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will Where a protein lacking any signal peptides be localized (this addresses what the "default fate" of a protein is)? What about an artificially designed peptide containing both an ER signal peptide and a NLS? Explain. A protein without any signal sequence will simply be translated by ribosomes in the cytosol and will remain there. A protein with an ER signal will be transported into the ER in spite of a NLS. This is because of ER transport typically being a co-translational process; the ER signal will be recognized and the ribosome will dock to the ER. The fully formed peptide will be in the ER and not have any access to the nucleus or the nuclear import machinery. Why would the expression of another acyl transferase having an ER signal peptide (this enzyme catalyzes the addition of glycerol 3-phosphate to fatty acids in the membrane) still not allow for even membrane growth in the absence of scramblase? The fatty acids and head group modifying enzymes are all available on the cytosolic side of the ER. Targeting the acyl transferase to the inside of the ER will not help matters because of the lack of substrates to work with. [This is the same idea behind why plasma membrane proteins are only glycosylated on the extra cellular side.] If the N-terminal half of PhyB is localized to the cytosol does that indicate the presence of signaling peptides? Does the C-terminal half of PhyB localizing to the nucleus indicate the presence of a NLS? Explain. It is likely that a protein lacking a NLS will remain in the cytosol, so the Nterminal localization does not tell us much. Likewise, it is not clear that even if the C-terminal portion of PhyB localizes to the nucleus that it has a NLS. This half of the protein could be binding to or being bound by another protein that has the NLS, allowing for import of the PhyB fragment. How might suppression of calnexin or calreticulin allow lung tissue to get around one of the main issues of mutant CFTR proteins? Why would this avenue of treatment for cystic fibrosis be a "bad idea?" One of the more common problems with CFTR in cystic fibrosis is that it is recognized as misfolded, retained in the ER, and exported to the cytosol for destruction by the proteosome, even if the misformed CFTR could still have some function. If calnexin and calreticulin were suppressed, this could theoretically allow these malformed but functional CFTR proteins to escape the ER and travel to the plasma membrane to function. However, this is not really a good idea because suppression of these chaperone proteins would allow the forward progress of many other misfolded proteins that could damage the cell through aggregation or simple lack of functioning proteins. What ensures that Sar1 and Rab proteins will associate with specific membranes? (In another way to think about it, why won't Sar1 or Rabs randomly associate with any membrane after activation?) This has to do with the exposure of the amphipathic or hydrophobic region of these monomeric GTPases on conversion to the GTP-bound form. Having these regions exposed to the aqueous environment of the cytosol is not favorable so they will want to associate with membranes as soon as possible. By localizing specific GEFs to specific membranes we can fairly simply ensure that an activated Sar1 or Rab protein will associate with the same membrane the GEF is in. Distortion of membranes and the concentration of solutes into vesicles are both energetically unfavorable processes. Where does the energy to drive these processes come from? Explain both processes. Both of these processes take advantage of the favorable energy changes of associating protein coats. Either of the following specific examples or a generic explanation would be fine to answer the question: 1) In the case of clathrin, the association of triskelions subunits provides the free energy to pull on the membrane as well as bring together cargo receptors. By concentrating receptors through their association with clathrin and adaptins we in turn concentrate the cargo molecules. 2) The other example would be with COP-coated vesicles. Here, the activation of Sar1 provides the energy to associate the COP protein coat. Again, the coat formation distorts the membrane. The favorably assembling coat proteins also bind to cargo receptors, concentrating cargo. What will happen to protein "X" in the ER if it contains no specific retention signals and is not recognized by any particular transport receptor proteins (i.e. what is the default pathway for proteins entering the ER)? How does this compare to the BiP chaperone? Proteins entering the ER and not containing any other specific signals well exit the ER [and ultimately head through the Golgi to the PM]. A BiP chaperone, in contrast, will remain in the ER because it is both an ER resident protei Explain how the transport of nucleotide activated sugars into the Golgi does not require ATP pumps or additional ion gradients. Recall that this process is an antiport involving free nucleosides and nucleotide activated sugars. Free nucleosides are released as the nucleotide-sugars are added to the growing oligosaccharide chains, increasing the concentration of nucleosides inside the Golgi. The high nucleoside concentration can be