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You are watching: Phagocytosis, pinocytosis, and receptor-mediated endocytosis all involve

Cooper GM. The Cell: A molecular Approach. Second edition. Sunderland (MA): Sinauer Associates; 2000.

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The carrier and also channel proteins disputed in the coming before section transport little molecules with the phospholipid bilayer. Eukaryotic cells are likewise able to take it up macromolecules and particles native the surrounding medium by a distinct process called endocytosis. In endocytosis, the material to be internalized is surrounded by an area that plasma membrane, i beg your pardon then sprout off within the cabinet to kind a vesicle containing the ingested material. The term “endocytosis” was coined through Christian deDuve in 1963 to include both the gulp down of large particles (such as bacteria) and the uptake of fluids or macromolecules in little vesicles. The former of these activities is recognized as phagocytosis (cell eating) and the latter as pinocytosis (cell drinking).


During phagocytosis, cell engulf large particles such together bacteria, cabinet debris, or even intact cells (Figure 12.34). Binding of the bit to receptors on the surface ar of the phagocytic cabinet triggers the expansion of pseudopodia—an actin-based motion of the cabinet surface, discussed in chapter 11. The pseudopodia eventually surround the particle and their membranes fuse to form a huge intracellular vesicle (>0.25 μm in diameter) dubbed a phagosome. The phagosomes climate fuse through lysosomes, developing phagolysosomes in which the ingested material is spend by the action of lysosomal mountain hydrolases (see thing 9). Throughout maturation that the phagolysosome, some of the internalized membrane proteins space recycled to the plasma membrane, as disputed in the following section for receptor-mediated endocytosis.


Figure 12.34

Phagocytosis. Binding of a bacterium to the cell surface ar stimulates the expansion of a pseudopodium, which ultimately engulfs the bacterium. Combination of the pseudopodium membrane then outcomes in formation of a big intracellular vesicle (a phagosome). (more...)

The gulp down of huge particles by phagocytosis plays distinctive roles in different kinds of cell (Figure 12.35). Numerous amoebas use phagocytosis to capture food particles, such as bacteria or other protozoans. In multicell animals, the major roles that phagocytosis space to carry out a defense against invading microorganisms and also to eliminate aged or damaged cells from the body. In mammals, phagocytosis is the duty of primarily two varieties of white blood cells, macrophages and also neutrophils, i m sorry are typically referred to as “professional phagocytes.” Both macrophages and neutrophils play an important roles in the body"s defense systems by eliminating microorganisms from infected tissues. In addition, macrophages remove aged or dead cell from organization throughout the body. A striking example of the scope of this activity is provided by the macrophages the the person spleen and also liver, which room responsible for the disposal of more than 1011 age blood cell on a everyday basis.


Figure 12.35

Examples the phagocytic cells. (A) one amoeba engulfing one more protist. (B) Macrophages eat red blood cells. False color has been included to the micrograph. (A, R. N. Band and H. S. Pankratz/Biological picture Service; B, courtesy that Joel Swanson.)

Receptor-Mediated Endocytosis

In comparison to phagocytosis, which plays only devoted roles, pinocytosis is common amongst eukaryotic cells. The best-characterized type of this procedure is receptor-mediated endocytosis, which gives a mechanism for the selective uptake of certain macromolecules (Figure 12.36). The macromolecules to it is in internalized very first bind to particular cell surface receptors. These receptors are concentrated in committed regions that the plasma membrane, referred to as clathrin-coated pits. This pits bud native the membrane to type small clathrin-coated vesicles containing the receptors and their tied macromolecules (ligands). The clathrin-coated vesicles then fuse with early on endosomes, in i m sorry their materials are sorted for carry to lyosome or recycling come the plasma membrane.


Figure 12.36

Clathrin-coated vesicle formation. (A) Extracellular macromolecules (ligands) tie to cell surface receptors that are focused in clathrin-coated pits. This pits bud indigenous the plasma membrane to type intracellular clathrin-coated vesicles. (B) Electron (more...)

The uptake of cholesterol by mammalian cell has noted a vital model for expertise receptor-mediated endocytosis in ~ the molecule level. Cholesterol is transported through the bloodstream in the kind of lipoprotein particles, the most typical of i m sorry is referred to as low-density lipoprotein, or LDL (Figure 12.37). Research studies in the laboratories the Michael Brown and also Joseph Goldstein demonstrated the the uptake of LDL through mammalian cells needs the binding of LDL to a details cell surface ar receptor that is focused in clathrin-coated pits and also internalized by endocytosis. As debated in the following section, the receptor is then recycled to the plasma membrane while LDL is transported to lysosomes, where cholesterol is released for usage by the cell.


Figure 12.37

Structure the LDL. Each fragment of LDL contains about 1500 molecule of cholesteryl esters in one oily core. The core is surrounded by a coat include 500 molecule of cholesterol, 800 molecule of phospholipid, and one molecule the apoprotein (more...)

