Physiological functions of endosomal proteolysis

Acidic pH as a physiological regulator of human cathepsin L activity

Human cysteine protease cathepsin L was inactivated at acid pH by a first-order process. The inactivation rate decreased with increasing concentrations of a small synthetic substrate, suggesting that substrates stabilize the active conformation. The substrate-independent inactivation rate constant increased with organic solvent content of the buffer, consistent with internal hydrophobic interactions, disrupted by the organic solvent, also stabilizing the enzyme. Circular dichroism showed that the inactivation is accompanied by large structural changes, a decrease in alpha-helix content being especially pronounced. The high activation energy of the reaction at pH 3.0 (200 kJ.mol-1) supported such a major conformational change occurring. The acid inactivation of cathepsin L was irreversible, consistent with the propeptide being needed for proper folding of the enzyme. Aspartic protease cathepsin D was shown to cleave denatured, but not active cathepsin L, suggesting a potential mechanism for in-vivo regulation and turnover of cathepsin L inside lysosomes.

Uptake by rat liver and intracellular fate of plasmid DNA complexed with poly-L-lysine or poly-D-lysine

Efficiency of transfection is probably dependent on the rate of intracellular degradation of plasmid DNA. When a non-viral vector is used, it is not known to what extent the plasmid DNA catabolism is subordinated to the catabolism of the vector. In the work reported here, the problem was approached by following the intracellular fate in rat liver, of plasmid [35S]DNA complexed with a cationic peptide poly-L-lysine that can be hydrolyzed by cellular peptidases or with its stereoisomer, poly-D-lysine, that cannot be split by these enzymes. Complexes of DNA with poly-L-lysine and poly-D-lysine are taken up to the same extent by the liver, mainly by Kupffer cells, but the intracellular degradation of nucleic acid molecules is markedly quicker when poly-L-lysine is injected. The association of DNA with the polycations inhibits DNA hydrolysis in vitro by purified lysosomes but similarly for poly-L-lysine and poly-D-lysine. The intracellular journey followed by [35S]DNA complexed with poly-L- or poly-D-lysine was investigated using differential and isopycnic centrifugation. Results indicate that [35S]DNA is transferred more slowly to lysosomes, the main site of intracellular degradation of endocytosed macromolecules, when it is given as a complex with poly-D-lysine than with poly-L-lysine. They suggest that the digestion of the vector in a prelysosomal compartment is required to allow endocytosed plasmid DNA to rapidly reach lysosomes. Such a phenomenon could explain why injected plasmid DNA is more stable in vivo when it is associated with poly-D-lysine.

Transport of residual endocytosed products into terminal lysosomes occurs slowly in rat liver endothelial cells

Receptor-mediated endocytosis of circulating collagen is a major physiological scavenger function of the liver endothelial cell and an important catabolic event in the complete turnover of this abundant connective tissue protein. In the present study, transport of collagen through the endocytic pathway was investigated in cultured liver endothelial cells. Collagen conjugated to fluorescein isothiocyanate, to allow detection of the ligand by fluorescence and immunoelectron microscopy, was found sequentially in three different organelles that compose the basic degradative endocytic pathway of eukaryotic cells: early endosomes, late endosomes, and terminal lysosomes. Early endosomes were identified as vesicles positive for early endosome antigen 1 (EEA1). Late endosomes were distinguished as structures positive for the late endosomal/lysosomal marker rat lysosomal membrane glycoprotein 120, but negative for EEA1 and lysosomally targeted BSA-gold. Lysosomes were defined by their content of BSA-gold, injected 24 hours before isolation of cells. Coated pits and coated vesicles mediated an extremely rapid internalization. Shortly after internalization and during the first 20 minutes, ligand was found in early endosomes. From 20 minutes on, ligand started to appear in late endosomes (23%), and by 2 hours the transfer was largely complete (82.5%). Only 2.5% of ligand was transferred to the lysosomes after 2 hours, and this number slowly increased to 21% and 53% after 6 and 16 hours, respectively. We conclude that 1) EEA1 is a useful marker for tracing early events of endocytosis in liver endothelial cells; 2) in contrast to the rapid internalization, transit of internalized ligand through early sorting endosomes generally takes from 20 minutes to 2 hours; and 3) exit from the late endosomes is very slow, requiring several hours.

