Lysosomes in general are meeting places in which several streams of intracellular traffic

13 Lysosomes in general are meeting places in which several streams of intracellular traffic

converge. Digestive enzymes are delivered to them by a route that leads outward from the ER via the Golgi apparatus, while substances to be digested are fed in by at least three paths, according to their source.

Of the three paths to degradation in lysosomes, the best studied is that followed by macromolecules taken up from the external medium by endocytosis. In brief (for the details will

be discussed later), the endocytosed molecules are initially delivered into small, irregularly shaped intracellular vesicles called early endosomes. From these, some of the ingested molecules are selectively retrieved and recycled to the plasma membrane, while others pass on into late endosomes. Here, by fusion of two streams of transport vesicles, the materials coming in for digestion first meet the lysosomal hydrolases coming out from the Golgi apparatus. The interior of the late endosomes is mildly acidic (pH~6), and it is thought to be the site where the hydrolytic digestion of the endocytosed molecules begins. Mature lysosomes form from the late endosomes, although it is not known precisely how this occurs. During the conversion process some distinct endosomal membrane proteins are lost, and there is a further decrease in internal pH.

A second pathway to degradation in lysosomes is used in all cell types for disposal of obsolete parts of the cell itself - a process called autophagy. In a liver cell, for example, an average mitochondrion has a lifetime of about 10 days, and electron microscopic images of normal cells reveal lysosomes containing (and presumably digesting) mitochondria as well as other organelles. The process seems to begin with the enclosure of an organelle by membranes derived from the ER, creating an autophagosome, which then fuses with a lysosome (or a late endosome). The process is highly regulated, and selected cell components can somehow be marked for destruction during cell remodeling: the smooth ER that proliferates in a liver cell in A second pathway to degradation in lysosomes is used in all cell types for disposal of obsolete parts of the cell itself - a process called autophagy. In a liver cell, for example, an average mitochondrion has a lifetime of about 10 days, and electron microscopic images of normal cells reveal lysosomes containing (and presumably digesting) mitochondria as well as other organelles. The process seems to begin with the enclosure of an organelle by membranes derived from the ER, creating an autophagosome, which then fuses with a lysosome (or a late endosome). The process is highly regulated, and selected cell components can somehow be marked for destruction during cell remodeling: the smooth ER that proliferates in a liver cell in

As we discuss later, the third pathway that provides materials to lysosomes for degradation occurs mainly in cells specialized for the phagocytosis of large particles and microorganisms. Such professional phagocytes (macrophages and neutrophils in vertebrates) engulf objects to form a phagosome, which is then converted to a lysosome in the manner described for the autophagosome. The three pathways are summarized in Figure 13-22.

S o m e Cy t o s o lic P ro t e in s Are D ire c t ly Tra n s p o rt e d in t o

Ly s o s o m e s fo r D e g ra d a t io n 14

There may be yet a fourth route for proteins to enter a lysosome for degradation: some proteins contain certain signals on their surface [called KFERQ sequences, KFERQ standing for lysine (K), phenylalanine (F), glutamate (E), arginine (R), and glutamine (Q)] that cause the proteins bearing them to be selectively delivered to lysosomes for degradation. It is possible that the KFERQ sequences attach these proteins to cytosolic organelles that are on the way to being autophago-cytosed, thereby dragging the proteins into the lysosome indirectly. Alternatively, there may be a specific transporter in the lysosomal membrane that recognizes these signals and transfers the proteins directly across the lysosomal membrane.

There are precedents for nonconventional mechanisms for moving proteins directly across membranes. A number of proteins that are secreted from cells, such as basic fibroblast growth factor or interleukin-1, for example, arrive at the cell surface without ever entering the classical secretory pathway through the ER and Golgi apparatus. In most cases it is not known which membrane the protein crosses or how its transmembrane transport is catalyzed. In the case of a small yeast peptide, the pheromone a-factor, the transport is known to be mediated directly across the plasma membrane by an ATP-driven peptide pump that belongs to the protein family of the ABC transporters (discussed in Chapter 11). Thus it is possible that similar pumps provide "private" transport systems, each specialized for the transfer of a small, specific subset of proteins across a particular membrane.