used to drive the import of the nucleotide-sugars. What are two ways that the ER is protected from the actions of the acid hydrolases it transports forward to the Golgi? The acid hydrolases are typically not active at the point of being in the ER. They need to be modified in some fashion, such as the removal of an inhibitory peptide to become functional. As well, the pH in the ER is not low enough for efficient functioning of the acid hydrolases. Do LDL particles taken up from the extracellular space require lysosomal targeting signals? If an LDL particle was assembled in the ER of the same cell it was to be used in, would it require a lysosomal targeting signal? Explain. LDL particles do not require lysosomal targeting signals because the default destination of materials taken up from the PM is the lysosome. Once released from the LDL receptor in the endosome they will be carried on to lysosomes. An LDL particle starting out in the ER would require a targeting signal for the lysosome though. The default pathway for materials in the ER is the plasma membrane, other detours will require some sort of specific sorting mechanism. Compare and contrast the process of glucose transporter delivery to the PM to that of neurotransmitter delivery. Glucose transporters are stored in recycling endosomes, where they wait for certain signals (insulin binding to insulin receptors) to induce their delivery to the PM. Neurotransmitters are stored in synaptic vesicles rather than recycling endosomes. These vesicles are also derived from the PM while endosome membrane may have a variety of sources. Synaptic vesicles also typically are docked at the PM ready to fuse when induced to. The delivery of glucose transporters requires the formation of new vesicles containing them, they are not simply waiting to fuse with the PM. Is the G positive, zero, or negative for filament growth at a concentration of subunits less than C C? Equal to CC? Greater than CC? Explain in terms of spontaneity of the process. Growth will not occur when the CC is less than the C so it would be a positive free energy change. Equal concentrations is a point of equilibrium where the free energy change is zero. In the last case it will be negative because growth will occur spontaneously. The centrosome helps to organize the cytoplasm by identifying the center of the cell. What abilities of microtubules allow for this to occur? Explain. Dynamic instability allows the microtubules to "explore" and push against the sides of the cell, resulting in the centrosome ending up in the center. The center is the point at which the microtubules will be pushing equally on all sides as they MT grow and shrink. Why does katanin require ATP while gelsolin can work with thermal energy inputs? Gelsolin only has to disrupt a few bonds in the actin filament while katanin needs to break many more [13 longitudinal associations]. More bonds being broken requires more energy. Compare and contrast the actions of formin to gelsolin. Formin binds to actin subunits to promote their assembly, either to form a new fiber or encourage the growth of an existing fiber. Gelsolin binds to actin as well, but it binds to existing fibers. While both interact with the plus ends of filaments, the gelsolin inhibits elongation while forming encourages it. What aspect of D-form tubulin do catastrophins exploit in order to disassemble microtubules that still have GTP caps? D-form tubulin forms curved protofilaments while the t-form in the cap forms straight filaments. Catastrophin bend the protofilaments in the GTP cap, causing them to look more like a D-form protofilament. This leads to the disassembly of the microtubule. Reviewing these homework problems may be helpful: HW4 Q5 and 7; HW5 Q2, 4, 9; HW6 Q3. Based on the signal sequences and structures of the following proteins, where would each protein localize in an animal cell? a. Amphipathic signal sequence only - Amphipathic signals target the mitochondria. Since there are no other listed qualities, we can assume that this protein is free in the matrix space of the mitochondria. b. Hydrophobic signal sequence, multiple hydrophobic sequences, KKXX - The hydrophobic signal targets the ER. The multiple hydrophobic sequences indicates that it will be a multipass transmembrane protein. The KKXX sequence is an ER retention signal for membrane proteins. Therefore we would expect to find this multipass transmembrane protein localized to the membrane of the ER. Cleavable hydrophobic signal sequence - This sequence again targets the ER. Since the sequence is cleavable, it will not serve to keep the protein anchored to the membrane. Therefore this protein is likely soluble. Since it has no other targeting signals it will follow the default pathway to the outside of the cell. c. Based on their localization in an animal cell, what might the signals sequences and structuring of each of these proteins look like? d. Lysosome (functioning there) - This would need to start in the ER so it would have a hydrophobic signal. It will have some additional signal to direct it to the lysosome however. Acid hydrolases have M6P tags, so that is one possibility. e. f. ER resident - Needs a hydrophobic sequence, probably cleavable. Soluble ER residents tend to use KDEL signals. Integral plasma