protein transport

8/11/2019 Protein Transport

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Protein transport

Eukaryotic cells posses distinct membrane bound organelles which are absent in prokaryotic cells. The

membrane bound organelles have different functions and these organelles provide discrete compartments inwhich specific cellular activities take place. The complex internal organization of eukaryotic cells generate

hardship for transport of proteins to their destinations.

Many proteins destined for the endoplasmic reticulum, the Golgi apparatus, lysosomes, the plasma

membrane and secretion from the cell are synthesized on ribosomes that are bound to the membrane of

endoplasmic reticulum (Figure-1).

Figure 1: Transport of proteins synthesized on membrane bound ribosomes

In contrast, proteins synthesized on free ribosomes either remain in the cytosol or are transported to the

nucleus, mitochondria, chloroplasts, or peroxisomes (Figure-2). The first step in the biosynthesis of secretoryproteins, plasma membrane proteins, and many other proteins in a eukaryotic cell involves the transport of at

least portions of the polypeptides across the endoplasmic reticulum membrane. Parts of the polypeptide chainsserve as signals that direct the translocation across and the integration into the endoplasmic reticulummembrane and also determine the orientation of membrane proteins. Processing of the proteins takes place in

the endoplasmic reticulum and from the endoplasmic reticulum proteins are transported in vesicles to the Golgi

apparatus, where the proteins are further processed and sorted for transport to lysosomes, the plasma

membrane, or secretion from the cell.


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Figure 2: Transport of proteins synthesized on free ribosomes

The Endoplasmic Reticulum

The endoplasmic reticulum (ER) is an organelle of cells in eukaryotic organisms that forms an interconnectednetwork of tubules, vesicles, and cisternae. It is the largest organelle of most eukaryotic cells. The general

structure of an endoplasmic reticulum is a membranous network of cisternae (sac-like structures) held togetherby the cytoskeleton. The phospholipid membrane encloses a space, the cisternal space (or lumen), which is

continuous with the perinuclear space but separate from the cytosol. Two types of endoplasmic reticulum can

be distinguished i.e. rough endoplasmic reticulum and smooth endoplasmic reticulum (Figure-3).

Figure 3: Smooth and rough endoplasmic reticulum with nucleus


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Rough endoplasmic reticula are involved in the synthesis of proteins and is also a membrane factory for

the cell, while smooth endoplasmic reticula are involved in the synthesis of lipids, including oils,

phospholipids and steroids, metabolism of carbohydrates, regulation of calcium concentration and

detoxification of drugs and poisons.

Protein targeting to the endoplasmic reticulum

In mammalian cells, most proteins are transferred into the endoplasmic reticulum while they are being

translated on membrane-bound ribosomes. In all eukaryotes, there are two pathways by which proteins can betranslocated into the endoplasmic reticulum i.e., a co-translational pathwayand a post-translational

pathway. In the co-translational pathway, transport occurs while the polypeptide chain is being synthesized ona membrane-bound ribosome, whereas in the post-translational pathway, the polypeptide chain is completed in

the cytoplasm before being transported into the endoplasmic reticulum. In prokaryotes, ribosomes don't seem

to be tightly bound to the membrane and most proteins may be transported post-translationally. Bothtranslocation modes require that polypeptides destined for translocation be specifically targeted to the

membrane.

In the co-translational pathway, first ribosomes are associated with the endoplasmic reticulum. Ribosomes

are targeted for binding to the endoplasmic reticulum by the amino acid sequence of the polypeptide chainbeing synthesized rather than by the intrinsic properties of the ribosome itself. Proteins which are destined for

secretion are targeted to the endoplasmic reticulum by a signal sequence at the amino terminus of the growing

polypeptide chain. These signal sequences are short stretches of hydrophobic amino acids which are usually

cleaved from the polypeptide chain during its transfer into the lumen of the endoplasmic reticulum.