Ly s o s o m a l En z y m e s Are S o rt e d fro m Ot h e r P ro t e in s in t h e Tr a n s Go lg i N e t w o rk b y a Me m b ra n e - b o u n d Re c e p t o r

P ro t e in Th a t Re c o g n iz e s Ma n n o s e 6 - P h o s p h a t e 15

We now consider more closely the system that delivers the other half of the traffic into lysosomes - the specialized lysosomal hydrolases and membrane proteins. Both classes of proteins are synthesized in the rough ER and transported through the Golgi apparatus. Transport vesicles that deliver these proteins to late endosomes - which later form lysosomes - bud from the trans Golgi network, incorporating lysosomal proteins while excluding the many other proteins being packaged into different transport vesicles for delivery elsewhere.

How are lysosomal proteins recognized and selected with the required accuracy? For the lysosomal hydrolases the answer is known. They carry a unique marker in the form of mannose 6- phosphate (M6P) groups, which are added exclusively to the N-linked oligosaccharides of these How are lysosomal proteins recognized and selected with the required accuracy? For the lysosomal hydrolases the answer is known. They carry a unique marker in the form of mannose 6- phosphate (M6P) groups, which are added exclusively to the N-linked oligosaccharides of these

Th e Ma n n o s e 6 - P h o s p h a t e Re c e p t o r S h u t t le s B a c k a n d

Fo rt h B e t w e e n S p e c ific Me m b ra n e s 16

The M6P receptor protein binds its specific oligosaccharide at pH 7 in the trans Golgi network and releases it at pH 6, which is the pH in the interior of late endosomes. Thus in the late endosomes the lysosomal hydrolases dissociate from the M6P receptor and can begin to digest the endocytosed material delivered from early endosomes. Having released their bound enzymes, the M6P receptors are retrieved into transport vesicles that bud from late endosomes and return to the membrane of the trans Golgi network for reuse (Figure 13-23). It is not clear whether transport back to the Golgi apparatus requires a specific signal peptide in the cytoplasmic tail of the M6P receptor or whether it will occur by default. This process of membrane recycling from late endosome back to the Golgi apparatus resembles the recycling that occurs between other subcompartments of the secretory and endocytic pathways that we discuss later.

The sorting of lysosomal hydrolases from other proteins is presently the best-understood example of the many sorting processes mediated by transport vesicles in a eucaryotic cell. Although an oligosaccharide marker is not likely to be used elsewhere, the general strategy is probably typical of other vesicle-mediated sorting processes. Cargo molecules are recognized and picked up by membrane-bound cargo receptors during the budding of specific clathrin-coated vesicles. These loaded vesicles move off and fuse with a specific target membrane, the cargo molecules are released in the target compartment, and the empty receptors are recycled back to their original compartment.

Not all of the cargo that is tagged for delivery to lysosomes gets to its proper destination. It seems that some of the ly sosomal hydrolase molecules escape the normal packaging process in the trans Golgi network and instead are transported via the default pathway to the cell surface, where they are secreted into the extracellular fluid. Some M6P receptors, however, also take a detour to the plasma membrane, where they help undo the error in lysosomal hydrolase routing by recapturing the escaped enzymes and returning them by receptor-mediated endocytosis to lysosomes via early and late endosomes.

