Limited and selective transfer of plasma membrane glycoproteins to membrane of secondary lysosomes

Lysosome and related protein degradation technologies

Recently, targeted protein degradation technologies based on lysosomal pathways have been developed. Lysosome-based targeted protein degradation technology has a broad range of substrates and the potential to degrade intracellular and extracellular proteins, protein aggregates, damaged organelles and non-protein molecules. Thus, they hold great promise for drug R&D. This study has focused on the biogenesis of lysosomes, their basic functions, lysosome-associated diseases and targeted protein degradation technologies through the lysosomal pathway. In addition, we thoroughly examine the potential applications and limitations of this technology and engage in insightful discussions on potential avenues for future research. Our primary objective is to foster preclinical research on this technology and facilitate its successful clinical implementation.

Kinetic evidence that newly-synthesized endogenous lysosome-associated membrane protein-1 (LAMP-1) first transits early endosomes before it is delivered to lysosomes

After de novo synthesis of lysosome-associated membrane proteins (LAMPs), they are sorted in the trans-Golgi network (TGN) for delivery to lysosomes. Opposing views prevail on whether LAMPs are targeted to lysosomes directly, or indirectly via prelysosomal stages of the endocytic pathway, in particular early endosomes. Conflicting evidence is based on kinetic measurements with too limited quantitative data for sufficient temporal and organellar resolution. Using cells of the mouse macrophage cell line, P338D(1), this study presents detailed kinetic data that describe the extent of, and time course for, the appearance of newly-synthesized LAMP-1 in organelles of the endocytic pathway, which had been loaded selectively with horse-radish peroxidase (HRP) by appropriate periods of endocytosis. After a 5-min pulse of metabolic labelling, LAMP-1 was trapped in the respective organelles by HRP-catalyzed crosslinking with membrane-permeable diaminobenzidine (DAB). These kinetic observations provide sufficient quantitative evidence that in P338D(1) cells the bulk of newly-synthesized endogenous LAMP-1 first appeared in early endosomes, before it was delivered to late endosomes and lysosomes about 25 min later.

The many niches and strategies used by pathogenic mycobacteria for survival within host macrophages

A major virulence factor of pathogenic mycobacteria is their ability to parasitize the host's scavenger cells and more particularly macrophages. The present overview discusses the known cellular and molecular mechanisms of intracellular survival of Mtb and other pathogenic mycobacteria within different intracellular niches, i.e. the macrophage in which they replicate and the granuloma in which they persist in a non-replicating state. After phagocytic uptake by macrophages, mycobacteria reside in phagosomes which they prevent from maturing and, as a result, from fusing with acidic and hydrolase-rich lysosomes. Two major points are highlighted: (i) the requirement for a close apposition between the phagosome membrane and the mycobacterial surface all around, and (ii) the ability for mycobacteria targeted to phagolysosomes to avoid degradation and to be rescued from this cytolytic environment to again reside in non-maturing phagosomes with a closely apposed membrane in which they can replicate. Concerning Mtb in granulomatous lesions, this review discusses the occurence of mycobacteria in lipid-rich foamy macrophages in which they persist in a non-replicating state. This overview highlights the major contribution of host cholesterol and/or fatty acids (triacylglycerol) in both prevention of phagosome maturation and persistence in granulomatous lesions.

Cholesterol depletion in Mycobacterium avium-infected macrophages overcomes the block in phagosome maturation and leads to the reversible sequestration of viable mycobacteria in phagolysosome-derived autophagic vacuoles