The an essential insights into this procedure came from researches of patients through the inherited disease known together familial hypercholesterolemia. Patients through this condition have very high level of serum cholesterol and suffer heart attacks early in life. Brown and Goldstein discovered that cell of these patients space unable come internalize LDL native extracellular fluids, bring about the buildup of high levels of cholesterol in the circulation. Further experiments demonstrated that cells of normal people possess a receptor for LDL, i beg your pardon is focused in coated pits, and that familial hypercholesterolemia outcomes from inherited mutations in the LDL receptor. These mutations room of 2 types. Cells from most patients through familial hypercholesterolemia simply failed to tie LDL, demonstrating the a details cell surface ar receptor was forced for LDL uptake. In addition, a couple of patients were established whose cells bound LDL but were can not to internalize it. The LDL receptor of this patients failed to concentrate in coated pits, providing straight evidence because that the main role that coated pits in receptor-mediated endocytosis.

The mutations that prevent the LDL receptor native concentrating in coated pits lie within the cytoplasmic tail of the receptor and also can it is in as subtle as the readjust of a solitary tyrosine to cysteine (Figure 12.38). Further studies have identified the internalization signal that the LDL receptor together a succession of 6 amino acids, including the necessary tyrosine. Similar internalization signals, frequently including tyrosine residues, are discovered in the cytoplasmic tails of other receptors taken increase via clathrin-coated pits. This internalization signals bind to adaptor proteins, which subsequently bind clathrin on the cytosolic next of the membrane (Figure 12.39), similar to the means in which clathrin-coated vesicles kind during the deliver of lysosomal hydrolases indigenous the trans Golgi network (see number 9.31). Clathrin assembles right into a basketlike structure that distorts the membrane, creating invaginated pits. A GTP-binding protein, dubbed dynamin, assembles into rings roughly the necks of these invaginated pits, eventually leading to the relax of coated vesicles inside the cell.

Figure 12.38

The LDL receptor. The LDL receptor contains 700 extracellular amino acids, a transmembrane α helix of 22 amino acids, and also a cytoplasmic tail the 50 amino acids. The N-terminal 292 amino acids constitute the LDL-binding domain. 6 amino mountain within (more...)

Figure 12.39

Formation of clathrin-coated pits. (A) Adaptor proteins tie both to clathrin and to the internalization signals current in the cytoplasmic tails of receptors. (B) Electron micrograph the a clathrin-coated pit reflecting the basketlike framework of the clathrin (more...)

Receptor-mediated endocytosis is a significant activity the the plasma membrane of eukaryotic bio cells. Much more than 20 various receptors have been presented to be selectively internalized by this pathway. Extracellular fluids are additionally incorporated into coated vesicles as they bud native the plasma membrane, so receptor-mediated endocytosis outcomes in the nonselective absorb of extracellular fluids and their materials (fluid phase endocytosis), in enhancement to the internalization of certain macromolecules. Coated pits commonly occupy 1 to 2% that the surface ar area the the plasma membrane and also are approximated to have a lifetime of 1 to 2 minutes. From these figures, one can calculate the receptor-mediated endocytosis results in the internalization of one area of cell surface equivalent to the entire plasma membrane about every 2 hours.

A range of studies indicate that cells likewise possess clathrin-independent endocytosis pathways. Because that example, fluids and also some membrane-bound molecules continue to it is in endocytosed under experimental problems that inhibit endocytosis from clathrin-coated pits. One pathway that clathrin-independent endocytosis requires the absorb of molecule in small invaginations that the plasma membrane (50 come 80 nm in diameter) called caveolae (Figure 12.40). Caveolae are enriched in lipid rafts of cholesterol and sphingolipids and also possess a distinct coat formed by a protein referred to as caveolin. They have been implicated in cabinet signaling and a range of move processes, consisting of endocytosis. In addition, large vesicles (0.15 come 5.0 μm in diameter) have the right to mediate the uptake of fluids in a process known as macropinocytosis. Thus, while clathrin-dependent endocytosis plainly provides a significant pathway because that the absorb of both fluids and specific macromolecules, cells also use several feasible clathrin-independent mechanisms.

Figure 12.40

Caveolae. Electron micrographs of caveolae. (A, courtesy of R. G. W. Anderson/University the Texas Southwestern clinical School-Dallas; B, courtesy of john E. Heuser, Washington University school of Medicine.)

Protein trafficking in Endocytosis

Following their internalization, clathrin-coated motor rapidly melted their coats and also fuse with at an early stage endosomes, which room vesicles through tubular expansions located in ~ the perimeter of the cell. The specificity of combination of endocytic vesicles through endosomes is established by interactions between complementary bag of transmembrane protein of the vesicle and also target membranes (v-SNAREs and t-SNAREs) and by Rab GTP-binding proteins, as questioned in chapter 9. The beforehand endosomes serve as a sorting compartment, indigenous which molecule taken up by endocytosis are either recycle to the plasma membrane or transported come lysosomes because that degradation. In addition, the at an early stage endosomes of polarized cells deserve to transfer endocytosed proteins between different domain names of the plasma membrane—for example, in between the apical and also basolateral domains of epithelial cells.

