Sequential processing of lysosomal acid phosphatase by a cytoplasmic thiol proteinase and a lysosomal aspartyl proteinase

Unconventional Trafficking of Mammalian Phospholipase D3 to Lysosomes

Variants in the phospholipase D3 (PLD3) gene have genetically been linked to late-onset Alzheimer's disease. We present a detailed biochemical analysis of PLD3 and reveal its endogenous localization in endosomes and lysosomes. PLD3 reaches lysosomes as a type II transmembrane protein via a (for mammalian cells) uncommon intracellular biosynthetic route that depends on the ESCRT (endosomal sorting complex required for transport) machinery. PLD3 is sorted into intraluminal vesicles of multivesicular endosomes, and ESCRT-dependent sorting correlates with ubiquitination. In multivesicular endosomes, PLD3 is subjected to proteolytic cleavage, yielding a stable glycosylated luminal polypeptide and a rapidly degraded N-terminal membrane-bound fragment. This pathway closely resembles the delivery route of carboxypeptidase S to the yeast vacuole. Our experiments reveal a biosynthetic route of PLD3 involving proteolytic processing and ESCRT-dependent sorting for its delivery to lysosomes in mammalian cells.

Acid phosphatase 2 (ACP2) is required for membrane fusion during influenza virus entry

Influenza viruses exploit host factors to successfully replicate in infected cells. Using small interfering RNA (siRNA) technology, we identified six human genes required for influenza A virus (IAV) replication. Here we focused on the role of acid phosphatase 2 (ACP2), as its knockdown showed the greatest inhibition of IAV replication. In IAV-infected cells, depletion of ACP2 resulted in a significant reduction in the expression of viral proteins and mRNA, and led to the attenuation of virus multi-cycle growth. ACP2 knockdown also decreased replication of seasonal influenza A and B viruses and avian IAVs of the H7 subtype. Interestingly, ACP2 depletion had no effect on the replication of Ebola or hepatitis C virus. Because ACP2 is known to be a lysosomal acid phosphatase, we assessed the role of ACP2 in influenza virus entry. While neither binding of the viral particle to the cell surface nor endosomal acidification was affected in ACP2-depleted cells, fusion of the endosomal and viral membranes was impaired. As a result, downstream steps in viral entry were blocked, including nucleocapsid uncoating and nuclear import of viral ribonucleoproteins. Our results established ACP2 as a necessary host factor for regulating the fusion step of influenza virus entry.

Spatial and temporal expression of lysosomal acid phosphatase 2 (ACP2) reveals dynamic patterning of the mouse cerebellar cortex

The Acp2 gene encodes lysosomal acid phosphatase 2 (ACP2), an isoenzyme that hydrolyzes orthophosphoric monoesters to alcohol and phosphate. Mutations in this gene compromise lysosomal function and cause acid phosphatase deficiency. Loss of Acp2 in the brain causes defects in the cerebellum. Here, we performed an in-depth protein expression analysis in the mouse cerebellum to understand how Acp2 controls cellular function in the developing and adult brain. We have found that during development, ACP2 expression marks the caudal midbrain and cerebellum, two regions that are linked by multiple signaling mechanisms during embryogenesis. By around P8, ACP2 was localized predominantly to the somata of Purkinje cells, the principal neurons of the cerebellar cortex. During the second postnatal week, we found that ACP2 expression expanded into the dendrites and axon terminals of Purkinje cells. However, at 2 weeks of age, only a subset of Purkinje cells strongly express ACP2. Further expression analyses revealed that in the mature cerebellum, ACP2 expression divided Purkinje cells into a pattern of molecular zones that are associated with the functional topography of sensory-motor circuitry. These data suggest that ACP2 expression is dynamically regulated during development, and in the adult, it may function within a complex architecture that is linked to cerebellar modular organization.

Purkinje cell compartmentation in the cerebellum of the lysosomal Acid phosphatase 2 mutant mouse (nax - naked-ataxia mutant mouse)

The Acp2 gene encodes the beta subunit of lysosomal acid phosphatase, which is an isoenzyme that hydrolyzes orthophosphoric monoesters. In mice, a spontaneous mutation in Acp2 results in severe cerebellar defects. These include a reduced size, abnormal lobulation, and an apparent anterior cerebellar disorder with an absent or hypoplastic vermis. Based on differential gene expression in the cerebellum, the mouse cerebellar cortex can normally be compartmentalized anteroposteriorly into four transverse zones and mediolaterally into parasagittal stripes. In this study, immunohistochemistry was performed using various Purkinje cell compartmentation markers to examine their expression patterns in the Acp2 mutant. Despite the abnormal lobulation and anterior cerebellar defects, zebrin II and PLCβ4 showed similar expression patterns in the nax mutant and wild type cerebellum. However, fewer stripes were found in the anterior zone of the nax mutant, which could be due to a lack of Purkinje cells or altered expression of the stripe markers. HSP25 expression was uniform in the central zone of the nax mutant cerebellum at around postnatal day (P) 18–19, suggesting that HSP25 immunonegative Purkinje cells are absent or delayed in stripe pattern expression compared to the wild type. HSP25 expression became heterogeneous around P22–23, with twice the number of parasagittal stripes in the nax mutant compared to the wild type. Aside from reduced size and cortical disorganization, both the posterior zone and nodular zone in the nax mutant appeared less abnormal than the rest of the cerebellum. From these results, it is evident that the anterior zone of the nax mutant cerebellum is the most severely affected, and this extends beyond the primary fissure into the rostral central zone/vermis. This suggests that ACP2 has critical roles in the development of the anterior cerebellum and it may regulate anterior and central zone compartmentation.