Phagocytic entry of mycobacteria into macrophages requires the presence of cholesterol in the plasma membrane. This suggests that pathogenic mycobacteria may require cholesterol for their subsequent intra-cellular survival in non-maturing phagosomes. Here we report on the effect of cholesterol depletion on pre-existing phagosomes in mouse bone marrow-derived macrophages infected with Mycobacterium avium. Cholesterol depletion with methyl-beta-cyclodextrin resulted in a loosening of the close apposition between the phagosome membrane and the mycobacterial surface, followed by fusion with lysosomes. The resulting phagolysosomes then autonomously executed autophagy, which did not involve the endoplasmic reticulum. After 5 h of depletion, intact mycobacteria had accumulated in large auto-phagolysosomes. Autophagy was specific for phagolysosomes that contained mycobacteria, as it did not involve latex bead-containing phagosomes in infected cells. Upon replenishment of cholesterol, mycobacteria became increasingly aligned to the lysosomal membrane, from where they were individually sequestered in phagosomes with an all-around closely apposed phagosome membrane and which no longer fused with lysosomes. These observations indicate that, cholesterol depletion (i) resulted in phagosome maturation and fusion with lysosomes and (ii) caused mycobacterium-containing phagolysosomes to autonomously undergo autophagy. Furthermore, (iii) mycobacteria were not killed in auto-phagolysosomes, and (iv) cholesterol replenishment enabled mycobacterium to rescue itself from autophagic phagolysosomes to again reside individually in phagosomes which no longer fused with lysosomes.

Mycobacterium tuberculosis and Mycobacterium avium modify the composition of the phagosomal membrane in infected macrophages by selective depletion of cell surface-derived glycoconjugates

The growth of pathogenic mycobacteria in phagosomes of the host cell correlates with their ability to prevent phagosome maturation. The underlying molecular mechanism remains elusive. In a previous study, we have shown that Mycobacterium avium depletes the phagosome membrane of cell surface-derived glycoconjugates (de Chastellier and Thilo, Eur. J. Cell Biol. 81, 17-25, 2002). We now extended these quantitative observations to the major human pathogen, Mycobacterium tuberculosis (H37Rv). At increasing times after infection of mouse bone marrow-derived macrophages, cell-surface glycoconjugates were labelled enzymatically with [3H]galactose. Subsequent endocytic membrane traffic resulted in a redistribution of this label from the cell surface to endocytic membranes, including phagosomes. The steady-state distribution was measured by quantitative autoradiography at the electron microscope level. Relative to early endosomes, with which phagosomes continued to fuse and rapidly exchange membrane constituents, the phagosome membrane was depleted about 3-fold, starting during infection and in the course of 9 days thereafter. These results were in quantitative agreement with our previous observations for Mycobacterium avium. For the latter case, we now showed by cell fractionation that the depletion was selective, mainly involving glycoproteins in the 110-210 kDa range. Together, these results indicated that pathogenic mycobacteria induced and maintained a bulk change in phagosome membrane composition that could be of special relevance for survival of pathogenic mycobacteria within phagosomes.

Kinetics of endocytosis and recycling of the GPI-anchored variant surface glycoprotein inTrypanosoma brucei

The dense coat of glycosylphosphatidylinositol (GPI)-anchored variant surface glycoprotein (VSG) covering parasitic African trypanosomes is essential for survival in mammalian hosts. VSG is internalised and recycled exclusively via a specialised part of the plasma membrane, the flagellar pocket. Direct measurement of the kinetics of VSG endocytosis and recycling shows that the VSG cell-surface pool is turned over within 12 minutes. Correspondingly, the turnover of the intracellular pool (9±4% of total VSG) requires only 1 minute, and this is an exceptionally high rate considering that endocytosis and exocytosis are limited to only 5% of the cell surface area. Kinetic 3D co-localisation analysis using biotinylated VSG and a panel of compartmental markers provides consistent evidence for the itinerary of VSG through the cell: VSG is endocytosed in large clathrin-coated vesicles, which bud from the flagellar pocket membrane at a rate of 6-7 vesicles per second, and is then delivered to RAB5-positive early endosomes. From there, VSG is recycled to RAB11-positive recycling endosomes at two stages, either directly or via RAB7-positive, late endosomes. Small clathrin-coated vesicles carrying fluid-phase cargo and being depleted of VSG bud from early and recycling endosomes. These vesicles are postulated to deliver their content to late endosomes and/or the lysosome. The recycling endosomes give rise to RAB11-positive exocytic carriers that fuse with the flagellar pocket and thereby return VSG to the cell surface. VSG recycling provides an interesting model for studies on the cellular trafficking and sorting of GPI-anchored proteins.