membrane protein - This needs a hydrophobic signal to get to the ER and will then either retain this signal or have more hydrophobic regions. There is no other special signal needed to arrive at the PM. Sketch out the default pathway of a protein that has entered the ER. What changes need to be made and/or what other systems must be engaged to send a protein in the ER to non-default locations? This is a practice problem, the act of constructing the pathway is more important that making sure you got every specific detail (although the details are important). Sketch the default pathway for an "unmarked" protein entering the cell through endocytosis. What changes need to be made and/or what other systems must be engaged to send a protein in the ER to non-default locations? This is a practice problem, the act of constructing the pathway is more important that making sure you got every specific detail (although the details are important). As was started in class, design a chart comparing and contrasting the three classes of cytoskeletal filaments (actin microfilaments, microtubules, intermediate filaments). Use what you know to come up with the categories. Not all categories will be applicable to all filaments (but that's important information too!). Some categories to get you started: Soluble subunit, fiber polarity, associated nucleotide, soluble subunit interacting proteins. This is a practice problem, the act of constructing the chart is more important that making sure you got every specific detail (although the details are important). How does myosin compare to kinesin in terms of processivity and speed? How can a single class of motor proteins be different in terms of processivity and speed. Myosins are much faster than kinesins, but kinesins have better processivity than myosins. Myosins vary in their speed through differences in their lever arm, and hence their step size. Aspects like time spent attached to the fiber and rate of nucleotide hydrolysis can also affect speed (less time bound = faster). However, the less time a motor protein spends bound to a fiber can increase the likelihood of dissociation so this has a negative impact on processivity. How does the functional movement of a platelet compare (and contrast) to that of a crawling keratocyte? Both systems are actin-based and as a consequence also utilize myosin motor proteins. They have very similar spreading/protrusion steps because this is based on the polymerization of a sheet of actin filaments. Platelets do so omnidirectionally while keratocytes do so directionally. Keratocytes undergo treadmilling of their entire extruded actin network while platelets do not. Filamen is primarily responsible for the network formed in platelets while the nucleating ability of ARPs forms the tree like mesh at the leading edge of the crawling cell. Also, platelets utilize gelsolin to fragment existing actin filaments and this is not a major behavior seen in a locomoting keratocyte. In both cases myosin proteins are responsible for the contraction of the cell body, although again in a crawling keratocyte this is more directional. We didn't really discuss it much, but the attachment of a keratocyte and a platelet to a substrate is going to be fairly similar as well, involving focal adhesions or similar structures. A spherical keratocyte is placed on a substrate including an attractant to encourage directional movement. If it is also exposed to a compound that inhibits myosin II, what will happen and why? Be specific about changes in cell movement, cell shape, and the cytoskeleton. The cell will attempt to move directionally. It will spread out and form a directed lamellepodium (as opposed to a flat disk). The cell will not continue forward however because myosin II is required for forward movement by contraction at the back end. The actin cytoskeleton will assemble just fine at the leading edge in a flat mesh-like array that a crawling cell normally forms. Dynein and dynein-like motors are used to orient the Golgi in T-cells and in the movement of flagella. Which one of these processes could kinesin (specifically kinesin, not one of the KRPs) substitute for and why? Flagellar movement only requires the motor protein to push against a neighboring immobilized microtubule. Orientation of the Golgi in a T-cell requires directional movement however. The minus end directed dyneins are capable of pulling the MTOC and hence the Golgi into position. Kinesins are plus end directed and would end up pushing the MTOC away from the desired location. So kinesins could substitute for flagellar movement but not Golgi orientation. Most soluble cytoskeletal subunits are unavailable for incorporation into a filament in vivo. Why is this useful to the cell? Are ARPs and -TuRCs more or less important in light of this situation? Explain your answer. The natural concentration of soluble subunits that are free is quite low, most units get incorporated into a fiber. Keeping them unavailable allows the cell to have a lot of subunits around for needed changes without having a lot of polymerized fibers. The nucleation proteins are more important in this situation because the local concentrations of available subunits will be low, causing longer delays in the nucleation process. How is the structural organization