically makes one round trip into the cell and back every 10 minutes, making a total of several hundred trips over its 20-hour life-span.
During a pulse-chase experiment with secreted proteins, the proteins are synthesized for a short "pulse" time with radioactive or fluorescent amino acids to label the proteins. During the "chase" period, unlabeled amino acids are added, so any additional proteins synthesized are not labeled. The labeled proteins can then be monitored over time. You complete a pulse-chase experiment to monitor the secretion of a protein from the cell. Which of the following correctly lists the order of locations of the protein during the chase period?
endoplasmic reticulum → transport vesicle → Golgi → transport vesicle → secreted Secreted proteins are synthesized at the rough endoplasmic reticulum, where ribosomes can be found. During constitutive secretion, proteins accumulate at random locations in the endoplasmic reticulum membrane network and are packaged into transport vesicles. The vesicles fuse into transport intermediates that travel along microtubule tracks to the Golgi apparatus. From the Golgi, proteins are again packaged into new transport vesicles for travel along additional microtubules to the plasma membrane, where they are secreted to the outside of the cell.
Botulism is a potentially fatal foodborne disease caused by the bacterium Clostridium botulinum. C. botulinum produces different toxins, several of which are proteases that cleave neuronal SNARE proteins. What normal process is blocked by cleavage and inhibition of SNARE proteins?
fusion of vesicles with target membranes- Many different proteins help vesicles dock and fuse with the correct target membrane (see below). Rabs interact with tethering proteins during the docking phase, bringing v-SNAREs and t-SNAREs into close proximity. The SNAREs then intertwine, aiding vesicle fusion with the target membrane. In neurons, the t-SNARE synaptosomal nerve-associated protein 25 (SNAP-25) is important for the fusion of neurotransmitter-containing vesicles with the plasma membrane.
Insulin is synthesized in the form of a precursor protein that requires cleavage of two different peptide segments before the mature protein is secreted from β cells in the pancreas. The first peptide is removed when the protein enters the lumen of the ER. To find out when the second cleavage event takes place, investigators prepare a pair of antibodies: one recognizes the pro-insulin precursor, the other the mature insulin protein. They tag the antibody that binds to the precursor protein with a red fluorescent marker; the antibody that binds to mature insulin is tagged with a green fluorescent marker. When both markers are present, the sample fluoresces yellow. The investigators then incubate an isolated β cell with both antibodies at the same time and monitor the fluorescence in its various membrane-bound compartments. The data are shown in the table below. (Google this question to get the table!) Based on these observations, where is the second peptide removed from the pro-insulin precursor protein?
immature secretory vesicles- The fluorescence associated with the ER and Golgi apparatus is red, which suggests that only the precursor protein is present and no appreciable cleavage has yet taken place. However, the fluorescence in mature secretory vesicles is green, indicating that by the time the protein is aggregated for secretion, cleavage is complete. The fluorescence in immature secretory vesicles is yellow, which means that both the mature protein and its precursor are present at once. (For fluorescent markers, the combination of red and green produces yellow.) Therefore, cleavage must be taking place in these immature vesicles. The absence of fluorescence in lysosomes indicates that the protein is not being routed to this compartment for processing or degradation. And the lack of fluorescence in the nucleus and mitochondria should not come as a surprise.
Which of the following components of receptor-mediated endocytosis of LDL is incorrectly matched with its function?
lysosome: releases LDL from the receptor When LDL binds to the specific LDL receptor on the plasma membrane, an adaptin binds to the receptor on the cytosol side. Adaptin recruits clathrin proteins, which form the coated vesicle. After vesicle formation, the clathrin is released and the naked transport vesicle is transported to the endosome. The endosome is the specific compartment where the LDL is released from the receptor as a result of the compartment's low pH. The empty LDL receptors are then recycled back to the plasma membrane, where the process can repeat. The LDL particles travel from the endosome to the lysosome where the LDL is broken down, finally releasing cholesterol to the cytosol for use in the cell.
If a phospholipid is located in the outer layer of the bilayer in a vesicle, where will it end up when the vesicle fuses with the plasma membrane?
the cytosolic face of the bilayer- When a vesicle fuses with the plasma membrane, the lipids on the outside of the vesicle will end up in the phospholipid layer on the inside (cytosolic face) of the plasma membrane. Lipids on the inside of the vesicle membrane will face the extracellular fluid in the plasma membrane.
All of the carbohydrates in the plasma membrane face the cell exterior. Which direction do the carbohydrates on internal cell membranes face?
the lumen of the vesicle or organelle- The lipids that show the most dramatically lopsided distribution in cell membranes are the glycolipids, which are located mainly in the plasma membrane and only in the noncytosolic half of the bilayer. The sugar groups of these membrane lipids face the cell exterior, where they form part of a continuous coat of carbohydrate (called the glycocalyx), which surrounds and protects animal cells. Glycolipid molecules acquire their sugar groups in the Golgi apparatus, where the enzymes that engineer this chemical modification are confined. These enzymes are oriented such that sugars are added only to lipid molecules in the noncytosolic half of the bilayer. Once a glycolipid molecule has been created in this way, it remains trapped in this monolayer, as there are no flippases that transfer glycolipids to the cytosolic side. Thus, when a glycolipid molecule is finally delivered to the plasma membrane, it displays its sugars to the exterior of the cell. On internal cell membranes, the noncytosolic half of the lipid bilayer faces the lumen of the vesicle or organelle. For an internal cell membrane, half of the bilayer that faces toward the plasma membrane would face the cytosol, as would the part of the membrane that faces the direction of the cell exterior.
How are newly made lipids supplied to the plasma membrane?
via the constitutive pathway of exocytosis- In all eukaryotic cells, a steady stream of vesicles buds from the trans Golgi network and fuses with the plasma membrane in the process of exocytosis. This constitutive exocytosis pathway supplies the plasma membrane with newly made lipids and proteins, enabling the plasma membrane to expand prior to cell division and refreshing old lipids and proteins in nonproliferating cells. The regulated exocytosis pathway also adds phospholipids to the plasma membrane; however, this pathway only operates in cells specialized for secretion.
Which molecule is displaced when a vesicle and its target membrane fuse?
water- Whereas docking requires only that the two membranes come close enough for the SNAREs protruding from the two lipid bilayers to interact, fusion requires a much closer approach: the two bilayers must come within 1.5 nanometers (nm) of each other so that their lipids can intermix. For this close approach, water must be displaced from the hydrophilic surfaces of the membranes—a process that is energetically highly unfavorable and thus prevents membranes from fusing randomly. All membrane fusions in cells must therefore be catalyzed by specialized proteins that assemble to form a fusion complex that provides the means to cross this energy barrier. For vesicle fusion, the SNARE proteins themselves catalyze the process: when fusion is triggered, the v-SNAREs and t-SNAREs wrap around each other tightly, thereby acting like a winch that pulls the two lipid bilayers into close proximity.