Structure of Acid phosphatases

Acid phosphatases are enzymes that have been studied extensively due to the fact that their dysregulation is associated with pathophysiological conditions. This characteristic has been exploited for the development of diagnostic and therapeutic methods. As an example, prostatic acid phosphatase was the first marker for metastatic prostate cancer diagnosis and the dysregulation of tartrate resistant acid phosphatase is associated with abnormal bone resorption linked to osteoporosis. The pioneering crystallization studies on prostatic acid phosphatase and mammalian tartrate-resistant acid phosphatase conformed significant milestones towards the elucidation of the mechanisms followed by these enzymes (Schneider et al., EMBO J 12:2609-2615, 1993). Acid phosphatases are also found in nonmammalian species such as bacteria, fungi, parasites, and plants, and most of them share structural similarities with mammalian acid phosphatase enzymes. Acid phosphatase (EC 3.1.3.2) enzymes catalyze the hydrolysis of phosphate monoesters following the general equation. Phosphate monoester + H2O -->/<-- alcohol + phosphate. The general classification "acid phosphatase" relies only on the optimum acidic pH for the enzymatic activity in assay conditions using non-physiological substrates. These enzymes accept a wide range of substrates in vitro, ranging from small organic molecules to phosphoproteins, constituting a heterogeneous group of enzymes from the structural point of view. These structural differences account for the divergence in cofactor dependences and behavior against substrates, inhibitors, and activators. In this group only the tartrate-resistant acid phosphatase is a metallo-enzyme whereas the other members do not require metal-ion binding for their catalytic activity. In addition, tartrate-resistant acid phosphatase and erythrocytic acid phosphatase are not inhibited by L-(+)-tartrate ion while the prostatic acid phosphatase is tartrate-sensitive. This is an important difference that can be exploited in in vitro assays to differentiate between different kinds of phosphatase activity. The search for more sensitive and specific methods of detection in clinical laboratory applications led to the development of radioimmunoassays (RIA) for determination of prostatic acid phosphatase in serum. These methods permit the direct quantification of the enzyme regardless of its activity status. Therefore, an independent structural classification exists that helps to group these enzymes according to their structural features and mechanisms. Based on this we can distinguish the histidine acid phosphatases (Van Etten, Ann N Y Acad Sci 390:27-51, 1982), the low molecular weight protein tyrosine acid phosphatases and the metal-ion dependent phosphatases. A note of caution is worthwhile mentioning here. The nomenclature of acid phosphatases has not been particularly easy for those new to the subject. Unfortunately, the acronym PAP is very common in the literature about purple acid phosphatases and prostatic acid phosphatase. In addition, LPAP is the acronym chosen to refer to the lysophosphatidic acid phosphatase which is a different enzyme. It is important to bear in mind this distinction while reviewing the literature to avoid confusion.

Proteolytic cleavage of the disease-related lysosomal membrane glycoprotein CLN7

CLN7 is a polytopic lysosomal membrane glycoprotein of unknown function and is deficient in variant late infantile neuronal ceroid lipofuscinosis. Here we show that full-length CLN7 is proteolytically cleaved twice, once proximal to the used N-glycosylation sites in lumenal loop L9 and once distal to these sites. Cleavage occurs by cysteine proteases in acidic compartments and disruption of lysosomal targeting of CLN7 results in inhibition of proteolytic cleavage. The apparent molecular masses of the CLN7 fragments suggest that both cleavage sites are located within lumenal loop L9. The known disease-causing mutations, p.T294K and p.P412L, localized in lumenal loops L7 and L9, respectively, did not interfere with correct lysosomal targeting of CLN7 but enhanced its proteolytic cleavage in lysosomes. Incubation of cells with selective cysteine protease inhibitors and expression of CLN7 in gene-targeted mouse embryonic fibroblasts revealed that cathepsin L is required for one of the two proteolytic cleavage events. Our findings suggest that CLN7 is inactivated by proteolytic cleavage and that enhanced CLN7 proteolysis caused by missense mutations in selected luminal loops is associated with disease.