Cloning and expression of mouse legumain, a lysosomal endopeptidase

Legumain, a recently discovered mammalian cysteine endopeptidase, was found in all mouse tissues examined, but was particularly abundant in kidney and placenta. The distribution in subcellular fractions of mouse and rat kidney showed a lysosomal localization, and activity was detectable only after the organelles were disrupted. Nevertheless, ratios of legumain activity to that of cathepsin B differed considerably between mouse tissues. cDNA encoding mouse legumain was cloned and sequenced, the deduced amino acid sequence proving to be 83% identical to that of the human protein [Chen, Dando, Rawlings, Brown, Young, Stevens, Hewitt, Watts and Barrett (1997) J. Biol. Chem. 272, 8090-8098]. Recombinant mouse legumain was expressed in human embryonic kidney 293 cells by use of a vector containing a cytomegalovirus promoter. The recombinant enzyme was partially purified and found to be an asparagine-specific endopeptidase closely similar to naturally occurring pig kidney legumain.

Endocytosed ricin and asialoorosomucoid follow different intracellular pathways in hepatocytes

Earlier studies have suggested that fluid phase endocytosis in rat hepatocytes takes place via a clathrin-independent mechanism [1,2]. This observation suggests that a relatively large amount of plasma membrane outside coated pits may be involved in hepatic endocytosis. Ricin, which binds to galactose residues on glycoproteins and glycolipids, has, in this report, been used as a general marker for the plasma membrane of hepatocytes. The endocytosis of ricin was compared with that of asialoorosomucoid (AOM) which is taken up exclusively via clathrin-coated pits. Hypertonic medium has been shown to inhibit uptake via coated pits more effectively than clathrin-independent uptake [3-5]. It was found, in this study, that the addition of 100 mM sucrose to the incubation medium inhibited the uptake of 125I-tyramine-cellobiose-asialoorosomucoid (125I-TC-AOM) more extensively than that of 125I-tyramine-cellobiose-ricin (125I-TC-ricin), compatible with the notion that the two probes are internalised via different mechanisms. Subcellular fractionation experiments indicated that 125I-TC-ricin entered a denser endocytic organelle than that receiving 125I-TC-AOM. To determine whether the separation of the two probes was due to a different transport kinetics (i.e. that 125I-TC-ricin is transported more rapidly to a later, denser compartment than 125I-TC-AOM) the cells were incubated at 18 degreesC to allow a slower internalisation/transport of the labelled probes. The results obtained showed, again, that the early endosomes containing 125I-TC-ricin were significantly denser than those containing 125I-TC-AOM. We also employed the horseradish peroxidase (HRP)-diaminobenzidine (DAB) density shift technique of Courtoy et al. [6] to determine whether 125I-TC-ricin and 125I-TC-AOM were in separate endosomes early after their uptake. The results showed that early endosomes containing 125I-TC-AOM were density shifted whereas those containing 125I-TC-ricin were unaffected by the density shift procedure. The use of probes labelled with 125I-TC allowed us to identify compartments involved in the degradation of 125I-TC-AOM and 125I-TC-ricin, by measuring acid soluble radioactivities in the gradient fractions. It was found that 125I-TC-ricin was degraded mainly in endosomes, whereas 125I-TC-AOM, as expected, was degraded mainly in lysosomes.

Retrograde traffic out of the yeast vacuole to the TGN occurs via the prevacuolar/endosomal compartment

A large number of trafficking steps occur between the last compartment of the Golgi apparatus (TGN) and the vacuole of the yeast Saccharomyces cerevisiae. To date, two intracellular routes from the TGN to the vacuole have been identified. Carboxypeptidase Y (CPY) travels through a prevacuolar/endosomal compartment (PVC), and subsequently on to the vacuole, while alkaline phosphatase (ALP) bypasses this compartment to reach the same organelle. Proteins resident to the TGN achieve their localization despite a continuous flux of traffic by continually being retrieved from the distal PVC by virtue of an aromatic amino acid-containing sorting motif. In this study we report that a hybrid protein based on ALP and containing this retrieval motif reaches the PVC not by following the CPY sorting pathway, but instead by signal-dependent retrograde transport from the vacuole, an organelle previously thought of as a terminal compartment. In addition, we show that a mutation in VAC7, a gene previously identified as being required for vacuolar inheritance, blocks this trafficking step. Finally we show that Vti1p, a v-SNARE required for the delivery of both CPY and ALP to the vacuole, uses retrograde transport out of the vacuole as part of its normal cellular itinerary.