This scavenger pathway was originally discovered through studies of cells from humans who are genetically defective in a specific lysosomal hydrolase. An example is Hurler's disease, in which the enzyme required for breakdown of glycosaminoglycans is missing. In these mutant individuals the lysosomes accumulate massive quantities of the particular molecules that cannot be digested. For this reason these diseases are called lysosomal storage diseases. The same cellular abnormality is seen when cells from the mutant individuals are grown in culture. If the mutant cells are co-cultured with cells from a normal individual, however, the abnormality is no This scavenger pathway was originally discovered through studies of cells from humans who are genetically defective in a specific lysosomal hydrolase. An example is Hurler's disease, in which the enzyme required for breakdown of glycosaminoglycans is missing. In these mutant individuals the lysosomes accumulate massive quantities of the particular molecules that cannot be digested. For this reason these diseases are called lysosomal storage diseases. The same cellular abnormality is seen when cells from the mutant individuals are grown in culture. If the mutant cells are co-cultured with cells from a normal individual, however, the abnormality is no

A S ig n a l P a t c h in t h e P o ly p e p t id e Ch a in P ro v id e s t h e Cu e fo r Ta g g in g a Ly s o s o m a l En z y m e w it h Ma n n o s e 6 -

Phosphate 17

The sorting system that segregates lysosomal hydrolases and dispatches them to late endosomes works because M6P groups are added to only the appropriate glycoproteins in the Golgi apparatus. This requires specific recognition of the hydrolases by the Golgi enzyme responsible for adding M6P. Since all glycoproteins leave the ER with identical N-linked oligosaccharide chains, the signal for adding the M6P units to oligosaccharides must reside somewhere in the polypeptide chain of each hydrolase.

Two enzymes act sequentially to catalyze the addition of M6P groups to lysosomal hydrolases (Figure 13-24). The first is a phosphotransferase with a recognition site that specifically binds the hydrolase and a separate catalytic site for the phosphotransferase reaction; the signal recognized by the recognition site is a conformation-dependent signal patch in the hydrolase rather than a signal peptide (see Figure 12-8). Once the hydrolase is bound, the phosphotrans-ferase adds GlcNAc-phosphate to one or two of the mannose residues on each oligosaccharide chain (Figure 13-25). A second enzyme then cleaves off the GlcNAc residue, creating the mannose 6- phosphate marker (see Figure 13-24).

Since most lysosomal hydrolases have multiple oligosaccharides, they acquire many M6P residues, providing a strong and easily recognized signal for the M6P receptor. While a lysosomal hydrolase typically binds to the recognition site of the phosphotransferase with an affinity

constant (K a ) of about 10 5 liters/mole, the multiply phosphorylated hydrolase binds to the M6P

receptor with a K a of about 10 9 liters/mole, a 10,000-fold increase in affinity.

D e fe c t s in t h e Glc N Ac P h o s p h o t ra n s fe ra s e Ca u s e a

Ly s o s o m a l S t o ra g e D is e a s e in Hu m a n s 18

Lysosomal storage diseases are caused by genetic defects that affect one or more of the lysosomal hydrolases and result in accumulation of their undigested substrates in lysosomes, with severe pathological consequences. Most often, there is a mutation in a structural gene that codes for an individual lysosomal hydrolase; this is the case in Hurler's disease, mentioned above. The most dramatic form of lysosomal storage disease, however, is a very rare disorder called inclusion-cell disease (I-cell disease). In this disease almost all of the hydrolytic enzymes are missing from the lysosomes of fibroblasts, and their undigested substrates accumulate in lysosomes, which consequently form large "inclusions" in the patients' cells. I-cell disease is due to a single gene defect, and like most genetic enzyme deficiencies, it is recessive - that is, it is seen only in individuals in whom both copies of the gene are defective.

In these individuals all the hydrolases missing from lysosomes are found in the blood; because they fail to be sorted properly in the Golgi apparatus, the hydrolases are secreted rather than transported to lysosomes. The missorting has been traced to a defective or missing GlcNAc- phosphotransferase. Because lysosomal enzymes are not phosphorylated in the cis Golgi network, they are not segregated by M6P receptors into the appropriate transport vesicles in the trans Golgi network and instead are carried to the cell surface and secreted by the default pathway. This was, in fact, the first evidence for such a default pathway; and it was through a biochemical comparison of normal lysosomal hydrolases with those from patients with I-cell disease that mannose 6-phosphate was discovered to be the lysosomal sorting signal and the whole lysosomal hydrolase-sorting pathway was elucidated.