Mannose 6 dephosphorylation of lysosomal proteins mediated by acid phosphatases Acp2 and Acp5

Mannose 6-phosphate (Man6P) residues represent a recognition signal required for efficient receptor-dependent transport of soluble lysosomal proteins to lysosomes. Upon arrival, the proteins are rapidly dephosphorylated. We used mice deficient for the lysosomal acid phosphatase Acp2 or Acp5 or lacking both phosphatases (Acp2/Acp5(-/-)) to examine their role in dephosphorylation of Man6P-containing proteins. Two-dimensional (2D) Man6P immunoblot analyses of tyloxapol-purified lysosomal fractions revealed an important role of Acp5 acting in concert with Acp2 for complete dephosphorylation of lysosomal proteins. The most abundant lysosomal substrates of Acp2 and Acp5 were identified by Man6P affinity chromatography and mass spectrometry. Depending on the presence of Acp2 or Acp5, the isoelectric point of the lysosomal cholesterol-binding protein Npc2 ranged between 7.0 and 5.4 and may thus regulate its interaction with negatively charged lysosomal membranes at acidic pH. Correspondingly, unesterified cholesterol was found to accumulate in lysosomes of cultured hepatocytes of Acp2/Acp5(-/-) mice. The data demonstrate that dephosphorylation of Man6P-containing lysosomal proteins requires the concerted action of Acp2 and Acp5 and is needed for hydrolysis and removal of degradation products.

Characterization of acid phosphatase from the tick Haemaphysalis longicornis

The full-length cDNA encoding acid phosphatase (HL-3) from Haemaphysalis longicornis was obtained by 5' rapid amplification of cDNA ends (RACE). The cDNA contained a 1137 bp open reading frame (ORF) coding for 356 amino acids with a predicted theoretical isoelectric point (pI) of 6.35 and molecular weight of 41.0 kDa. The recombinant protein was expressed in Escherichia coli. The enzyme could hydrolyze para-nitrophenyl phosphate (pNPP) substrate at an optimum pH of 5.0. Real-time RT-PCR analysis showed that the HL-3 transcripts were expressed in various stages of unfed ticks and were significantly induced by blood feeding. Furthermore, the expression of HL-3 in midguts was significantly higher than in other tested tissues of partially fed adult ticks. The transcripts of the HL-3 mRNA in lipopolysaccharide (LPS)-injected ticks were 1.75 times of the PBS-injected control; Theileria sergenti infected larvae expressed 3.86 more times than that of uninfected ones. Western blot analysis showed that rabbit antiserum against the recombinant rHL-3 could recognize a native protein of approximately 41.0 kDa in the lysates from different stages of ticks. Vaccination of rabbits with the rHL-3 conferred partial protective immunity against ticks, resulting in 28% mortality and 10.6% reduction in engorgement weight of adult ticks, respectively. These results suggested that the HL-3 was involved in tick innate immunity and could be used as a potential candidate antigen for the development of anti-tick vaccines.

Analysis of protein transport to lysosomes

Lysosomes are terminal degradative organelles that are found in all higher eukaryotic cells. The biogenesis of lysosomes involves the transport of various acid hydrolases and transmembrane glycoproteins from their site of synthesis in the endoplasmic reticulum through the biosynthetic and endocytic pathways. Protein transport to lysosomes can be studied by a combination of techniques based on the separation of intracellular organelles. Percoll density gradient centrifugation has long been the method of choice for separating lysosomes from other organelles in cell homogenates, and accordingly, this unit describes protocols for obtaining reasonably pure lysosomal fractions from mammalian cells using Percoll density gradient separation.

Increased expression of lysosomal acid phosphatase in CLN3-defective cells and mouse brain tissue

Juvenile neuronal ceroid lipofuscinosis (Batten disease) is a neurodegenerative disorder caused by defective function of the lysosomal membrane glycoprotein CLN3. The activity of the lysosomal acid phosphatase (LAP/ACP2) was found to be significantly increased in the cerebellum and brain stem of Cln3-targeted mice during the early stages of postnatal life. Histochemical localization studies revealed an increased LAP/ACP2 staining intensity in neurons of the cerebral cortex of 48-week-old Cln3-targeted mice as compared with controls. Additionally, the expression of another lysosomal membrane protein LAMP-2 was increased in all brain areas. Knockdown of CLN3 expression in HeLa cells by RNA interference also resulted in increased LAP/ACP2 and LAMP-2 expression. Finally in fibroblasts of two juvenile neuronal ceroid lipofuscinosis patients elevated levels of LAP/ACP2 were found. Both activation of gene transcription and increased protein half-life appear to contribute to increased LAP/ACP2 protein expression in CLN3-deficient cells. The data suggest that lysosomal dysfunction and accumulation of storage material require increased biogenesis of LAP/ACP2 and LAMP-2 positive membranes which makes LAP/ACP2 suitable as biomarker of Batten disease.