DR/CLIP (Class II-Associated Invariant Chain Peptides) and DR/Peptide Complexes Colocalize in Prelysosomes in Human B Lymphoblastoid Cells

In APCs, MHC class II molecules (MHC class II) bind antigenic peptides after HLA-DM mediated removal of CLIP. To characterize intracellular sites of peptide loading in human B lymphoblastoid cell lines, we conducted immunoelectron microscopy studies with Abs recognizing MHC class II associated with CLIP or bound peptide, respectively, together with Abs to HLA-DM and endocytic markers. The distribution of these molecules indicates that peptide binding occurs in compartments with characteristics of normal late endosomes, and in compartments that show characteristics of late endosomes, but are not detectably accessed by endocytosed BSA-gold. The latter compartments may represent or give rise to recycling vesicles that deliver peptide-loaded class II molecules to the cell surface. In addition, we have compared cells in which HLA-DM and HLA-DR interaction is defective with cells in which this interaction is intact, and find that DM/DR interaction is not required for the proper localization of either molecule to peptide-loading compartments.

Lysosomal enzyme trafficking between phagosomes, endosomes, and lysosomes in J774 macrophages. Enrichment of cathepsin H in early endosomes

In this study we take advantage of recently developed methods using J774 macrophages to prepare enriched fractions of early endosomes, late endosomes, dense lysosomes, as well as phagosomes of different ages enclosing 1-micron latex beads to investigate the steady state distribution and trafficking of lysosomal enzyme activity between these organelles. At steady state these cells appear to possess four different cellular structures, in addition to phagolysosomes, where acid hydrolases were concentrated. The first site of hydrolase concentration was the early endosomes, which contained the bulk of the cellular cathepsin H. This enzyme was acquired by phagosomes significantly faster than the other hydrolases tested. The second distinct site of lysosomal enzyme concentration was the late endosomes which contain the bulk of cathepsin S. The third and fourth large pools of hydrolases were found in two functionally distinct types of dense lysosomes, only one of which was found to be secreted in the presence of chloroquine or bafilomycin. Among this secreted pool was soluble furin, generally considered only as a membrane-bound trans-Golgi network resident protein. Thus, the organelles usually referred to as "lysosomes" in fact encompass a growing family of highly dynamic but functionally distinct endocytic organelles.

Fusion of lysosomes with late endosomes produces a hybrid organelle of intermediate density and is NSF dependent

Using a cell-free content mixing assay containing rat liver endosomes and lysosomes in the presence of pig brain cytosol, we demonstrated that after incubation at 37 degrees C, late endosome-lysosome hybrid organelles were formed, which could be isolated by density gradient centrifugation. ImmunoEM showed that the hybrids contained both an endocytosed marker and a lysosomal enzyme. Formation of the hybrid organelles appeared not to require vesicular transport between late endosomes and lysosomes but occurred as a result of direct fusion. Hybrid organelles with similar properties were isolated directly from rat liver homogenates and thus were not an artifact of cell-free incubations. Direct fusion between late endosomes and lysosomes was an N-ethylmaleimide-sensitive factor-dependent event and was inhibited by GDP-dissociation inhibitor, indicating a requirement for a rab protein. We suggest that in cells, delivery of endocytosed ligands to an organelle where proteolytic digestion occurs is mediated by direct fusion of late endosomes with lysosomes. The consequences of this fusion to the maintenance and function of lysosomes are discussed.