Dynamin-dependent transferrin receptor recycling by endosome-derived clathrin-coated vesicles

Previously we described clathrin-coated buds on tubular early endosomes that are distinct from those at the plasma membrane and the trans-Golgi network. Here we show that these clathrin-coated buds, like plasma membrane clathrin-coated pits, contain endogenous dynamin-2. To study the itinerary that is served by endosome-derived clathrin-coated vesicles, we used cells that overexpressed a temperature-sensitive mutant of dynamin-1 (dynamin-1(G273D)) or, as a control, dynamin-1 wild type. In dynamin-1(G273D)-expressing cells, 29-36% of endocytosed transferrin failed to recycle at the nonpermissive temperature and remained associated with tubular recycling endosomes. Sorting of endocytosed transferrin from fluid-phase endocytosed markers in early endosome antigen 1-labeled sorting endosomes was not inhibited. Dynamin-1(G273D) associated with accumulated clathrin-coated buds on extended tubular recycling endosomes. Brefeldin A interfered with the assembly of clathrin coats on endosomes and reduced the extent of transferrin recycling in control cells but did not further affect recycling by dynamin-1(G273D)-expressing cells. Together, these data indicate that the pathway from recycling endosomes to the plasma membrane is mediated, at least in part, by endosome-derived clathrin-coated vesicles in a dynamin-dependent manner.

Pathogenic Mycobacterium avium remodels the phagosome membrane in macrophages within days after infection

As part of their strategy for intracellular survival, mycobacteria prevent maturation of the phagosomes in which they reside inside macrophages. The molecular basis for this inhibition is only now beginning to emerge, by way of the molecular characterisation of the phagosome membrane when it encloses virulent mycobacteria. Our own work has shown that at 15 days after the phagocytic uptake of Mycobacterium avium by mouse bone marrow-derived macrophages, the phagosome membrane is depleted about 4-fold for cell surface-derived membrane glycoconjugates, labelled by exogalactosylation, in comparison to the membrane of early endosomes with which it continues to interact. Here we asked whether this depletion occurred at early or late stages after infection. We found that only about half of the depletion had occurred at about 5 hours after the beginning of phagocytic uptake, with the remainder becoming established thereafter, with a half-time of about 2.5 days. Phagosomes became depleted in relation to early endosomes with which they continued to exchange membrane constituents. Early endosomes themselves became gradually depleted by about 30% during the 15-day post-infection period. In contrast, late endosomes/lysosomes remained unchanged, with a concentration of surface-derived glycoconjugates between that of early endosomes and of phagosomes at day 15 post infection. In view of the slowness of the post-infection change of phagosome membrane composition, we proposed that this change did not play a role in preventing maturation immediately after phagosome formation, but rather correlated with the process of maintaining the phagosomes in an immature state.

Maturation of early endosomes and vesicular traffic to lysosomes in relation to membrane recycling

The controversy whether endocytic processing occurs by organellar maturation or by vesicular traffic has not been resolved. It is also not clear whether maturation continues to the stage of lysosomes, to what extent it involves a decrease in organellar fusogenicity, and how it relates to membrane recycling. Maturation and vesicular traffic imply distinct kinetics for the intermingling of endocytic markers after sequential endocytic uptake. We have studied the kinetics of intermingling of fluid-phase markers (fluorescein-labelled dextran and horseradish peroxidase) and cell surface-derived membrane (labelled by galactosylation) in organelles at early and late stages of the endocytic pathway in macrophage-like P388D1 cells. Intermingling declined by sigmoid kinetics, indicating that endosomes matured within about 3 minutes to become non-fusogenic towards early endosomes. During maturation about 60% of internalized membrane was recycled with T1/2 approximately 2 minutes. Whereas matured endosomes were non-fusogenic towards early endosomes and towards each other, a second phase of intermingling was observed upon delivery to lysosomes. This intermingling occurred by a first-order process (T1/2 approximately 4 minutes), concurrent with recycling of the remaining 40% of internalized membrane marker. These kinetic observations suggest a model for endocytic processing which reconciles maturation of early endosomes with the known function of carrier vesicles: Endocytic carrier vesicles do not bud off from permanent early endosomes as proposed for vesicular traffic, but are derived, together with recycling vesicles, from the maturation of early endosomes which are consumed by this process; these carrier vesicles subsequently mediate delivery to lysosomes by vesicular traffic during which the remaining surface-derived membrane is recycled.