A dileucine motif and a cluster of acidic amino acids in the second cytoplasmic domain of the batten disease-related CLN3 protein are required for efficient lysosomal targeting

The juvenile form of ceroid lipofuscinosis (Batten disease) is a neurodegenerative lysosomal storage disorder caused by mutations in the CLN3 gene. CLN3 encodes a multimembrane-spanning protein of unknown function, which is mainly localized in lysosomes in non-neuronal cells and in endosomes in neuronal cells. For this study we constructed chimeric proteins of three CLN3 cytoplasmic domains fused to the lumenal and transmembrane domains of the reporter proteins LAMP-1 and lysosomal acid phosphatase to identify lysosomal targeting motifs and to determine the intracellular transport and subcellular localization of the chimera in transfected cell lines. We report that a novel type of dileucine-based sorting motif, EEEX(8)LI, present in the second cytoplasmic domain of CLN3, is sufficient for proper targeting to lysosomes. The first cytoplasmic domain of CLN3 and the mutation of the dileucine motif resulted in a partial missorting of chimeric proteins to the plasma membrane. At equilibrium, 4-13% of the different chimera are present at the cell surface. Analysis of lysosome-specific proteolytic processing revealed that lysosomal acid phosphatase chimera containing the second cytoplasmic domain of CLN3 showed the highest rate of lysosomal delivery, whereas the C terminus of CLN3 was found to be less efficient in lysosomal targeting. However, none of these cytosolic CLN3 domains was able to interact with AP-1, AP-3, or GGA3 adaptor complexes. These data revealed that lysosomal sorting motifs located in an intramolecular cytoplasmic domain of a multimembrane-spanning protein have different structural requirements for adaptor binding than sorting signals found in the C-terminal cytoplasmic domains of single- or dual-spanning lysosomal membrane proteins.

Mutation in the gene encoding lysosomal acid phosphatase (Acp2) causes cerebellum and skin malformation in mouse

We report a novel spontaneous mutation named nax in mice, which exhibit delayed hair appearance and ataxia in a homozygote state. Histological analyses of nax brain revealed an overall impairment of the cerebellar cortex. The classical cortical cytoarchitecture was disrupted, the inner granule cell layer was not obvious, the Purkinje cells were not aligned as a Purkinje cell layer, and Bergmann glias did not span the molecular layer. Furthermore, histological analyses of skin showed that the hair follicles were also abnormal. We mapped the nax locus between marker D2Mit158 and D2Mit100 within a region of 800 kb in the middle of chromosome 2 and identified a missense mutation (Gly244Glu) in Acp2, a lysosomal monoesterase. The Glu244 mutation does not affect the stability of the Acp2 transcript, however it renders the enzyme inactive. Ultrastructural analysis of nax cerebellum showed lysosomal storage bodies in nucleated cells, suggesting progressive degeneration as the underlying mechanism. Identification of Acp2 as the gene mutated in nax mice provides a valuable model system for studying the role of Acp2 in cerebellum and skin homeostasis.

Todarepsin, a new cathepsin D from hepatopancreas of Japanese common squid (Todarodes pacificus)

An intracellular aspartic proteinase obtained from the hepatopancreas (liver) of Japanese common squid (Todarodes pacificus) was purified to homogeneity. The molecular mass of the enzyme was 36,500 Da on SDS-PAGE, and the isoelectric point was 8.29 by isoelectric focusing. The enzyme activity was optimal at pH 3.5, pH 2.2 and pH 3.0 for the substrates acid-denatured hemoglobin, acid-denatured casein, and MOCAc-GKPILFFRLK(Dnp)-D-R-NH2, respectively. Enzyme activity decreased rapidly at 50 degrees C. The Km and kcat values of the enzyme were estimated to be 3.2 mM and 46 s(-1) with MOCAc-GKPILFFRLK(Dnp)-D-R-NH2, and 1.7 mM and 1.1 s(-1) with MOCAc-SEVNLDAEFRK(Dnp)RR-NH2. The enzyme activity was strongly inhibited by pepstatin A, but only partially inhibited by DAN and EPNP. The Ki values for pepstatin A, DAN and EPNP were 0.5 nM, 0.5 mM and 0.2 mM, respectively. A cDNA encoding the enzyme was cloned by RT-PCR and subjected to nucleotide sequencing. The entire open reading frame was 1179 bp and coded for a protein of 392 amino acid residues. The mature enzyme consisted of 334 amino acids. The deduced amino acid sequence of the enzyme showed a high degree of identity to the sequences of cathepsins D found in various species.