Different endosomal proteolysis requirements for antigen processing of two T-cell epitopes of the M5 protein from viable Streptococcus pyogenes

We studied endosomal proteolysis of the surface fibrillar M5 protein from viable Streptococcus pyogenes as an essential step involved in major histocompatibility complex class II-restricted antigen processing of two immunodominant CD4(+) T-cell epitopes (17-31/Ed and 308-319/Ad). Intracellular proteolysis of viable streptococci for presentation of 17-31, bound by serine proteinase cleavage sites, was mediated by serine proteinases, whereas processing of soluble recombinant M5 protein required in addition cysteine proteinases. Furthermore, processing of 17-31 was resistant to ammonium chloride and thus was not dependent on endosome acidification. Cysteine and serine proteinase cleavage sites were located adjacent to 308-319, and its processing was dependent on serine, cysteine, and aspartic proteinases, as well as on endosomal acidification. The data suggest that antigen processing of two major T-cell epitopes on streptococcal M5 protein occurred in different endosomal compartments by different classes of intracellular proteinases.

Degradation of phagosomal components in late endocytic organelles

Phagosomes are formed when phagocytic cells ingest particles such as bacteria, viruses or synthetic beads of different kinds. The environment within the phagosome gradually changes to generate degradative conditions. These changes require multiple interactions between the maturing phagosomes and the endocytic and the biosynthetic pathway. The phagosomes probably communicate with endocytic organelles by a transient fusion event, often referred to as the ‘kiss-and-run’ hypothesis. We have studied the role of endocytic organelles in the phagocytic pathway of J774 cells, a mouse macrophage cell line. We have used magnetic Dynabeads coated with 125ITC-IgG and 125ITC-OVA as phagocytic probes and were able to isolate the phagosomal fraction by means of a magnet. To separate lysosomes from other organelles in the endocytic pathway we allowed the cells to endocytose a pulse of colloidal gold particles complexed with ovalbumin. By combining this density shift technique with subcellular fractionation of a postnuclear supernatant in Percoll gradients we could isolate three endocytic fractions corresponding to early endosomes (the light Percoll fraction), late endosomes (the dense Percoll fraction) and lysosomes (the gold fraction). We observed that the proteins linked to the ingested beads are initially cleaved in the phagosomes. This cleavage is inhibited by leupeptin, a thiol-protease inhibitor, and requires an acidic environment. However, efficient communication between the phagosomes and the endocytic pathway leads to the transfer of dissociated phagocytosed peptides of different sizes to late endosomes and lysosomes for further processing. Consequently, the late endosomes and the lysosomes may be involved in the degradation of phagocytosed compounds.

Dense core lysosomes can fuse with late endosomes and are re-formed from the resultant hybrid organelles

Electron microscopy was used to evaluate the function and formation of dense core lysosomes. Lysosomes were preloaded with bovine serum albumin (BSA)-gold conjugates by fluid phase endocytosis using a pulse-chase protocol. The gold particles present in dense core lysosomes and late endosomes were flocculated, consistent with proteolytic degradation of the BSA. A second pulse of BSA-gold also accumulated in the pre-loaded dense core lysosomes at 37 degrees C, but accumulation was reversibly blocked by incubation at 20 degrees C. Time course experiments indicated that mixing of the two BSA-gold conjugates initially occurred upon fusion of mannose 6-phosphate receptor-positive/lysosomal glycoprotein-positive late endosomes with dense core lysosomes. Treatment for 5 hours with wortmannin, a phosphatidyl inositide 3-kinase inhibitor, caused a reduction in number of dense core lysosomes preloaded with BSA-gold and prevented a second pulse of BSA-gold accumulating in them. After wortmannin treatment the two BSA-gold conjugates were mixed in swollen late endosomal structures. Incubation of NRK cells with 0.03 M sucrose resulted in the formation of swollen sucrosomes which were morphologically distinct from preloaded dense core lysosomes and were identified as late endosomes and hybrid endosome-lysosome structures. Subsequent endocytosis of invertase resulted in digestion of the sucrose and re-formation of dense core lysosomes. These observations suggest that dense core lysosomes are biologically active storage granules of lysosomal proteases which can fuse with late endosomes and be re-formed from the resultant hybrid organelles prior to subsequent cycles of fusion and re-formation.