of actin in microvillus related its to form? Why would filamin or spectrin by unsuitable for organizing actin in this manner? A microvillus is an elongated protrusion. The actin in them are bundled into long straight structures of even length. Filamin and spectrin are both bad for organizing actin in this way because filamin's two binding sites are angled and spectrin is fairly long. Both of these binding arrangements lead to gel actin associations, not bundled actin fibers. Actin is bound to profilin in blood platelets, yet severing of the actin filaments is not sufficient for inducing actin filament growth. Why? Severing of actin filaments is done by gelsolin. After severing, gelsolin remains bound to the fiber and acts as a cap. Filament caps prevent both growth and shrinkage of the filament. W) Internal hydrophobic signal sequence with multiple hydrophobic sequences Integral membrane protein, would follow default pathway to PM X) Hydrophobic signal sequence with a KDEL sequence ER resident protein Y) Hydrophobic signal sequence only would follow default pathway, outside cell Z) Hydrophobic signal sequence with an M6P signal patch M6P is a lysosomal targeting marker. Label the above figure with where you would expect to find the final location of each protein (use the letters W-Z). Be sure to give explanatory text if your localization is not perfectly clear. What will be the final location of proteins X' and Y'? Explain your answer. The KDEL sequence of protein X will still allow it to be retrieved back to the ER. However, protein Y has no signal sequences to direct it after being absorbed in an endosome so it will follow the default pathway to the lysosome (Y must have been absorbed from outside the cell because the secretory pathway does not require endosomes). What signal sequence(s) would you expect to find on protein V? Why? Protein V is in the nucleus so will require a nuclear localization signal (NLS). NLSs are sufficient for nuclear import so no other sequences are needed. What special steps would need to be taken to break down and recycle protein W in the lysosome? Since protein W is a transmembrane protein, it will need to be formed into a multivesicular body. It specifically needs a single ubiquitin tag and then the endosome will be induced to invaginate around protein W. Which protein(s) (W-Z) could have N-linked glycosylations? Which one(s) would retain their signal sequence? Explain your answers. All of them have the potential to be glycosylated because they all at least pass through the ER. Only protein W would retain its signal sequence because it is internal. All of the rest would be cleaved off because this automatically occurs for imported proteins (single pass proteins have a second internal hydrophobic region that allows them to insert into the membrane so even those have their ER targeting sequence removed too). Golgi apparatus is a major site of carbohydrate synthesis, as well as a sorting and dispatching station for products of the ER. Proteins leave the ER in COPII-coated transport vesicles, which bud from specialized regions of the ER call ER exit sites, whose membrane lacks bound ribosomes. It is thought that these cargo membrane proteins display exit signals on their cytosolic surface that components of the COPII coat recognize. Soluble cargo proteins have exit signals that attach them to transmembrane cargo receptors, which in turn bind through exit signals in their cytoplasmic tails to components of the COPII coat. Proteins that are misfolded or incompletely assembled remain in the ER, where they are bound to chaperone proteins such as BiP or calnexin. The structures formed when ER-derived vesicles fuse with one another are called vesicular tubular clusters. As soon as they form, they begin to bud off transport vesicles of their own, which are COPI-coated and carry back to the ER resident proteins that have escaped. Thus, the clusters continuously mature, gradually changing their composition. KKXX sequence ER retrieval signal on resident ER membrane proteins that binds directly to COPI coats for retrograde delivery to the ER. KDEL sequence ER retrieval signal on resident ER soluble proteins, such as BiP. This sequence then binds to a KDEL receptor, a multipass transmembrane protein that packages protein into COPI-coated vesicles. Receptor must have a high affinity to KDEL sequence in Golgi and low affinity in the ER. Might be related to the different ionic conditions and pH in the compartment membrane. While retrieval signals increase the efficiency of the retrieval process, some proteins randomly enter budding vesicles destined for the ER and are returned to the ER at a slower rate. Mechanism independent of KDEL signal anchors ER resident proteins and that only those proteins that escape this retention mechanism are captured and returned via the KDEL receptor. A suggested retention mechanism is that ER resident proteins bind to one another, thus forming complexes that are too big to enter transport vesicle efficiently. The oligosaccharide intermediates created by the trimming reactions serve to help proteins fold and to help transport misfolded proteins to the cytosol for degradation. If the oligosaccharide is accessible to the processing enzymes in the Golgi, it is likely to be converted