Food vacuole plasmepsins are processed at a conserved site by an acidic convertase activity in Plasmodium falciparum

Intraerythrocytic Plasmodium falciparum digests vast amounts of hemoglobin within an acidic food vacuole (FV). Four homologous aspartic proteases participate in hemoglobin degradation within the FV. Plasmepsin (PM) I and II are thought to initiate degradation of the native hemoglobin molecule. PM IV and histo-aspartic protease (HAP) act on denatured globin further downstream in the pathway. PM I and II have been shown to be synthesized as zymogens and activated by proteolytic removal of a propiece. In this study, we have determined that the proteolytic processing of FV plasmepsins occurs immediately after a conserved Leu-Gly dipeptidyl motif with uniform kinetics and pH and inhibitor sensitivities. We have developed a cell-free in vitro processing assay that generates correctly processed plasmepsins. Our data suggest that proplasmepsin processing is not autocatalytic, but rather is mediated by a separate processing enzyme. This convertase requires acidic conditions and is blocked only by the calpain inhibitors, suggesting that it may be an atypical calpain-like protease.

Receptor downregulation and multivesicular-body sorting

The sorting of proteins into the inner vesicles of multivesicular bodies is required for many key cellular processes, which range from the downregulation of activated signalling receptors to the proper stimulation of the immune response. Recent advances in our understanding of the multivesicular-body sorting pathway have resulted from the identification of ubiquitin as a signal for the efficient sorting of proteins into this transport route, and from the discovery of components of the sorting and regulatory machinery that directs this complex process.

Intracellular distribution of lysosomal sialidase is controlled by the internalization signal in its cytoplasmic tail

Sialidase (neuraminidase), encoded by the neu-1 gene in the major histocompatibility complex locus catalyzes the intralysosomal degradation of sialylated glycoconjugates. Inherited deficiency of sialidase results in sialidosis or galactosialidosis, both severe metabolic disorders associated with lysosomal storage of oligosaccharides and glycopeptides. Sialidase also plays an important role in cellular signaling and is specifically required for the production of cytokine interleukin-4 by activated T lymphocytes. In these cells, neu-1-encoded sialidase activity is increased on the cell surface, suggesting that a specific mechanism regulates sorting of this enzyme to the plasma membrane. We investigated that mechanism by first showing that sialidase contains the internalization signal found in lysosomal membrane proteins targeted to endosomes via clathrin-coated pits. The signal consists of a C-terminal tetrapeptide (412)YGTL(415), with Tyr(412) and Leu(415) essential for endocytosis of the enzyme. We further demonstrated that redistribution of sialidase from lysosomes to the cell surface of activated lymphocytes is accompanied by increased reactivity of the enzyme with anti-phosphotyrosine antibodies. We speculate that phosphorylation of Tyr(412) results in inhibition of sialidase internalization in activated lymphocytes.

Overlapping functions of lysosomal acid phosphatase (LAP) and tartrate-resistant acid phosphatase (Acp5) revealed by doubly deficient mice

To date, two lysosomal acid phosphatases are known to be expressed in cells of the monocyte/phagocyte lineage: the ubiquitously expressed lysosomal acid phosphatase (LAP) and the tartrate-resistant acid phosphatase-type 5 (Acp5). Deficiency of either acid phosphatase results in relatively mild phenotypes, suggesting that these enzymes may be capable of mutual complementation. This prompted us to generate LAP/Acp5 doubly deficient mice. LAP/Acp5 doubly deficient mice are viable and fertile but display marked alterations in soft and mineralised tissues. They are characterised by a progressive hepatosplenomegaly, gait disturbances and exaggerated foreshortening of long bones. Histologically, these animals are distinguished by an excessive lysosomal storage in macrophages of the liver, spleen, bone marrow, kidney and by altered growth plates. Microscopic analyses showed an accumulation of osteopontin adjacent to actively resorbing osteoclasts of Acp5- and LAP/Acp5-deficient mice. In osteoclasts of phosphatase-deficient mice, vacuoles were frequently found which contained fine filamentous material. The vacuoles in Acp5- and LAP/Acp5 doubly-deficient osteoclasts also contained crystallite-like features, as well as osteopontin, suggesting that Acp5 is important for processing of this protein. This is further supported by biochemical analyses that demonstrate strongly reduced dephosphorylation of osteopontin incubated with LAP/Acp5-deficient bone extracts. Fibroblasts derived from LAP/Acp5 deficient embryos were still able to dephosphorylate mannose 6-phosphate residues of endocytosed arylsulfatase A. We conclude that for several substrates LAP and Acp5 can substitute for each other and that these acid phosphatases are essential for processing of non-collagenous proteins, including osteopontin, by osteoclasts.

The AP-3-dependent targeting of the melanosomal glycoprotein QNR-71 requires a di-leucine-based sorting signal

The Quail Neuroretina clone 71 gene (QNR-71) is expressed during the differentiation of retinal pigmented epithelia and the epidermis. It encodes a type I transmembrane glycoprotein that shares significant sequence homologies with several melanosomal proteins. We have studied its intracellular traffic in both pigmented and non-pigmented cells. We report that a di-leucine-based sorting signal (ExxPLL) present in the cytoplasmic domain of QNR-71 is necessary and sufficient for its proper targeting to the endosomal/premelanosomal compartments of both pigmented and non-pigmented cells. The intracellular transport of QNR-71 to these compartments is mediated by the AP-3 assembly proteins. As previously observed for the lysosomal glycoproteins LampI and LimpII, overexpression of QNR-71 increases the amount of AP-3 associated with membranes, and inhibition of AP-3 synthesis increases the routing of QNR-71 towards the cell surface. In addition, expression of QNR-71 induces a misrouting of endogenous LampI to the cell surface. Thus, the targeting of QNR-71 might be similar to that of the lysosomal integral membrane glycoproteins LampI and LimpII. This suggests that sorting to melanosomes and lysosomes requires similar sorting signals and transport machineries.