In I-cell disease the lysosomes in some cell types, such as hepatocytes, contain a normal complement of lysosomal enzymes, implying that there is another pathway for directing hydrolases to lysosomes that is used by some cell types but not others. The nature of this M6P- independent pathway is unknown. Similarly, the lysosomal membrane proteins are sorted from the trans Golgi network to late endosomes by an M6P-independent pathway in all cells. It is unclear why cells should need more than one sorting pathway to construct a lysosome, although it is perhaps not surprising that different mechanisms should operate for soluble and membrane- bound proteins.

S u m m a ry

Lysosomes are specialized for intracellular digestion. They contain unique membrane proteins and a wide variety of hydrolytic enzymes that operate best at pH 5, the internal pH of lysosomes, which is maintained by an ATP-driven H + pump in the lysosomal membrane. Newly synthesized lysosomal proteins are transferred into the lumen of the ER, transported through the Golgi apparatus, and then carried from the trans Golgi network to late endosomes by means of transport vesicles.

The lysosomal hydrolases contain N- linked oligosaccharides that are covalently modified in a unique way in the cis Golgi network so that their mannose residues are phosphorylated. These mannose 6-phosphate (M6P) groups are recognized by an M6P receptor protein in the trans Golgi network that segregates the hydrolases and helps to package them into budding transport vesicles, which deliver their contents to late endosomes, and thereby to lysosomes. These transport vesicles act as shuttles that move the M6P receptor back and forth between the trans Golgi network and late endosomes. The low pH in the late endosome dissociates the lysosomal hydrolases from this receptor, making the transport of the hydrolases unidirectional

Figure 13-17. Lysosomes. The acid hydrolases are hydrolytic enzymes that are active under acidic conditions. The lumen is maintained at an acidic pH by an H + ATPase in the membrane that pumps H + into the lysosome.

Figure 13-18. The low pH in lysosomes and endosomes. Proteins labeled with a pH-sensitive fluorescent probe (fluorescein) and then endocytosed by cells can be used to measure the pH in endosomes and lysosomes. The different colors reflect the pH that the fluorescent probe encounters in these organelles. The pH in lysosomes (red) is about 5, while the pH in various types of endosomes (blueand green) ranges from 5.5 to 6.5. This method was originally developed in the 1890s by Metchnikoff, who fed litmus particles to phagocytic cells and observed Figure 13-18. The low pH in lysosomes and endosomes. Proteins labeled with a pH-sensitive fluorescent probe (fluorescein) and then endocytosed by cells can be used to measure the pH in endosomes and lysosomes. The different colors reflect the pH that the fluorescent probe encounters in these organelles. The pH in lysosomes (red) is about 5, while the pH in various types of endosomes (blueand green) ranges from 5.5 to 6.5. This method was originally developed in the 1890s by Metchnikoff, who fed litmus particles to phagocytic cells and observed

Figure 13-19. Histochemical visualization of lysosomes. Electron micro-graphs of two sections of

a cell stained to reveal the location of acid phos-phatase, a marker enzyme for lysosomes. The larger membrane-bounded organelles, containing dense precipitates of lead phosphate, are lysosomes, whose diverse morphology reflects variations in the amount and nature of the material they are digesting. The precipitates are produced when tissue fixed with glutaraldehyde (to fix the enzyme in place) is incubated with a phosphatase substrate in the presence of lead ions. Two small vesicles thought to be carrying acid hydrolases from the Golgi apparatus are indicated by red arrowsin the top panel. (Courtesy of Daniel S. Friend.)

Figure 13-20. The plant cell vacuole. This electron micrograph of cells in a young tobacco leaf shows that the cytosol is confined by the enormous vacuole to a thin layer, containing chloroplasts, pressed against the cell wall. The membrane of the vacuole is called the tonoplast. (Courtesy of J. Burgess.)