Increased neuronal endocytosis and protease delivery to early endosomes in sporadic Alzheimer's disease: neuropathologic evidence for a mechanism of increased beta-amyloidogenesis

The early endosome is the first vacuolar compartment along the endocytic pathway. It is the site of internalization and initial processing of amyloid precursor protein (APP) and apolipoprotein E (ApoE), two proteins of etiological importance in Alzheimer's disease, and a putative site of beta-amyloid peptide (Abeta) formation. Here, we identify early endosomes in human pyramidal neurons, using specific compartmental markers and morphometry, and show that in Alzheimer's disease individual endosomes display up to 32-fold larger volumes than the normal average. Endosomal enlargement contributed to an average 2.5-fold larger total endosomal volume per neuron, implying a marked increase in endocytic activity. Endosomal alterations were evident in most pyramidal neurons in Alzheimer brain, detectable at early stages of the disease but absent in several other neurodegenerative disorders examined. In addition, mature and proenzyme forms of the proteases cathepsin B and cathepsin D, a candidate APP secretase, were identified in most early endosomes in Alzheimer brains but were detectable in only a minor proportion of endosomes in normal brain. Expression of the cation-dependent 46 kDa mannose 6-phosphate receptor was elevated in pyramidal neurons of Alzheimer brains, which could be a possible basis for the altered cathepsin trafficking pattern. Enhanced endocytic activity, coupled with increased trafficking to endosomes of proteases, which may have the ability under pathological conditions to generate Abeta, constitutes a potential mechanism by which beta-amyloidogenesis may become accelerated in sporadic AD and also be subject to influences by ApoE.

Increased neuronal endocytosis and protease delivery to early endosomes in sporadic Alzheimer's disease: neuropathologic evidence for a mechanism of increased beta-amyloidogenesis

The early endosome is the first vacuolar compartment along the endocytic pathway. It is the site of internalization and initial processing of amyloid precursor protein (APP) and apolipoprotein E (ApoE), two proteins of etiological importance in Alzheimer's disease, and a putative site of beta-amyloid peptide (Abeta) formation. Here, we identify early endosomes in human pyramidal neurons, using specific compartmental markers and morphometry, and show that in Alzheimer's disease individual endosomes display up to 32-fold larger volumes than the normal average. Endosomal enlargement contributed to an average 2.5-fold larger total endosomal volume per neuron, implying a marked increase in endocytic activity. Endosomal alterations were evident in most pyramidal neurons in Alzheimer brain, detectable at early stages of the disease but absent in several other neurodegenerative disorders examined. In addition, mature and proenzyme forms of the proteases cathepsin B and cathepsin D, a candidate APP secretase, were identified in most early endosomes in Alzheimer brains but were detectable in only a minor proportion of endosomes in normal brain. Expression of the cation-dependent 46 kDa mannose 6-phosphate receptor was elevated in pyramidal neurons of Alzheimer brains, which could be a possible basis for the altered cathepsin trafficking pattern. Enhanced endocytic activity, coupled with increased trafficking to endosomes of proteases, which may have the ability under pathological conditions to generate Abeta, constitutes a potential mechanism by which beta-amyloidogenesis may become accelerated in sporadic AD and also be subject to influences by ApoE.

The renal and hepatic distribution of Bence Jones proteins depends on glycosylation: a scintigraphic study in rats

The aim of the present study was to evaluate renal and liver distribution of two monoclonal immunoglobulin light chains. The chains were purified individually from the urine of patients with multiple myeloma and characterized as lambda light chains with a molecular mass of 28 kDa. They were named BJg (high amount of galactose residues exposed) and BJs (sialic acid residues exposed) on the basis of carbohydrate content. A scintigraphic study was performed on male Wistar rats weighing 250 g for 60 min after i.v. administration of 1 mg of each protein (7.4 MBq), as the intact proteins and also after carbohydrate oxidation. Images were obtained with a Siemens gamma camera with a high-resolution collimator and processed with a MicroDelta system. Hepatic and renal distribution were established and are reported as percent of injected dose. Liver uptake of BJg was significantly higher than liver uptake of BJs (94.3 vs 81.4%) (P < 0.05). This contributed to its greater removal from the intravascular compartment, and consequently lower kidney accumulation of BJg in comparison to BJs (5.7 vs 18.6%) (P < 0.05). After carbohydrate oxidation, there was a decrease in hepatic accumulation of both proteins and consequently a higher renal overload. The tissue distribution of periodate-treated BJg was similar to that of native BJs: 82.7 vs 81.4% in the liver and 17.3 vs 18.6% in the kidneys. These observations indicate the important role of sugar residues of Bence Jones proteins for their recognition by specific membrane receptors, which leads to differential tissue accumulation and possible toxicity.