Mechanism: The signal sequences cover about twenty amino acids, including a stretch of hydrophobic

residues, usually at the amino terminus of the polypeptide chain. As soon as the signal sequences of the

growing polypeptide chain emerge from the ribosome, they are recognised and bound by a signal recognitionparticle (SRP)consisting of six polypeptides and a small cytoplasmic RNA (srp RNA). Then the complex

containing the growing polypeptide chain, ribosome, and SRP is specifically targeted to the endoplasmicreticulum membrane by an interaction with a membrane-bounrd receptor, the SRP receptor or docking protein

(Figure-4). In the next step, the SRP is released from both the ribosome and the signal sequence, where GTP

(guanosine triphosphate) plays a key role. The ribosome then binds to a protein translocation complex in themembrane of the endoplasmic reticulum, and the signal sequence is inserted into a membrane channel or

translocon. The translocons are complexes of three transmembrane proteins, known as Sec61 proteins.

Transfer of the ribosome from the SRP to the translocon allows translation to resume, and the growingpolypeptide chain is transferred directly into the translocon channel and across the membrane of the

endoplasmic reticulum as translation proceeds. As translocation proceeds, the signal sequence is cleaved by the

signal peptidase and the polypeptide is released into the lumen of the endoplasmic reticulum. Finally, GTP

hydrolysis leads to the dissociation of the SRP from its receptor, and a new targeting cycle can begin. The

actual transfer of the polypeptide through the membrane does not require the SRP or its receptor and

commences only after their disengagement. Two basic functions are done by the SRP, where first it targets the

polypeptide chain to the Endoplasmic reticulum membrane by interacting both with the signal sequence and

with the translocation apparatus and secondly it keeps the bound signal sequence segregated from the rest of

the polypeptide chain and thereby prevents aberrant, premature folding.


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Figure 4:The cotranslational pathway of transport of secretory proteins to the endoplasmic reticulum

Some proteins in mammals and many proteins in yeast are transported through post-translational pathway.

These proteins are synthesized on free cytosolic ribosomes and these proteins do not require a signal

recognition particle (SRP) for their transport. Their signal sequences are recognised by distinct receptor

proteins associated with the translocon in the endoplasmic reticulum membrane. The polypeptide chains are

remained in an unfolded conformation by the cytosolic Hsp70 chaperones.

Proteins inserted into the membrane of endoplasmic

reticulum

The proteins which are to be incorporated into the plasma membrane or the membrane of these compartmentsare initially inserted into the membrane of endoplasmic reticulum instead of into its lumen. The proteins from

the membrane of endoplasmic reticulum travel the same pathway as secretory proteins. But along this pathway

these proteins are transported as membrane components.

Integral membrane proteins are embedded in the membrane by the hydrophobic regions that span the

phospholipid bilayer. The membrane spanning portions of the integral membrane proteins are usually -helical

regions consisting of 20-25 hydrophobic amino acids. Hydrogen bonding between the peptide bonds gets


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maximized by the formation of an -helix and the hydrophobic amino acid side chains interact with the fatty

acid tails of the phospholipids in the bilayer. But there are different arrangement of different membrane

proteins such as some membrane proteins span the membrane only once whereas some span the membrane

more than once. Some proteins are oriented in the membrane with their amino terminus facing towards the

cytosol whereas some proteins are oriented with their carboxy terminus facing towards the cytosol. These

orientations of proteins inserted into the endoplasmic reticulum, Golgi bodies, lysosomes and plasma

membranes are established as the growing polypeptide chains are translocated into the endoplasmic reticulum.

After insertion into the endoplasmic reticulum membrane the transmembrane proteins are oriented with

their carboxy termini exposed to the cytosol. Before insertion of these transmembrane proteins into the

endoplasmic reticulum membrane, the normal amino-terminal signal sequence of these proteins, is cleaved bysignal peptidase during translocation of the polypeptide chain across the endoplasmic reticulum membrane

through the translocon. A second membrane-spanning -helix brings the polypeptide chain in the membrane.This transmembrane sequence is called a stop-transfer sequence which signals closure of the translocon

channel. The carboxy-terminal portion of the growing polypeptide chain is synthesized in the cytosol as further

translocation of the polypeptide chain across the membrane of the endoplasmic reticulum is blocked.

Internal signal sequences that are not cleaved by signal peptidase also help in anchoring of proteins in the

endoplasmic reticulum membrane. These signal sequences act as transmembrane helices that exit thetranslocon and anchor proteins in the membrane of the endoplasmic reticulum. Proteins inserted into themembrane by the help of the internal signal sequences can have either their carboxy or amino terminus

exposed to the cytosol, because it depends upon the orientation of the signal sequences. Proteins that span themembrane multiple times are thought to be inserted asa result of an alternating series of internal signal

sequences and transmembrane stop-transfer sequences.