Cytochemical analysis of acid phosphatase activity in the venom secretory cells of Bothrops jararaca

A study of the histochemical reaction for acid phosphatase (AcPase) in venom gland secretory cells from Bothrops jararaca was done to investigate the distribution of lysosomes and related structures in stages of high- and low-protein synthesis. From this analysis, it was expected to gain insight into the cellular pathway by which AcPase is secreted into the venom. Two subtypes of AcPase reactivities were detected in the venom gland secretory cells: one was found in lysosomes and related structures and in some trans-Golgi network (TGN) elements and reacts with beta-glycerophosphate (betaGP) as substrate; the other was found in secretory vesicles, apical plasmalemma, lysosomes and related structures, and in some TGN elements, and reacts with cytidine monophosphate (CMP). The results are compatible with the possibility that there is a secretory via for AcPase in the venom gland of B. jararaca and that the elements composing this pathway are noted only when CMP is used as substrate. Large autophagosomes reactive to both betaGP and to CMP were commonly observed in the basal region of the secretory cells, and they were more abundant in the glands during the stage of low activity of protein synthesis.

Unique properties of lamp2a compared to other lamp2 isoforms

Lamp2a acts as a receptor in the lysosomal membrane for substrate proteins of chaperone-mediated autophagy. Using antibodies specific for the cytosolic tail of lamp2a and others recognizing all lamp2 isoforms, we found that in rat liver lamp2a represents 25% of lamp2s in the lysosome. We show that lamp2a levels in the lysosomal membrane in rat liver and fibroblasts in culture directly correlate with rates of chaperone-mediated autophagy in a variety of physiological and pathological conditions. The concentration of other lamp2s in the lysosomal membrane show no correlation under the same conditions. Furthermore, substrate proteins bind to lamp2a but not to other lamp2s. Four positively-charged amino acids uniquely present in the cytosolic tail of lamp2a are required for the binding of substrate proteins. Lamp2a also distributes to an unique subpopulation of perinuclear lysosomes in cultured fibroblasts in response to serum withdrawal, and lamp2a, more than other lamp2s, tends to multimerize. These characteristics may be important for lamp2a to act as a receptor for chaperone-mediated autophagy.

Purification and partial characterisation of tentatively classified acid phosphatase from the earthworm Eisenia veneta

In order to use leakage of lysosomal acid phosphatase (AP) as a biomarker of stress to earthworms, more information about AP's in earthworms are needed. This paper describes the details about tentatively classified APs in the earthworm Eisenia veneta. Two isoenzymes (enzyme I and II) of acid phosphatase (AP) and one alkaline phosphatase (enzyme III) from the earthworm E. veneta were separated by gel filtration. All three enzymes were further purified and concentrated on a Con A Sepharose 4B column. Enzyme I was inhibited by tartrate, showed an optimal pH range between 4.0 and 5.0 and was assumed to be of lysosomal origin. Enzyme II was the major enzyme showing the highest activity of the three enzymes. It was expected to be a lysosomal AP under physiological conditions. Enzyme II had a molecular mass 113 kDa and was composed of apparently identical polypeptide chains of 36 kDa each. This enzyme was inhibited by tartrate, showed an optimal pH in the range 6.0-7.5 and was slowly degraded at temperatures above 40 degrees C. Enzyme III is not inhibited by tartrate and has a pH-optimum > 9. The subcellular location under physiological conditions was assumed to be the cytosol.

Identification of senescence-associated genes from daylily petals

The petals of daylily (Hemerocallis hybrid) have a genetically based program that leads to senescence and cell death ca. 24 h after the flower opens. In order to determine the components of this program, six cDNAs, whose levels increase during petal senescence, were isolated and sequenced and designated DSA3, 4, 5, 6, 12 and 15. All six DSAs are members of gene families and all but DSA5 and DSA6 have one to three other very similar genes. GenBank database homology searches indicate that DSA3 is most similar at the amino acid level to an in-chain fatty acid hydroxylase which is bound to cytochrome P450, DSA4 may be an aspartic proteinase, DSA5 is as yet unidentified, DSA6 is a putative S1-type nuclease, DSA12 is very similar to a cytochrome P450-containing allene oxide synthase, and DSA15 may be a fatty acid elongase. Except for DSA12, the genes are expressed at low levels in daylily roots. Levels of the DSA mRNAs in leaves are less than 4% of the maximum detected in petals, and there are no clear differences between younger and older leaves. With the exception of DSA4, accumulation of the DSA mRNAs is increased 3.2 to 43 times by a concentration of abscisic acid that causes premature senescence of the petals. The relationship of the putative DSA gene products to senescence and cell death of daylily petals is discussed.