The A-1 antigen: a novel marker in experimental peripheral nerve injury

Expression of the Schwann cell phenotype is regulated by signals from the adjoining axon. After axotomy, the Schwann cell ceases the production and maintenance of the myelin sheath and assumes phagocytic properties necessary to digest its own myelin. The molecular mechanisms responsible for this behavior remain unclear. A monoclonal antibody termed BIKS was produced after the immunization of mice with guinea pig lymphoid tissue. This antibody recognizes a cytoplasmic vesicle-associated molecule (A-1 antigen) which is abundant in all tissue macrophages but is also expressed in small amounts in normal Schwann cells. Following axotomy, the A-1 antigen appears to be translocated from a perinuclear site to accumulate in large quantities around myelin ovoids in Schwann cells, as well at the nodes of Ranvier-sites where Wallerian degeneration is known to commence. The level of the antigen remains high when axons are prevented from regeneration. During repair of crush injury, however, the level of antigen drops concomitant with the ingrowth of regenerating axons, suggesting axonal control of A-1 antigen expression.

Plasticity of early endosomes

We observed that the structural organization of early endosomes was significantly modified after cell surface biotinylation followed by incubation in the presence of low concentrations of avidin. Under these conditions early endosomes increased in size to form structures which extended over several micrometers and which had an intra-luminal content with a characteristic electron-dense appearance. The modified early endosomes were not formed when either avidin or biotinylation was omitted, suggesting that they resulted from the cross-linking of internalized biotinylated proteins by avidin. Accumulation of a fluid-phase tracer was increased after the avidin-biotin treatment (145% after 45 min). Both recycling and transport to the late endosomes still occurred, albeit to a somewhat lower extent than in control cells. Quantitative electron microscopy showed that the volume of the endosomal compartment was increased approximately 1.5-fold but that the surface area of the compartment decreased relative to its volume after avidin-biotin treatment. Finally, overexpression of a rab5 mutant, which is known to inhibit early endosome fusion in vitro, prevented the formation of these structures in vivo and caused early endosome fragmentation. Altogether, our data suggest that early endosomes exhibit a high plasticity in vivo. Cross-linking appears to interfere with this dynamic process but does not arrest membrane traffic to/from early endosomes.

Membrane recapture and early triggered secretion from the newly formed endocytotic compartment in bovine chromaffin cells

1. Recycling of secretory vesicles in cultured bovine adrenal medullary cells was investigated. 2. Extracellular horseradish peroxidase (HRP), a fluid phase marker, was taken up into cultured adrenal medullary cells following carbamylcholine-induced secretion of catecholamine. 3. The endocytosed HRP remained compartmentalized within the cell, migrating to a low density band on a Percoll density gradient. The endocytotic compartment was distinct from the major pool of catecholamine-containing chromaffin granules, which were found at much higher densities on the Percoll gradient. 4. The chromaffin granule membrane marker dopamine beta-hydroxylase was associated with the endocytosed HRP compartment as well as with the heavier chromaffin granules. 5. A subsequent challenge of the cells with carbamylcholine triggered the release of up to forty per cent of the endocytosed HRP. 6. The time course for secretion of the fluid phase marker was similar to that for catecholamine secretion. 7. Triggered release of HRP was dependent on extracellular calcium. The dependence on the extracellular calcium concentration was similar to that of catecholamine release. 8. Release of HRP could be triggered from electropermeabilized cells by raising the intracellular Ca2+ into the micromolar range. The intracellular Ca2+ dependence of triggered HRP release was similar to that for catecholamine release. 9. HRP could be secreted as early as 5 min, and as late as 2 h after endocytosis. 10. These data provide evidence that endocytotic vesicles can rapidly re-enter the secretory cycle. Endocytosed vesicles may therefore not have to recycle via the trans-Golgi reticulum to form high-density chromaffin granules in order to re-enter the regulated secretory pathway.