Figure 13-21. The role of the vacuole in controlling the size of plant cells. A large increase in cell volume can be achieved without increasing the volume of the cytosol. Localized weakening of the cell wall orients a turgor-driven cell enlargement that accompanies the uptake of water into an expanding vacuole (see Figure 19-67). The cytosol is eventually confined to a thin peripheral layer that is connected to the nuclear region by strands of cytosol, which are stabilized by bundles of actin filaments (not shown).

Figure 13-22. Three pathways to degradation in lysosomes. Each pathway leads to the intracellular digestion of materials derived from a different source. The compartments resulting from the three pathways can sometimes be distinguished morphologically - hence the terms "autophagolysosome," "phago-lysosome," and so on. Such lysosomes, however, may differ only because of the different materials they are digesting

Figure 13-23. The transport of newly synthesized lysosomal hydrolases to lysosomes. The precursors of lysosomal hydrolases are covalently modified by the addition of mannose 6- phosphate groups (M6P) in the cis Golgi network. They then become segregated from all other types of proteins in the trans Golgi network because a specific class of transport vesicles (called clathrin-coated vesicles) budding from the trans Golgi network concentrates mannose 6- phosphate-specific receptors, which bind the modified lysosomal hydrolases. These vesicles subsequently fuse with late endosomes. At the low pH of the late endosome the hydrolases dissociate from the receptors, which are recycled to the Golgi apparatus for further rounds of transport. In late endosomes the phosphate is removed from the mannose on the hydrolases, further ensuring that the hydrolases do not return to the Golgi apparatus with the receptor

Figure 13-24. Synthesis of the mannose 6-phosphate marker on a lysosomal hydrolase. The synthesis occurs in two steps. First, GlcNAc phosphotransferase transfers GlcNAc-P to the 6 position of several mannoses on the N-linked oligosaccharides of the lysosomal hydrolase. Second, a phospho-glycosidase cleaves off the GlcNAc, creating the mannose 6-phosphate marker. The first enzyme is specifically activated by a signal patch present on lysosomal Figure 13-24. Synthesis of the mannose 6-phosphate marker on a lysosomal hydrolase. The synthesis occurs in two steps. First, GlcNAc phosphotransferase transfers GlcNAc-P to the 6 position of several mannoses on the N-linked oligosaccharides of the lysosomal hydrolase. Second, a phospho-glycosidase cleaves off the GlcNAc, creating the mannose 6-phosphate marker. The first enzyme is specifically activated by a signal patch present on lysosomal

Figure 13-25. The recognition of a lysosomal hydrolase. The GlcNAc phosphotransferase enzyme that recognizes lysosomal hydrolases in the Golgi apparatus has separate catalytic and recognition sites. The catalytic site binds both high-mannose N-linked oligosaccharides and UDP- GlcNAc. The recognition site binds to a signal patch that is present only on the surface of lysosomal hydrolases

13. Vesicular Traffic in the Secretory and Endocytic Pathways Introduction Transport from the ER Through the Golgi Apparatus Transport from the Trans Golgi Network to Lysosomes

Transport from the Plasma Membrane via Endosomes: Endocytosis

Transport from the Trans Golgi Network to the Cell Surface: Exocytosis The Molecular Mechanisms of Vesicular Transport and the Maintenance of

Compartmental Diversity References

Transport from the Plasma Membrane via Endosomes: Endocytosis

Specialized Phagocytic Cells Can Ingest Large Particles

Pinocytic Vesicles Form from Coated Pits in the Plasma Membrane

Clathrin-coated Pits Can Serve as a Concentrating Device for Internalizing Specific Extracellular Macromolecules

Cells Import Cholesterol by Receptor-mediated Endocytosis Endocytosed Materials Often End Up in Lysosomes Specific Proteins Are Removed from Early Endosomes and Returned to the Plasma Membrane The Relationship Between Early and Late Endosomes Is Uncertain Epithelial Cells Have Two Distinct Early Endosomal Compartments But a Common Late

Endosomal Compartment Summary

Tra n s p o rt fro m t h e P la s m a Me m b ra n e v ia En d o s o m e s : En d o c y t o s is 19

Introduction The routes that lead inward to lysosomes from the cell surface start with the process of

endocytosis, by which cells take up macromolecules, particulate substances, and, in specialized cases, even other cells.