to a complex oligosaccharide; if it is inaccessible because its sugars are tightly held to the protein's surface, it is likely to remain in a high-mannose form. Many proteoglycans are secreted and become components of the extracellular matrix, while others remain anchored to the extracellular face of the plasma membrane. Others form a major component of slimy materials, such as mucus. Complex carbohydrates require a different enzyme at each step, each product being recognized as the exclusive substrate for the next enzyme in the series. N-linked glycosylation promotes protein folding by making more folding intermediates more soluble, thereby preventing their aggregation, and the sequential modifications of the N-linked oligosaccharide establish a "glycol-code" that marks the progression of protein folding and mediates the binding of the protein to chaperones and lectins, such as guiding ER-to-Gogi transport. Because chains of sugars have limited flexibility, even a small N-linked oligosaccharide protruding from the surface of a glycoprotein can limit the approach of other macromolecules to the protein surface. This tends to make a glycoprotein more resistant to digestion by proteolytic enzymes. Mucus has freedom to change shape and move compared to rigid cell wall. Vesicular transport model Golgi is a relatively static structure, with its enzymes held in place, while the molecules in transit move through the cisternae in sequence, carried by transport vesicles. Although both forward-moving and retrograde types of vesicles are likely to be COPI-coated, the coats may contain different adaptor proteins that confer selectivity on the packaging of cargo molecules. Or instead there is no direction at all, and directional flow then occurs because of the continual input at the cis cisterna and output at the trans cisterna. Cisternal maturation model views the Golgi as a dynamic structure in which the cisternae themselves move. This network progressively matures to become a cis cisterna, medial cisterna, and so on. Distribution of Golgi enzymes can be explained by: Everything moves continuously forward with the maturing cisterna, including the processing enzymes that belong in the early Golgi. Budding COPI-coated vesicles continually collect the appropriate enzymes, almost all of which are membrane proteins, and carry them back to the earlier cisterna where they function. Golgi matrix proteins help organize the stack Most of the lysosomal membrane proteins are unusually highly glycosylated, which helps to protect them from the lysosomal proteases in the lumen H+ gradient provides a source of energy that drives the transport of small metabolites across the organelle membrane. Lysosomes are heterogeneous the diversity reflects the wide variety of digestive functions that acid hydrolases mediates, including the breakdown of intra- and extracellular debris, the destruction of phagocytosed microorganisms, and the production of nutrients for the cell. It also reflects the way they form: late endosomes contain material received from both the plasma membrane by endocytosis and newly synthesized lysosomal hydrolases. The vacuole is important as a homeostatic device, enabling the plant cells to withstand wide variations in their environment. A route that leads outwards from the ER via the Golgi delivers most digestive enzymes while at least three paths from different sources feed substance into lysososmes for digestion: endocystosis, autophagy, and phagocytosis. Autophagy you have the enclosure of an organelle by a double membrane of unknown origin. 1) nucleation and extension of a delimiting membrane into a crescent-shaped structure that engulfs a portion of the cytoplasm. 2) closure of the autophagosome into a sealed double-membrane compartment. 3) fusion of the new compartment with Lysosomes. 4) digestion of the inner membrane of the autophagosome and its contents. Lysosomal proteins are recognized and selected in the TGN because they carry an M6P group, which is added exclusively to the N-linked oligosaccharides of these soluble lysosomal enzymes as they pass through the lumen of the cis Golgi network. Transmembrane M6P receptor proteins (present in TGN) recognize the groups and bind to adaptor proteins in assembling clathrin coats on the cytosolic side. Contents delivered to early endosomes. As the pH drops during endosomal maturation, the lysosomal hydrolases dissociate from the M6P receptor and eventually begin to digest the material delivered by endocytosis. Acid phosphatase removes the phosphate group from the mannose, thereby destroying the sorting signal and contributing to the release of the lysosomal hydrolases from the M6P receptor. M6P receptors then are returned for reuse in TGN. Transport in either direction requires signals in the cytoplasmic tail of the M6P receptor that direct this protein to the endosome or back to the Golgi. Some hydrolases tagged with M6P escape normal packaging process and are transported "by default" to the cell surface, but some M6P receptors also take a detour to the plasma membrane, where they recaputure the escaped hydrolases and return them by receptor-mediated endocytosis to lysosmes via early and late endosomes. The recognition signal to add M6P group to hydrolase is a cluster of neighboring