Localization of major histocompatibility complex class II molecules in phagolysosomes of murine macrophages infected with Leishmania amazonensis

Leishmania-infected macrophages are potential antigen-presenting cells for CD4+ T lymphocytes, which recognize parasite antigens bound to major histocompatibility complex class II molecules (Ia). However, the intracellular sites where Ia and antigens may interact are far from clear, since parasites grow within the modified lysosomal compartment of the host cell, whereas Ia molecules seem to be targeted to endosomes. To address this question, the expression and fate of Ia molecules were studied by immunocytochemistry in Leishmania amazonensis-infected murine macrophages stimulated with gamma interferon. In uninfected macrophages, Ia molecules were localized on the plasma membrane and in perinuclear vesicles, but they underwent a dramatic redistribution after infection, since most of the intracellular staining was then associated with the periphery of the parasitophorous vacuoles (p.v.) and quite often polarized towards amastigote-binding sites. The Ii invariant chain, which is transiently associated with Ia during their intracellular transport, although well expressed in infected macrophages, apparently did not reach the p.v. Similar findings were observed with macrophages from mice either resistant or highly susceptible to Leishmania infection. In order to determine the origin of p.v.-associated Ia, the fate of plasma membrane, endosomal, and lysosomal markers, detected with specific antibodies, was determined after infection. At 48 h after infection, p.v. was found to exhibit a membrane composition typical of mature lysosomes. Overall, these data suggest that (i) Ia located in p.v. originate from secondary lysosomes involved in the biogenesis of this compartment or circulate in several endocytic organelles, including lysosomes and (ii) p.v. could play a role in antigen processing and presentation. Alternatively, the presence of high amounts of Ia in p.v. could be due to a Leishmania-induced mechanism by means of which this organism may evade the immune response.

Rapid intramolecular turnover of N-linked glycans in plasma membrane glycoproteins. Extension of intramolecular turnover to the core sugars in plasma membrane glycoproteins of hepatoma

Plasma membrane glycoproteins of rat hepatocytes undergo a rapid terminal deglycosylation in that the terminal sugars of the oligosaccharide side chains are rapidly removed from the otherwise intact glycoproteins [Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl Acad. Sci. USA 80, 4026-4029]. The present paper demonstrates that this rapid intramolecular turnover of plasma membrane glycoproteins is not restricted to peripheral sugars but, in contrast to liver, in hepatoma the core sugars of the oligosaccharide chains are also involved. Intramolecular turnover was measured in Morris hepatoma 7777 in five plasma membrane glycoproteins with Mr of 85,000 (hgp85), 105,000 (hgp105), 115,000 (hgp115), 125,000 (hgp125), 175,000 (hgp175) (hgp = hepatoma glycoprotein) that were isolated and purified to homogeneity by concanavalin-A--Sepharose affinity chromatography and semipreparative SDS gel electrophoresis. Analysis of the carbohydrates of hgp85, hgp105, hgp115 and hgp125 revealed the presence of N-linked oligosaccharides containing L-fucose, D-galactose, D-mannose and N-acetyl-D-glucosamine, but only of trace amounts of N-acetyl-D-galactosamine; hgp175 additionally contained significant amounts of N-acetyl-D-galactosamine, indicating the presence of both N- and O-linked oligosaccharides. As shown by digestion with endoglucosaminidase H, the N-linked oligosaccharides of hgp105, hgp115, hgp125 and hgp175 were of the complex type, whereas hgp85 also contained oligosaccharides of the high-mannose type. Half-lives of the turnover of the oligosacharide chains and of the protein backbone of the five glycoproteins were measured in the plasma membrane in pulse-chase experiments in vivo, using L-[3H]fucose as a marker of terminal sugars, D-[3H]mannose as marker of a core sugar and L-[3H]leucine for labelling the protein backbone. Protein backbones of the five glycoproteins were degraded with individual half-lives ranging over 41-90 h with a mean of 66 h. Compared to the degradation of the polypeptide backbone, both the terminal sugar L-fucose and the core sugar D-mannose turned over with much shorter half-lives averaging about 20 h in the five glycoproteins. The data show that, conversely to liver, within plasma membrane glycoproteins of hepatoma not only peripheral sugars but also core sugars of the oligosaccharides are split off during the life-span of the protein backbone. It may therefore be assumed that this reprocessing of plasma membrane glycoproteins is sensitive to malignant transformation.