Mis-sorting of procathepsin D in metastogenic tumor cells is not due to impaired synthesis of the phosphomannosyl signal

Synthesis, processing and sorting of pro-cathepsin D (proCD), the precursor of a lysosomal protease involved in tumor-cell proliferation and invasion, were compared in various human tumor cell lines. In cultures of HepG2, HT29 and MCF7 cells the extracellular CD activity was up to 10 times higher than in cultures of U937 and MG63 cells. Ammonium chloride, which disrupts the pH-dependent receptor-mediated transport of lysosomal enzymes, exerted a differential effect on the secretion of CD activity in these cells. As judged by metabolic labeling and immunoprecipitation, the secretion of CD synthesized in 24 hr in MG63 and U937 cells was enhanced 4- to 10-fold by ammonium chloride, while it was only slightly increased in HepG2, HT29 and MCF7 cells. In all tumor cells examined, a portion of proCD was segregated into the endosomal-lysosomal pathway in the presence of ammonium chloride. However, the intralysosomal maturation of CD was only inhibited by this drug in HT29 and MCF7 cells. The Golgi-associated processing of proCD, leading to the synthesis of uncovered phosphomannosyl groups, proceeded with comparable efficiency in all tumor cells examined. These results suggest that hypersecretion of proCD in HepG2, HT29 and MCF7 cells is linked to transport mechanisms, as yet unidentified, which are independent of mannose-6-phosphate.

Uptake and intracellular stability of glycosylphosphatidylinositol-specific phospholipase D in neuroblastoma cells

Glycosylphosphatidylinositol-specific phospholipase D from mammalian serum has been described to be relatively stable towards the action of proteases in vitro, and it has been speculated that the enzyme may only be active on glycosylphosphatidylinositol-anchored substrates after its proteolytic processing in an intracellular compartment following uptake from body fluids. To test this hypothesis, we studied the possible uptake and intracellular processing of purified glycosylphosphatidylinositol-specific phospholipase D into the mouse neuroblastoma cell line N2A. We found that after incubation of neuroblastoma cells with glycosylphosphatidylinositol-specific phospholipase D at 37 degrees C the amount of cell-associated glycosylphosphatidylinositol-specific phospholipase D activity increased in a concentration- and time-dependent way. A similar uptake was also observed with 125I-labeled intact and trypsin-treated form of glycosylphosphatidylinositol-specific phospholipase D. We found that the incorporated radiolabeled proteins were processed intracellularly to distinct low molecular mass products, and that this process was in part inhibited by the presence of chloroquine during incubation.

A population of rat liver lysosomes responsible for the selective uptake and degradation of cytosolic proteins

Two populations of rat liver lysosomes can be distinguished on the basis of their density. A major difference between these populations is that one contains the heat shock cognate protein of 73 kDa (hsc73) within the lysosomal lumen. The lysosomal fraction containing hsc73 exhibits much higher efficiencies in the in vitro uptake and degradation of glyceraldehyde-3-phosphate dehydrogenase and ribonuclease A, two well established substrates of the selective lysosomal pathway of intracellular protein degradation. Preloading of the lysosomal population that is devoid of lumenal hsc73 with hsc73 isolated from cytosol activated the selective transport of substrate proteins into these lysosomes. Furthermore, treatment of animals with leupeptin, an inhibitor of lysosomal cathepsins, or 88 h of starvation also increased the amount of hsc73 within their lysosomal lumen, and these in vivo treatments also activated the selective transport of substrate proteins in vitro. Thus, the hsc73 located within lysosomes appears to be required for efficient uptake of cytosolic proteins by these organelles. The difference in hsc73 content between the lysosomal populations appears to be due to differences in their ability to take up hsc73 combined with differences in the intralysosomal degradation rates of hsc73. The increased stability of hsc73 in one population of lysosomes is primarily a consequence of this lysosomal population's more acidic pH.