Proteins are translocated across the endoplasmic reticulum membrane as unfolded polypeptide chains

during the progression of translation. These polypeptides fold into their three-dimensional conformation within

the endoplasmic reticulum by the help of molecular chaperones. Those proteins that cannot be correctly folded

are diverted from the secretory pathway and marked for degradation.

Transport of proteins from endoplasmic reticulum

Transport of proteins from endoplasmic reticulum to Golgi bodies occur through vesicles along the secretory

pathway. Vesicles are very small, membrane-enclosed sacs that carry cargo within the cell. Vesicles typicallytransport large molecules that can not pass through a membrane on their own or by using other transport

molecules. Vesicles gather their cargo in a process called budding as they move with their cargo through the

cell and then deliver their cargo by fusing with another membrane enclosed compartment or with the cell'splasma membrane. Vesicles bud-off from the endoplasmic reticulum and in the process capture the molecules

within the lumen of the endoplasmic reticulum. Proteins from the lumen of one organelle carried as budding

transport vesicle, released into the lumen of the recipient organelle following vesicle fusion. Vesicles that bud

from the transitional endoplasmic reticulum , carry their cargo first to the endoplasmic reticulum-Golgiintermediate compartment and then to the Golgi apparatus (Figure-5). From Golgi apparatus, vesiculartransport occurs to lysosomes or the plasma membrane. The membrane proteins also undergo the same

pathway and their topological orientation is maintained as they travel from one membrane-bound organelle to

another.


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Figure 5: Vesicular transport from the endoplasmic reticulum to the Golgi apparatus

In the transport pathway not all the proteins are transported but some proteins are retained within the

endoplasmic reticulum. Many proteins are retained in the endoplasmic reticulum lumen asa result of the

presence of the targeting sequence Lys-Asp-Glu-Leu (KDEL sequence) at their carboxy terminus. Theretention of some transmembane proteins in the endoplasmic reticulum is similarly detected by short C-

terminal sequences that contain two lysine residues (KKXX sequences). These signals cause residentendoplasmic reticulum proteins to be selectively retrieved from the endoplasmic reticulum-Golgi intermediatecompartment or the Golgi complex and returned to the endoplasmic reticulum through a recycling pathway

(Figure-6). Proteins bearing these sequences bind to specific recycling receptors in the membranes of thesecompartments and then selectively transported back to the endoplasmic reticulum.

The Golgi apparatus

The Golgi apparatus, also known as the Golgi complex or Golgi body, is an organelle found in most eukaryotic

cells. Part of the cellular endomembrane system, the Golgi apparatus packages proteins inside the cell before

they are sent to their destination; it is particularly important in the processing of proteins for secretion. TheGolgi apparatus is integral in modifying, sorting, and packaging these macromolecules for cell secretion

(exocytosis) or use within the cell. It primarily modifies proteins delivered from the rough endoplasmic

reticulum but is also involved in the transport of lipids around the cell, and the creation of lysosomes. The

Golgi plays an important role in the synthesis of proteoglycans, which are molecules present in the

extracellular matrix of animals. It is also a major site of carbohydrate synthesis. In plant cells, the Golgi

apparatus serves as the site at which the complex polysaccharides of the cell wall are synthesized.

Structure:The Golgi is composed of stacks of membrane-bound structures known as cisternae and associated


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vesicles. Each cisterna comprises a flat, membrane enclosed disc that includes special Golgi enzymes which

modify or help to modify cargo proteins that travel through it. The cisternae stack has four functional regions:

the cis-Golgi network, medial-Golgi, endo-Golgi, and trans-Golgi network (Figure-6).