The mammalian AP-3 adaptor-like complex mediates the intracellular transport of lysosomal membrane glycoproteins

In mammalian cells, the mannose 6-phosphate receptors (MPRs) and the lysosomal glycoproteins, lysosomal-associated membrane protein (LAMP) I, lysosomal integral membrane protein (LIMP) II, are directly transported from the trans-Golgi network to endosomes and lysosomes. While MPR traffic relies on the AP-1 adaptor complex, we report that proper targeting of LAMP I and LIMP II to lysosomes requires the AP-3 adaptor-like complex. Overexpression of these proteins, which contain either a tyrosine- or a di-leucine-based-sorting motif, promotes AP-3 recruitment on membranes. Inhibition of AP-3 function using antisense oligonucleotides leads to a selective misrouting of both LAMP I and LIMP II to the cell surface without affecting MPR trafficking. These results provide evidence that AP-3 functions in the intracellular targeting of transmembrane glycoproteins to lysosomes.

Infection of epithelial cells by pathogenic neisseriae reduces the levels of multiple lysosomal constituents

Members of our group reported recently that neisseria infection of human epithelial cells results in accelerated degradation of the major lysosomal integral membrane protein LAMP1 and that this is due to hydrolysis of this glycoprotein at its immunoglobulin A1 (IgA1)-like hinge by the neisseria type 2 IgA1 protease (L. Lin et al., Mol. Microbiol. 24:1083-1094, 1997). We also reported that the IgA1 protease plays a major role in the ability of the pathogenic neisseriae to survive within epithelial cells and hypothesized that this is due to alteration of lysosomes as a result of protease-mediated LAMP1 degradation. In this study, we tested the hypothesis that neisseria infection leads to multiple changes in lysosomes. Here, we report that neisseria infection also reduces the levels of three other lysosomal markers: LAMP2, lysosomal acid phosphatase (LAP), and CD63. In contrast, neither the epidermal growth factor receptor level nor the beta-tubulin level is affected. A detailed examination of LAMP2 indicated that the reduced LAMP2 levels are not the result of an altered biosynthetic rate or of cleavage by the IgA1 protease. Nevertheless, the protease plays a role in reducing LAMP2 and LAP activity levels, as these are partially restored in cells infected with an iga mutant. We conclude that neisseria infection results in multiple changes to the lysosomes of infected epithelial cells and that these changes are likely an indirect result of IgA1 protease-mediated cleavage of LAMP1.

Phytepsin, a barley vacuolar aspartic proteinase, is highly expressed during autolysis of developing tracheary elements and sieve cells

Vacuolarisation, formation of autophagocytotic vacuoles and tonoplast disruption have been reported in plant cells undergoing developmentally regulated programmed cell death (PCD), but little is known about the vacuolar proteins involved. In HeLa cells, cathepsin D, a lysosomal aspartic proteinase has been shown to mediate PCD. Based on immunohistochemical staining of barley roots, we show here that the previously well characterised barley vacuolar aspartic proteinase (phytepsin), a plant homologue to cathepsin D, is highly expressed both during formation of tracheary elements and during partial autolysis of sieve cells. In serial transverse sections of the vascular cylinder, starting from the root tip, phytepsin is expressed in root cap cells, in the tracheary elements of early and late metaxylem, and in the sieve cells of the protophloem and metaphloem. Aleurain, a barley vacuolar cysteine proteinase, is expressed similarly in root cap cells but differently in the tracheary elements of protoxylem and early metaxylem. This is the first evidence that a vacuolar aspartic proteinase, in analogy to cathepsin D in animals, may play a role in the active autolysis of plant cells.

Mice deficient in lysosomal acid phosphatase develop lysosomal storage in the kidney and central nervous system

Lysosomal acid phosphatase (LAP) is a tartrate-sensitive enzyme with ubiquitous expression. Neither the physiological substrates nor the functional significance is known. Mice with a deficiency of LAP generated by targeted disruption of the LAP gene are fertile and develop normally. Microscopic examination of various peripheral organs revealed progredient lysosomal storage in podocytes and tubular epithelial cells of the kidney, with regionally different ultrastructural appearance of the stored material. Within the central nervous system, lysosomal storage was detected to a regionally different extent in microglia, ependymal cells, and astroglia concomitant with the development of a progressive astrogliosis and microglial activation. Whereas behavioral and neuromotor analyses were unable to distinguish between control and deficient mice, approximately 7% of the deficient animals developed generalized seizures. From the age of 6 months onward, conspicuous alterations of bone structure became apparent, resulting in a kyphoscoliotic malformation of the lower thoracic vertebral column. We conclude from these findings that LAP has a unique function in only a subset of cells, where its deficiency causes the storage of a heterogeneously appearing material in lysosomes. The causal relationship of the enzyme defect to the clinical manifestations remains to be determined.