amino acids known as a signal patch. GlcNac phosphotransferase adds GlcNac-phosphate to one or two of the mannose residues on each oligosaccharide chain. Then another enzyme cleaves off the GlcNac residue, leaving behind a newly created M6P marker. Formin nucleates assembly and remains associated with the growing plus end, nucleates the growth of straight, unbranched actin filaments that can be cross-linked by other proteins to form parallel bundles. Ratchet-like. Remains associated with the rapidly growing plus end, while still allowing the binding of new subunits at that end to elongate the filment. ARP complex or gamma-TuRC remain stably bound to the minus end of the actin filament or microtubule and prevent both subunit addition or loss at this end. ARP complex nucleates assembly of actin to form a web and remains associated wit the minus end. Thymosin binds actin subunits, prevents assembly, actin monomer bound to this are in a locked state, cannot associate with either the plus or minus ends and can neither hydrolyze nor exchange their bound nucleotide. Profilin binds actin subunits, speeds elongation, binds to the face of the actin monomer opposite the ATP-binding cleft, blocking the side of the monomer that would normally associate with the filament minuse end, while leaving exposed the site on the monomer thata\ binds to the plus end. Profilin falls off once actin is attached to polymer. Actin filament growth depends even more strongly on profiling activation for those filaments whose plus end is associated with certain formins due to forming whiskers. Gelsolin severs actin filaments and binds to plus end, when high levels of cytosolic Ca are around. Tropomyosin stabilizes actin filaments Capping protein prevents assembly and disassembly at plus end of actin filaments Coflin binds ADP-actin filaments, accelerates disassembly, destabilizes actin filaments by forcing the filament to twist a little more tightly causing mechanical stress. Actin filaments can be protected by tropomyosin Stathmin binds subunits, prevents assembly of microtubules, prevents micortuble addition onto the ends of microtubules. Binds to two tubulin heterodimers so it decreases the effective concentration of tubulin subunits that are available for polymerization. Kinesin 13 enhances catastrophic disassembly at plus end of microtubule. Katanin severs microtubules using ATP. Accelerates the assembly of new filament structure. Promotes the shrinking of old filaments. One part of it directs it towards the centrosome MAPs - stabilizes tubules by binding along sides, stabilize microtubules against disassembly. MAP2 long projecting domain so widely spaced microtubules. Tau opposite binding so shortly spaced XMAP215 stabilizes plus ends and accelerates assembly of microtubules. They have two domains that link MAP-coated microtubules together. Tropomysin doesn't do this to actin. +TIPs remain associated with growing plus ends of microtubules and can link them to other structures, such as membranes gamma-TuRC nucleast assembly and remains associated with the minus end of microtubules. The binding of a plus end capping protein stabilizes an actin filament at is plus end, which greatly slows the rates of both filament growth and depolymerization by making the plus end inactive. At the minus end, an actin filament may be capped by remaining bound to the ARP complex. Fimbrin stiff, parallel close packing of actin filaments, excludes myosin so not contractile Alpha-actinin stiff, parallel looser packing of actin Filamin promotes loose and highly viscous gel of actin Spectrin long, flexible protein arranged so that the two actin filament binding sites are very far apart. It provides mechanical support by attaching actin to the plasma membrane; think capillaries and squeezing of RBC Plectin bundles intermediate filaments, and also links them to microtubules, actin filament bundles, and filaments of the motor protein myosin II. Filaggrin bundles keratin filaments in differentiating cells of the epidermis to give skin toughness Actin subunits assemble head to tail to generate filaments with a distinct structural polarity. Can be considered to consists of two parallel protofilaments that twist around each other in a right-handed helix. Relatively flexible and easily bend compared to microtubules. Intermediate filaments a pair of parallel dimmers associates in an antiparallel fashin to form a staggered tetramer. Its two ends are the same so no structural polarity. Pack together laterally. Rope like character. Microtubles hollow cylindrical structure built from 13 parallel protofilaments. Multiple contacts among subunits make them stiff and difficult to bend. Alpha at bottom and beta at top. Distinct polarity. GPI-anchored proteins are thought to be directed to the apical membrane because they associate with glycosphigolipids in the lipid rafts that form in the membrane of the TGN. Apical plasma membrane is greatly enriched in glycosphingolipids, which help to protect form digestive enzymes and low pH. Most synaptic vesicles are generated not from the Golgi membrane in the nerve cell body but by local recycling from the plasma membrane in the nerve terminals. You can have primed vesicles waiting to be released.

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