Figure 6: Structure of the Golgi apparatus showing vesicular transport

Vesicles from the endoplasmic reticulum are transported to the endoplasmic reticulum-Golgi intermediate

compartment and then enter the Golgi apparatus at thecis Golgi network. From the cisGolgi network, vesiclesprogress to the medial and trans compartments of the Golgi stack, where most metabolic activities of the

Golgi apparatus take place. The modified proteins, lipids and polysaccharides then move to the transGolgi

network, which acts as a sorting and distribution center directing to endosomes, lysosomes, the plasmamembrane and the cell exterior (Figure-6). Each region in the cisternae stack contains different enzymes which

selectively modify the contents depending on where they reside. The cisternae also carry structural proteinsimportant for their maintenance as flattened membranes which stack upon each other.


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Protein processing within the Golgi

Processing of proteins begins with interaction of the newly made peptide with chaparone proteins in the lumen

of the endoplasmic reticulum. Chaparone proteins are released as the protein assumes its proper configuration.For many secreted proteins (including cell surface proteins), the three dimensional structure of the protein is

supported by disulfide bridges that are formed between cysteine residues. Most proteins are modified in theendoplasmic reticulum by addition of polysaccharides and the process is known as glycosylation. There aretwo types of glycosylation i.e. N-linked glycosylationand O-linked glycosylation. The enzymes involved in

the glycosylation process are reside in the lumen of the endoplasmic reticulum. Initially in glycosylation, a pre-fabricated oligosaccharide unit consisting of N-acetyl-glucosamine, mannose and glucose residues is

transferred from a glycolipid. The glycolipid consists of the oligosaccharide chain attached to dolichol

phosphate (a phospholipid with an extremely long hydrophobic chain), which serves as a membrane anchor forthe oligosaccharide chain. The oligosaccharide chain is attached to an asparagine residue through its free

amino group. The oligosaccharides are assembled on the cytosol side of the membrane and then flipped to the

lumen (cisterna) side of the membrane. The N-linked oligosachharides are generally undergo modification inthe Golgi apparatus. Subsequent to the addition of the 14 sugar oligosaccharide chain to the protein, there are a

long series of modifications of the polysaccharide chain, where Some of these modifiacations take place in theER, others in the cisGolgi network, other in the cis Golgi, others in medial Golgi and yet others in the trans

Golgi and trans Golgi network. Each of these steps is carried by a different enzyme that is localized in aparticular region of the Golgi. The processing of N-linked oligosaccharides is specific for the proteins destinedfor specific regions, such as for plasma membrane or for secretion, and lysosomes.

N-linked oligosaccharides are processed in an ordered sequence of reactions within the Golgi apparatus.

The proteins which are destined for secretion or for the plasma membrane, are first modified by the removal ofthree additional mannose residues which is followed by the sequential addition of an N-acetylglucosamine, the

removal of two more mannoses, and the addition of a fructose with two more N-acetylglucosamines. Finallythere is an addition of two galactoses and three sialic acid residues is made to it.

The processing of N-linked oligosaccharides of lysosomal proteins differs from that of plasma membrane

and secreted proteins. The lysosomal proteins are modified by mannose phosphorylation. First, there is

addition of N-acetylglucosamine phosphates to specific mannose residues, and this happens probably while theprotein is still in the cis Golgi network. After this N-acetylglucosamine group is removed, leaving mannose-6-

phosphate residues on the N-linked oligosaccharide. The phosphorylated mannose residues are specifically

recognised by a mannose-6-phosphate receptor in the transGolgi network, which directs the transport of these

proteins to lysosomes.

Proteins can be modified by O-linked glycosylation, which involves the addition of carbohydrates to the

side chains of acceptor serine and thereonine residues within specific sequences of amino acids. Thesemodifications take place in the Golgi apparatus by the sequential addition of single sugar residues.

Transport of proteins from the Golgi apparatus

Proteins, lipids and polysaccharides are transported from the Golgi apparatus to their final destinations through

the secretory pathway. Proteins are sorted into different kinds of transport vesicles, which bud from the trans

Golgi network and deliver their contents to the appropriate cellular locations. Proteins that function within theGolgi apparatus must be retained within that organelle, rather than being transported along the secretory

pathway. The proteins retained within the Golgi complex are associated with the Golgi membrane. The signals

responsible for retention of some proteins within the Golgi apparatus have been localized to their


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transmembrane domains, which prevent the protein from being packaged in the transport vesicles that leave the

transGolgi network. Some proteins are carried from the Golgi apparatus to the plasma membrane by a

constitutive secretory pathway and some proteins are transported to the cell surface by a distinct pathway of

regulated secretion or are specifically targeted to other intracellular destinations, such as lysosomes in animal

cells or vacuoles in yeast.