An crucial feature of early endosomes is that they keep an acidic interior pH (about 6.0 come 6.2) together the result of the action of a membrane H+ pump. This acidic pH leader to the dissociation of countless ligands from their receptors in ~ the at an early stage endosome compartment. Adhering to this uncoupling, the receptors and their ligands can be transported to different intracellular destinations. A classic example is detailed by LDL, i m sorry dissociates from its receptor within early on endosomes (Figure 12.41). The receptor is then went back to the plasma membrane via deliver vesicles the bud native the tubular extensions of endosomes. In contrast, LDL is transported (along with various other soluble components of the endosome) come lysosomes, where its deterioration releases cholesterol.

Figure 12.41

Sorting in beforehand endosomes. LDL bound to its receptor is internalized in clathrin-coated vesicles, which burned their coats and fuse with at an early stage endosomes. In ~ the acidic pH of early on endosomes, LDL dissociates native its receptor and also the endocytosed materials (more...)

Recycling to the plasma membrane is the major fate the membrane proteins taken increase by receptor-mediated endocytosis, with numerous receptors (like the LDL receptor) being went back to the plasma membrane complying with dissociation of your bound ligands in beforehand endosomes. The recycling of this receptors outcomes in the continuous internalization of your ligands. Every LDL receptor, for example, renders a round-trip indigenous the plasma membrane to endosomes and back approximately every 10 minutes. The prominence of the recycling pathway is more emphasized by the magnitude of membrane web traffic resulting indigenous endocytosis. As currently noted, approximately 50% of the plasma membrane is internalized through receptor-mediated endocytosis every hour and must because of this be changed at an identical rate. Most of this instead of is the an outcome of receptor recycling; only around 5% the the cell surface is freshly synthesized per hour.

Ligands and membrane protein destined for destruction in lysosomes space transported from at an early stage endosomes to so late endosomes, i m sorry are situated near the nucleus (see number 12.41). Transfer from early to late endosomes is mediated through the motion of huge endocytic carrier vesicles along microtubules. The so late endosomes are an ext acidic than at an early stage endosomes (pH around 5.5 to 6.0) and, as debated in thing 9, room able come fuse through transport vesicles delivering lysosomal hydrolases native the Golgi apparatus. Late endosomes then mature right into lysosomes as they obtain a full enhance of lysosomal enzymes and also become still an ext acidic (pH around 5). In ~ lysosomes, the endocytosed materials are degraded by the action of mountain hydrolases.

Although plenty of receptors (like the LDL receptor) are recycled to the plasma membrane, others follow different fates. Some space transported to lysosomes and degraded along with their ligands. Because that example, the cell surface ar receptors for numerous growth factors (discussed in the next chapter) room internalized following growth factor binding and also eventually degraded in lysosomes. The result of this procedure is to remove the receptor-ligand complexes native the plasma membrane, thereby terminating the solution of the cell to growth factor stimulation—a phenomenon well-known as receptor down-regulation.

A specialized kind of recycling native endosomes plays an essential role in the infection of nerve impulses throughout synapses (Figure 12.42). As debated earlier in this chapter, the arrival of an activity potential in ~ the terminus of many neurons signals the blend of synaptic vesicles through the plasma membrane, release the neurotransmitters that carry the signal come postsynaptic cells. The empty synaptic vesicles space then recovered indigenous the plasma membrane in clathrin-coated vesicles, i beg your pardon fuse with early on endosomes. The synaptic vesicles are then regenerated directly by budding indigenous endosomes. Lock accumulate a new supply of neurotransmitter and also recycle come the plasma membrane, ready for the following cycle the synaptic transmission.

In polarized cells (e.g., epithelial cells), internalized receptor can additionally be transferred across the cell to the contrary domain of the plasma membrane—a procedure called transcytosis. Because that example, a receptor endocytosed native the basolateral domain the the plasma membrane can be sorted in at an early stage endosomes for move to the apical membrane. In part cells, this is crucial mechanism because that sorting membrane protein (Figure 12.43). Quite than gift sorted for shipment to the apical or basolateral domains in the infectious diseases worldwide Golgi network (see number 9.28), proteins are initially delivered to the basolateral membrane. Proteins targeted because that the apical membrane are then transferred to that website by transcytosis. In addition, transcytosis offers a system for the transfer of extracellular macromolecules across epithelial cell sheets. Because that example, numerous kinds the epithelial cells carry antibodies from the blood to a variety of secreted fluids, such as milk. The antibodies tie to receptor on the basolateral surface and are climate transcytosed along with their receptors to the apical surface. The receptors are then cleaved, releasing the antibodies into extracellular secretions.

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Figure 12.43

Protein sorting through transcytosis. A protein destined because that the apical domain of the plasma membrane is first transported indigenous the Golgi device to the basolateral domain. It is climate endocytosed and also selectively transported come the apical domain from at an early stage (more...)

By commitment with the publisher, this publication is available by the find feature, however cannot be browsed.