Material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first invaginates and then pinches off to form an intracellular vesicle containing the ingested substance or particle. Two main types of endocytosis are distinguished on the basis of the size of Material to be ingested is progressively enclosed by a small portion of the plasma membrane, which first invaginates and then pinches off to form an intracellular vesicle containing the ingested substance or particle. Two main types of endocytosis are distinguished on the basis of the size of

vesicles called phagosomes, generally > 250 nm in diameter. Although most eucaryotic cells are continually ingesting fluid and solutes by pinocytosis, large particles are ingested mainly by specialized phagocytic cells.

S p e c ia liz e d P h a g o c y t ic Ce lls Ca n I n g e s t La rg e P a rt ic le s 20

Phagocytosis is a special form of endocytosis in which large particles such as microorganisms and cell debris are ingested via large endocytic vesicles called phagosomes. In protozoa phagocytosis is a form of feeding: large particles taken up into phagosomes end up in lysosomes, and the products of the subsequent digestive processes pass into the cytosol to be utilized as food. Few cells in multicellular organisms are able to ingest large particles efficiently, however, and in the gut of animals, for example, large particles of food are broken down extracellularly before import into cells. Phagocytosis is important in most animals for purposes other than nutrition, and it is mainly carried out by specialized cells that are "professional" phagocytes. In mammals there are two classes of white blood cells that act as professional phagocytes - macrophages (which are widely distributed in tissues as well as in blood) and neutrophils. These two types of cells develop from a common precursor cell (discussed in Chapter 22), and they defend us against infection by ingesting invading microorganisms. Macrophages also play an important part in scavenging senescent and damaged cells and cellular debris. In quantitative terms the latter function is far more important: macrophages phagocytose more than 10

11 senescent red blood cells in each of us every day, for example. Whereas the endocytic vesicles involved in pinocytosis are small and uniform, phagosomes have

diameters that are determined by the size of the ingested particle, and they can be almost as large as the phagocytic cell itself (Figure 13-26). The phagosomes fuse with lysosomes, and the ingested material is degraded; indigestible substances will remain in lysosomes, forming residual bodies. Some of the internalized plasma membrane components are retrieved from the phagosome by transport vesicles and returned to the plasma membrane.

In order to be phagocytosed, particles must first bind to the surface of the phagocyte. Not all particles that bind are ingested, however. Phagocytes have a variety of specialized surface receptors that are functionally linked to the phagocytic machinery of the cell. Unlike pinocytosis, which is a constitutive process that occurs continuously, phagocytosis is a triggered process that requires that activated receptors transmit signals to the cell interior to initiate the response. The best-characterized triggers are antibodies, which protect us by binding to the surface of infectious microorganisms to form a coat in which the tail region of each antibody molecule (called the Fc region) is exposed on the exterior. This antibody coat is then recognized by specific Fc receptors on the surface of macrophages and neutrophils (see Figure 23-20). The binding of antibody- coated particles to these receptors induces the phagocytic cell to extend pseudopods that engulf the particle and fuse at their tips to form a phagosome (Figure 13-27).

Several other classes of receptors that promote phagocytosis have been characterized - those that recognize complement (a class of molecules that circulate in the blood and collaborate with antibodies in targeting undesirable cells for destruction, discussed in Chapter 23), for example, Several other classes of receptors that promote phagocytosis have been characterized - those that recognize complement (a class of molecules that circulate in the blood and collaborate with antibodies in targeting undesirable cells for destruction, discussed in Chapter 23), for example,