Proteins are sorted into the regulated secretory pathway in thetransGolgi network, where they arepackaged into specialized secretory vesicles. These immature secretory vesicles are larger than the transport

vesicles, often fuses with each other while further processing their protein contents. The sorting of proteins

into the regulated secretory pathway appears to involve the recognition of signal patches shared by multiple

proteins that enter this pathway.

In the process of selective transport of proteins to lysosomes, the lumenal lysosomal proteins are marked

by mannose-6-phosphates that are formed by modification of their N-linked oligosaccharides shortly after

entry into the Golgi apparatus. A specific receptor in the membrane of the transGolgi network then recognizesthese mannose-6-phosphate residues. The resulting complexes of receptor with the lysosomal enzymes are

packaged into transport vesicles destined for lysosomes.

The cisternal maturation model proposed for transport of proteins through the Golgi apparatus deals withthe fact that the cisternae of the Golgi apparatus move by being built at the cis face and destroyed at the transface. Vesicles from the endoplasmic reticulum fuse with each other to form a cisterna at the cisface,

consequently this cisterna would appear to move through the Golgi stack when a new cisterna is formed at the

cisface. This model is supported by the fact that structures larger than the transport vesicles, such as collagenrods, were observed microscopically to progress through the Golgi apparatus.

The vesicular transport model views the Golgi as a very stable organelle, divided into compartments in the

cisto transdirection. Membrane bound carriers transport material between the endoplasmic reticulum and the

different compartments of the Golgi. Experimental evidence includes the abundance of small vesicles alsoknown as shuttle vesicles in proximity to the Golgi apparatus. To direct the vesicles, actin filaments connect

packaging proteins to the membrane to ensure that they fuse with the correct compartment.

The cisternal maturation model and the vesicular transport model may actually work in conjuction witheach other and sometimes referred to as the combined model.

Vesicular transport

Transport vesicles play a central role in the traffic of molecules between different membrane-enclosed

compartments of the secondary pathway. The functional organization of the cell is maintained by theselectivity of the vesicular transport.

The vesicles that leave the rough endoplasmic reticulum are transported to the cis face of the Golgiapparatus, where they fuse with the Golgi membrane and empty their contents into the lumen. Once inside thelumen, the molecules are modified, then sorted for transport to their next destinations. The Golgi apparatus

tends to be larger and more numerous in cells that synthesise and secrete large amounts of substances; for

example, the plasma B cells and the antibody-secreting cells of the immune system have prominent Golgi

complexes. Those proteins destined for areas of the cell other than either the endoplasmic reticulum or Golgi

apparatus are moved towards the transface, to a complex network of membranes and associated vesiclesknown as the trans-Golgi network (TGN). This area of the Golgi is the point at which proteins are sorted and


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shipped to their intended destinations by their placement into one of at least three different types of vesicles,

depending upon the molecular marker they carry.

There are three different types of vesicles: exocytic vesicles(continous vesicles), secretory vesicles(

regulated vesicles) and lysosomal vesicles which take part in different types of secretion. Exocytic vesiclescontain proteins destined for extracellular release. The vesicles bud off after packaging and release their

contents into the extracellular space in a process known as constitutive secretion. Antibody release byactivated plasma B cells is an example of constitutive secretion. Secretory vesicles after packaging bud off

and are stored in the cell until a signal is given for their release. They move towards the membrane after

receiving the appropriate signal and fuse with the membrane to release their contents in a process also known

as regulated secretion. Release of neurotransmitters from neurons is an example of regulated secretion.

Lysosomal vesiclescontain proteins destined for the lysosome, an organelle of degradation containing many

acid hydrolases, or to lysosome-like storage organelles. These proteins include both digestive enzymes and

membrane proteins. The vesicle first fuses with the late endosome, and the contents are then transferred to the

lysosome through an unknown mechanism.