Decreased intramolecular turnover of L-fucose in membrane glycoproteins of rat liver during liver regeneration

In plasma membrane glycoproteins of rat liver L-fucose undergoes a rapid intramolecular turnover in that fucose residues are removed from the glycoproteins (Tauber, R., Park, C.S. & Reutter, W. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 4026-4029). The present paper demonstrates that the intramolecular turnover of L-fucose is markedly decreased during liver regeneration. Turnover half-lives of L-fucose were measured in regenerating liver by pulse-chase experiments in five plasma membrane glycoproteins (Mr 60,000 (gp60), 80,000 (gp80), 120,000 (gp120), 140,000 (gp140), and 160,000 (gp160). The glycoproteins were isolated from plasma membranes by concanavalin A-Sepharose affinity chromatography and semipreparative NaDodSO4 polyacrylamide gel electrophoresis. L-Fucose turned over in the five glycoproteins with heterogeneous half-lives ranging from 22 h (gp160) to 49 h (gp120). The protein moieties of the glycoproteins were degraded with half-lives ranging from 56 h (gp80) to 107 h (gp140). Relative to the half-life of the protein backbone the half-live of L-fucose was increased in the five membrane glycoproteins by 70% (gp60), 150% (gp80), 182% (gp120), 60% (gp140) and 16% (gp160) during liver regeneration when compared to normal liver. The data show that L-fucose turns over in different membrane glycoproteins with individual rates, and that loss of L-fucose from plasma membrane glycoproteins is reduced in rapidly proliferating liver after partial hepatectomy.

Endosomes differ from plasma membranes in the phospholipid molecular species composition

125I-Labeled epidermal growth factor was incorporated into and highly concentrated in endosomes of Chinese hamster V79-UF cells during incubation at 37 degrees C for 8 min after binding to its receptors on the cell surface at 4 degrees C. From the labeled cells, endosomes were isolated by isopycnic centrifugation on a Percoll density gradient and then sucrose density gradients. The isolated endosomes were mostly free from contamination by Golgi, endoplasmic reticulum, lysosome, plasma membrane and mitochondria. Endosome membranes were found to differ from plasma membranes in the phospholipid composition. Sphingomyelin and phosphatidylserine were enriched in endosomes, compared with plasma membranes. Diacylphosphatidylcholine and diacylphosphatidylethanolamine were major phospholipids of the membranes in both organelles. The contents of molecular species of diacylphosphatidylcholine and diacylphosphatidylethanolamine with two monoenoic fatty acids were lower in endosomes than in plasma membranes. The differences in the polar head group and molecular species compositions of phospholipids between endosomes and plasma membranes did not change, regardless of whether or not the proportions of phospholipid molecular species in plasma membranes changed. The significance of the lipids in endosomes is discussed.