P in o c y t ic Ve s ic le s Fo rm fro m Co a t e d P it s in t h e P la s m a

Me m b ra n e 21

Virtually all eucaryotic cells continually ingest bits of their plasma membrane in the form of small pinocytic (endocytic) vesicles that are later returned to the cell surface. The rate at which plasma membrane is internalized in this process of pinocytosis varies from cell type to cell type, but it is usually surprisingly large. A macrophage, for example, ingests 25% of its own volume of fluid each hour. This means that it must ingest 3% of its plasma membrane each minute, or 100% in about half an hour. Fibroblasts endocytose at a somewhat lower rate, whereas some amoebae ingest their plasma membrane even more rapidly. Since a cell's surface area and volume remain unchanged during this process, it is clear that as much membrane as is being removed by endocytosis is being added to the cell surface by exocytosisthe converse process, discussed later. In this sense endocytosis and exocytosis are linked processes that can be considered to constitute an endocytic-exocytic cycle.

The endocytic part of the cycle usually begins at specialized regions of the plasma membrane called clathrin-coated pits, which typically occupy about 2% of the total plasma membrane area. In electron micrographs of plasma membranes studied by the rapid-freeze, deep-etch technique, these pits appear as invaginations of the plasma membrane coated on their inner (cytosolic) surface with a densely packed material. These coats are made of the protein clathrin, which, with other proteins, forms a characteristic basket or cage, which we discuss later. The lifetime of clathrin-coated pits is short: within a minute or so of being formed, they invaginate into the cell and pinch off to form clathrin-coated vesicles (Figure 13-28). It has been estimated that about 2500 clathrin-coated vesicles leave the plasma membrane of a cultured fibroblast every minute. These coated vesicles are even more transient than the coated pits: within seconds of being formed, they shed their coat and are able to fuse with early endosomes. Since extracellular fluid is trapped in clathrin-coated pits as they invaginate to form coated vesicles, substances dissolved in the extracellular fluid are internalized - a process called fluid-phase endocytosis.

Cla t h rin - c o a t e d P it s Ca n S e rv e a s a Co n c e n t ra t in g D e v ic e

fo r I n t e rn a liz in g S p e c ific Ex t ra c e llu la r Ma c ro m o le c u le s 22

In most animal cells, clathrin-coated pits and vesicles provide an efficient pathway for taking up specific macromolecules from the extracellular fluid, a process called receptor-mediated endocytosis. The macromolecules bind to complementary cell-surface receptors (which are transmembrane proteins), accumulate in coated pits, and enter the cell as receptor- macromolecule complexes in clathrin-coated vesicles. The process is very similar to the packaging of lysosomal hydrolases in the Golgi apparatus. There too, as we have seen, the molecules to be transported bind to specific receptors in the membrane (the M6P receptors) and so become captured in membrane vesicles that detach from their original compartment and are released into the cytosol. Moreover, the budding of vesicles loaded with lysosomal enzymes from the Golgi apparatus also involves the formation of a clathrin coat (see Figure 13-23). As we In most animal cells, clathrin-coated pits and vesicles provide an efficient pathway for taking up specific macromolecules from the extracellular fluid, a process called receptor-mediated endocytosis. The macromolecules bind to complementary cell-surface receptors (which are transmembrane proteins), accumulate in coated pits, and enter the cell as receptor- macromolecule complexes in clathrin-coated vesicles. The process is very similar to the packaging of lysosomal hydrolases in the Golgi apparatus. There too, as we have seen, the molecules to be transported bind to specific receptors in the membrane (the M6P receptors) and so become captured in membrane vesicles that detach from their original compartment and are released into the cytosol. Moreover, the budding of vesicles loaded with lysosomal enzymes from the Golgi apparatus also involves the formation of a clathrin coat (see Figure 13-23). As we

Receptor-mediated endocytosis provides a selective concentrating mechanism that increases the efficiency of internalization of particular ligands more than 1000-fold, so that even minor components of the extracellular fluid can be specifically taken up in large amounts without taking in a correspondingly large volume of extracellular fluid. A particularly well-understood and physiologically important example is the process whereby mammalian cells take up cholesterol.

Ce lls I m p o rt Ch o le s t e ro l b y Re c e p t o r- m e d ia t e d En d o c y t o s is