Reconstituted vesicular transport

An understanding of the biochemical mechanisms that control the vesicular transport of proteins betweencompartments of the secretory pathway of eukaryotic cells will require successful reconstitution of individual

segments of the pathway using novel cell-free systems. It is now possible to measure, in vitro, the transport of

proteins from the endoplasmic reticulum to the Golgi, between Golgi cisternae, and the formation of transportvesicles en route from the trans Golgi network to the cell surface. The first cell-free transport system was

developed by James Rothman and collegues, who analyzed protein transport between compartments of theGolgi apparatus. The experiment exploited a mutant mammalian cell line that lacked the enzyme required to

transfer N-acetylglucosamine residues to the N-linked oligosaccharide at an early stage of its modification in

the Golgi apparatus, as a result of which the glycoproteins produced by this mutant cell line lacked added N-acetylglucosamine units. But when the Golgi stacks isolated from the mutant cell line were incubated with

stacks isolated from normal cells, N-acetylglucosamine residues were added to glycoproteins synthesized bythe mutant cells. The addition of N-acetylglucosamine provided a readily detectable marker for vesicular

transport in the reconstituted system. Similar reconstituted systems have been developed to analyze transport

between other compartments, including transport from endoplasmic reticulum to the Golgi, and transport fromthe Golgi to secretory vesicles, vacuoles, and the plasma membrane.

The process of vesicle budding

There are different processes in the secretory pathway where transmembrane proteins and lumenal proteins and

their receptors are packaged into vesicles (Figure-7). The cargo proteins are first sorted from proteins targeted

for other destinations and the proteins that need to stay behind. A cargo- containing bud must develop on themembrane and pinch off to form a transport vesicle and the transport vesicle must move to the target

membrane and fuse with it (Figure-7). The formation of most transport vesicles is regulated by small GTP-

binding proteins via adaptor proteins that directly interact with a vesicle coat protein. The sequential and

cooperative binding of GTP-binding proteins and adaptor proteins establishes a platform on the membrane for

a specific process, as a result of which assembly and budding of a transport vesicle directed from the transGolgi network to endosomes and lysosomes.


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Figure 7: The process of vesicle budding and fusion

Most transport vesicles are coated with cytosolic proteins and termes as coated vesicles. The clatharin

coated vesiclesare responsible for the uptake of extracellular molecules from the plasma membrane by

endocytosis as well as the transport of molecules from the trans Golgi network to endosomes and lysosomes.The formation of clatharin-coated vesicles on thetrans Golgi network requires clatharin, two types of adaptor

proteins, and a GTP-binding protein. Two other types of vesicles are also known as non clatharin-coated

vesicles (coat protein-coated vesicles) have been identified as budding from the endoplasmic reticulum andGolgi complex. One class of these vesicles (COPII-coated vesicles) buds from the endoplasmic reticulum and

carries its cargo forward along the secretory pathway to the Golgi apparatus. Another class of these vesicles(COPI-coated vesicles) bud from the endoplasmic reticulum-Golgi intermediate compartment and function in

the retrieval pathway that retain resident proteins in the Golgi and endoplasmic reticulum.

Vesicle fusion

For fusion of a transport vesicle with its target, the transport vesicle must recognise the correct target

membrane and the vesicle and target membranes must fuse and deliver the contents of the vesicle to the target

organelle (Figure-7). A hypothesis called SNARE hypothesiswas proposed by James Rothman and

colleagues, in which vesicle fusion is mediated by interactions between specific pairs of transmembrane

proteins, called SNAREs, on the vesicle(v-SNAREs) and target membranes (t-SNAREs). According to this

hypothesis, the formation of complexes between v-SNAREs on the vesicle and t-SNAREs on the target

membranes lead to membrane fusion.


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According to recent research, SNAREs are required for vesicle fusion with a target membrane and that

SNARE-SNARE pairing provides the energy to bring the two bilayers sufficiently close to destabilize them

and result in fusion.

Members of the Rab family of small GTP-binding proteins help in the docking of transport vesicles. Theyfunction in several steps of vesicle trafficking, including interacting with SNAREs to regulate and facilitate the

formation of SNARE/SNARE complexes. To initiate transport vesicle fusion, Rab/GTP on the transportvesicle recruits effector proteins and v-SNAREs to assemble a pre-fusion complex. A different Rab protein on

the target membrane similarly organizes other effector proteins and t-SNAREs. When the transport vesicle

encounters this target membrane, the effector proteins link the membranes by protein-protein interactions. This

tethering of the vesicle to the target membrane allows the v-SNAREs to contact the t-SNAREs. All SNARE

proteins have a long central coiled-coil domain and this domain binds strongly to other coiled coil domains and

so zips the SNAREs together, bringing the two membranes into nearly direct contact. This creates instability in

the lipid bilayers and as a result causing the fusion of the vesicles and the target membranes.