Differential targeting and processing of procathepsin D in normal and transformed murine 3T3 fibroblasts

The kinetics of transport and the processing of procathepsin D (proCD), the precursor of a lysosomal aspartyl protease involved in tumor-cell proliferation and metastasis, were compared in normal and SV-40- or benzo[a]pyrene-transformed 3T3 mouse fibroblasts. Sorting of newly synthesized proCD in normal cells was almost complete within 3 hr, while in transformed cells a fraction of the precursor survives a long time. In both normal and transformed 3T3 cultures, secretion of proCD started at 3 hr of chase. However, in normal cells secretion of proCD remained constant between 3 and 24 hr of chase, while in transformed cells it increased along with the chase incubation. The efficiency of formation of the mannose-6-phosphate group on proCD varied among the 3 cell types, being minimal in benzo[a]pyrene-transformed 3T3 cells. Ammonium chloride, a drug known to disrupt the segregation and to enhance the secretion of lysosomal proenzymes, was 2-fold more effective in normal than in transformed 3T3 cells. Despite vacuolar alkalinization, about one third of proCD was segregated into the endosomal-lysosomal pathway in normal and in transformed 3T3 fibroblasts, indicating the existence in these cells of alternative, mannose-6-phosphate receptor-independent mechanisms for targeting proCD. Thus, while hypersecretion of proCD and reduced sensitivity to vacuolar alkalinization are common features of both transformed cell types, the mechanisms responsible for inefficient segregation of proCD may differ between virally and chemically transformed 3T3 cells.

Isolation and characterization of early endosomes, late endosomes and terminal lysosomes: their role in protein degradation

Although endosomal proteolysis has been reported (e.g. for peptide hormones and lysosomal enzymes), lysosomes are believed to be the main site of degradation in the endocytic pathway. We have studied the separate roles of lysosomes and prelysosomal endocytic organelles in the degradation of ovalbumin in J774 cells. The ovalbumin was labelled with 125I-tyramine cellobiose (125I-TC-ova). The labelled degradation products formed from this probe are trapped at the site of formation. To separate lysosomes efficiently from prelysosomal endocytic organelles we allowed the cells to endocytose a pulse of colloidal gold particles complexed with ovalbumin. By combining this density shift technique with subcellular fractionation of a postnuclear supernatant in Percoll gradients we could isolate three fractions that were sequentially involved in the endocytic pathway: a light Percoll fraction, a dense Percoll fraction and a gold fraction. The light Percoll fraction contained early endosomes since it was transferrin positive and received endocytic markers such as ovalbumin and horseradish peroxidase (HRP) early (&lt; 5 minutes) after internalization. The dense Percoll fraction was transferrin negative, rab7 positive and received endocytic markers after 10–15 minutes of internalization. The gold-filled fraction was negative for both transferrin and rab7 but highly enriched in the lysosomal enzyme beta-hexosaminidase and was therefore defined as a lysosome. To study the role of endosomes and lysosomes in the degradation of endocytosed material we allowed the cells to take up (via the mannose receptor) 125I-TC-ova. It was found that the main degradation of 125I-TC-ova (measured as acid soluble radioactivity trapped in the organelle) took place in the late endosomes (and not in the lysosomes containing the bulk of the lysosomal enzymes). Our data therefore suggest that the late endosomes operate as an early lysosomal compartment. The terminal lysosomes may serve as storage bodies for acid hydrolases that may be called upon when needed (for instance during phagocytosis).

Separation of proteases: old and new approaches

All methods of protein separations can be applied to proteases. Some emphasis is put in this review on a powerful technique specific to proteases purification: cyclic peptide antibiotics may be seen as general affinity ligands for proteases. Also, some examples of affinity chromatography of proteases on ligands with narrower specificity are given. The special interest of hydrophobic interaction chromatography for proteases purification is discussed. The merits of immobilized dye chromatography for proteases purification and the interest in empirically screening many immobilized dyes, as well as several eluents are discussed.

The fates of proteins in cells

Nascent polypeptide chains are in a dangerous situation as soon as they leave their place of birth, the channel of the large ribosomal subunit: more than 20 different pathways for the degradation of proteins exist in cells. Chaperones protect and guide the young protein molecules and support their correct foldings. Targeting signals direct the proteins to the organelles of their destination. The lysosome is the site of random degradation, while the proteasome is highly selective. Although these two organelles provide the most important pathways for the degradation of long- and short-lived proteins, other pathways with roles in deciding the fate of cellular proteins must also be considered.