A quantitative model of traffic between plasma membrane and secondary lysosomes: evaluation of inflow, lateral diffusion, and degradation

We present here a mathematical model that accounts for the various proportions of plasma membrane constituents occurring in the lysosomal membrane of rat fibroblasts (Draye, J.-P., J. Quintart, P. J. Courtoy, and P. Baudhuin. 1987. Eur. J. Biochem. 170: 395-403; Draye, J.-P., P. J. Courtoy, J. Quintart, and P. Baudhuin. 1987. Eur. J. Biochem. 170:405-411). It is based on contents of plasma membrane markers in purified lysosomal preparations, evaluations of their half-life in lysosomes and measurements of areas of lysosomal and plasma membranes by morphometry. In rat fibroblasts, structures labeled by a 2-h uptake of horseradish peroxidase followed by a 16-h chase (i.e., lysosomes) occupy 3% of the cellular volume and their total membrane area corresponds to 30% of the pericellular membrane area. Based on the latter values, the model predicts the rate of inflow and outflow of plasma membrane constituents into lysosomal membrane, provided their rate of degradation is known. Of the bulk of polypeptides iodinated at the cell surface, only 4% reach the lysosomes every hour, where the major part (integral of 83%) is degraded with a half-life in lysosomes of integral to 0.8 h. For specific plasma membrane constituents, this model can further account for differences in the association to the lysosomal membrane by variations in the rate either of lysosomal degradation, of inflow along the pathway from the pericellular membrane to the lysosomes, or of lateral diffusion.

Relations between plasma membrane and lysosomal membrane. 1. Fate of covalently labelled plasma membrane protein

To quantify the kinetics of the plasma membrane flow into lysosomes, we covalently labelled at 4 degrees C the pericellular membrane of rat fibroblasts and followed label redistribution to the lysosomal membrane using purified lysosomal preparations. The polypeptides were, either labelled with 125I by the lactoperoxidase procedure, or conjugated to [3H]peroxidase using bisdiazobenzidine as a bifunctional reagent. Both labels were initially bound to plasma membrane, as indicated by their equilibrium density in sucrose or Percoll gradients and their displacement by digitonin, as well as by electron microscopy. Upon cell incubation at 37 degrees C, both covalent labels were lost from cells with diphasic kinetics: a minor component (35% of cell-associated labels) was rapidly released (half-life less than 1 h), and most label (65%) was released slowly (half-life was 20 h for incorporated 125I and 27 h for 3H). Immediately after labelling up to 30 h after incubation at 37 degrees C, the patterns of 125I-polypeptides quantified by autoradiography after SDS-PAGE were indistinguishable, indicating no preferential turnover for the major plasma membrane polypeptides. The redistribution of both labels to lysosomes was next quantified by cell fractionation. At equilibrium (between 6 and 25 h of cell incubation) 2-4% of cell-associated 125I label was recovered with the purified lysosomal membranes. By contrast, when 3H-labelled cells were incubated for 16 h, most of the label codistributed with lysosomes. However, only 6% of cell-associated 3H was bound to lysosomal membrane. These results indicate that in cultured rat fibroblasts, a minor fraction of plasma membrane polypeptides becomes associated with the lysosomal membrane and is constantly equilibrated by membrane traffic.

Cycling of the integral membrane glycoprotein, LEP100, between plasma membrane and lysosomes: kinetic and morphological analysis

LEP100 (an integral membrane glycoprotein, Mr = 100,000) occurs in three subcellular compartments: lysosome (approximately 90% of the molecules), endosome (5%-8%), and plasma membrane (2%-3%). Rate constants for movement to and from each compartment have been estimated. The movement of LEP100 from endosomes to lysosomes was blocked by chloroquine, causing redistribution to a new steady state in which about 30% of LEP100 molecules were localized in clathrin-coated patches on the cell surface, while intracellular LEP100 occurred in nearby endocytic vesicles. The cell-surface and endosomal pools of LEP100 remained in rapid equilibrium (t1/2 about 5 min). These results support the existence of a hitherto unappreciated pathway of membrane flow from lysosomes. The lysosome should not be considered simply a terminal target of membrane trafficking.