Endocytosis

Endocytosis is a process by which a cell engulfs some of its extracellular fluid including material dissolved or

suspended in it (Figure-8). Most substances that are important for cell are large polar molecules and so theycan not pass through the plasma membrane, because of which cells engulf them by endocytosis. In this

process the plasma membrane extends outward and surrounds the food particle. Endocytosis pathway can be

divided into major three categories:

Phagocytosis:If the material the cell takes in is particulate, such as a bacterium or a fragment of organicmatter, the process is called phagocytosis.

Pinocytosis:If the material the cell takes in is liquid, the process is called pinocytosis, otherwise known as cell

drinking process.

Receptor-mediated endocytosis:Specific molecules such as low-density lipoproteins are often transported

into eukaryotic cells through receptor mediated endocytosis. Molecules to be transported first bind to specific

receptors on the plasma membrane. The interior portion of the receptor protein is embedded in the membrane.The protein clathrin coats the inside of the membrane in the area of the pit. When an appropriate collection of

molecules gather in the coated pit, the pit deepens and seals off to form a coated vesicle, which carries the

molecules into the cell.

Figure 8: The process of endocytosis


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Components of an endocytic pathway

The endocytic pathway of mammalian cells consists of distinct membrane compartments that internalize

molecules from the plasma membrane and recycle them back to the surface or sort them to degradation.

Early endosomes:Early endosomes are often located in the periphery of the cell and receive most of typesof vesicles coming from the cell surface. They have a characteristic tubulo-vesicular morphology and a mildly

acid pH. They are principally sorting organelles where many ligands dissociate from their receptors in the acid

pH of the lumen and from which many of the receptors recycle to the cell surface.

Late endosomes:Late endosomes often contain many membrane vesicles or membrane lamellae andproteins characteristic of lysosomes, including lysosomal membrane glycoproteins and acid hydrolases. Lateendosomes are thought to mediate a final set of sorting events prior to delivery of material to lysosomes. Late

endosomes receive internalized material en route to lysosomes, usually from early endosomes in the endocytic

pathway, from trans-Golgi network (TGN) in the biosynthetic pathway, and from phagosomes in the

phagocytic pathway.

Lysosomes:Lysosomes are the last compartment of the endocytic pathway. These are membrane enclosedorganelles that contain an array of enzymes capable of breaking down all types of biological polymers i.e.,

proteins, nucleic acids, carbohydrates, and lipids. To do this task, These are then returned to the cytoplasm asnew cell-building materials. To accomplish the tasks, the lysosomes use different types of hydrolytic enzymes,

all of which are manufactured in the endoplasmic reticulum and modified in the Golgi apparatus. Lysosomes

function as the digestive system of the cell, serving both to degrade material taken up from outside the cell andto digest obsolete components of the cell itself.

The formation of endosomes and lysosomes thus represents an interaction between the secretory pathway,

through which lysosomal proteins are processed, and the endocytic pathway through which extracellular

molecules are taken up at the cell surface. Material from outside the cell is taken up by clathrin-coatedendocytic vesicles, which bud from the plasma membrane and then fuse with early endosomes. Membrane

components are then recycled to the plasma membrane and the early endosomes gradually mature into late

endosomes, which are the precursors to lysosomes. Acid hydrolases are targeted to lysosomes by mannose-6-

phosphate residues, which are recognised by mannose-6-phosphate receptors in the trans Golgi network and

packaged into clathrin-coated vesicles. After the removal of the clathrin coat, these transport vesicles fuse withlate endosomes, and the acidic internal pH causes the hydrolases to dissociate from the mannose-6-phosphate

receptor. Then the hydrolases are released into the lumen of the endosome, while the receptors remain in the

membrane and are recycled to the Golgi. Then late endosomes are matured into lysosomes, which digest themolecules taken up by endocytosis.

protein transport

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