Biosynthesis and maturation of the malaria aspartic hemoglobinases plasmepsins I and II

During the intraerythrocytic stage of infection, the malaria parasite Plasmodium falciparum digests most of the host cell hemoglobin. Hemoglobin degradation occurs in the acidic digestive vacuole and is essential for the survival of the parasite. Two aspartic proteases, plasmepsins I and II, have been isolated from the vacuole and shown to make the initial cleavages in the hemoglobin molecule. We have studied the biosynthesis of these two enzymes. Plasmepsin I is synthesized and processed to the mature form soon after the parasite invades the red blood cell, while plasmepsin II synthesis is delayed until later in development. Otherwise, biosynthesis of the plasmepsins is identical. The proplasmepsins are type II integral membrane proteins that are transported through the secretory pathway before cleavage to the soluble form. They are not glycosylated in vivo, despite the presence of several potential glycosylation sites. Proplasmepsin maturation appears to require acidic conditions and is reversibly inhibited by the tripeptide aldehydes N-acetyl-L-leucyl-L-leucyl-norleucinal and N-acetyl-L-leucyl-L-leucyl-methional. These compounds are known to inhibit cysteine proteases and the chymotryptic activity of proteasomes but not aspartic proteases. However, proplasmepsin processing is not blocked by other cysteine protease inhibitors, nor by the proteasome inhibitor lactacystin. Processing is also not blocked by aspartic protease inhibitors. This inhibitor profile suggests that unlike most other aspartic proteases, proplasmepsin maturation may not be autocatalytic in vivo, but instead could require the action of an unusual processing enzyme. Compounds that block processing are expected to be potent antimalarials.

Cloning, expression and chromosomal mapping of human lysosomal sialidase and characterization of mutations in sialidosis

Sialidase (neuraminidase, EC 3.2.1.18) catalyses the hydrolysis of terminal sialic acid residues of glyconjugates. Sialidase has been well studied in viruses and bacteria where it destroys the sialic acid-containing receptors at the surface of host cells, and mobilizes bacterial nutrients. In mammals, three types of sialidases, lysosomal, plasma membrane and cytosolic, have been described. For lysosomal sialidase in humans, the primary genetic deficiency results in an autosomal recessive disease, sialidosis, associated with tissue accumulation and urinary excretion of sialylated oligosaccharides and glycolipids. Sialidosis includes two main clinical variants: late-onset, sialidosis type I, characterized by bilateral macular cherry-red spots and myoclonus, and infantile-onset, sialidosis type II, characterized by skeletal dysplasia, mental retardation and hepatosplenomegaly. We report the identification of human lysosomal sialidase cDNA, its cloning, sequencing and expression. Examination of six sialidosis patients revealed three mutations, one frameshift insertion and two missense. We mapped the lysosomal sialidase gene to human chromosome 6 (6p21.3), which is consistent with the previous chromosomal assignment of this gene in proximity to the HLA locus.

Aspartic proteinase genes in the Brassicaceae Arabidopsis thaliana and Brassica napus

Active aspartic proteinase is isolated from Brassica napus seeds and the peptide sequence is used to generate primers for PCR. We present here cDNA and genomic clones for aspartic proteinases from the closely related Brassicaceae Arabidopsis thaliana and Brassica napus. The Arabidopsis cDNA represents a single gene, while Brassica has at least 4 genes. Like other plant aspartic proteases, the two Brassicaceae enzymes contain an extra protein domain of about 100 amino acids relative to the mammalian forms. The intron/exon arrangement in the Brassica genomic clone is significantly different from that in mammalian genes. As the proteinase is isolated from seeds, the same tissue where 2S albumins are processed, this implies expression of one of the aspartic proteinase genes there.

Molecular characterization of the lysosomal acid phosphatase from Drosophila melanogaster

In Drosophila, unlike humans, the lysosomal acid phosphatase (Acph-1) is a non-essential enzyme. It is also one of the most rapidly evolving gene-enzyme systems in the genus. In order to determine which parts of the enzyme are conserved and which parts are apparently under little functional constraint, we cloned the gene from Drosophila melanogaster via a chromosomal walk. Fragments from the gene were used to recover an apparently full-length cDNA. The cDNA was sub-cloned into a Drosophila transformation vector where it was under the control of the 5' promoter sequence of the hsp-70 gene. Three independent transformants were obtained; in each, Acph-1 expression from the cDNA was constitutive and not dependent on heat shock, as determined by densitometric analyses of the allozymic forms of the enzyme. The pattern of expression indicates the hsp-70 and endogenous Acph-1 promoters act together in some, but not all, tissues. The sequence of the cDNA was determined using deletions made with exonuclease III, and primers deduced from the cDNA sequence were used to sequence the genomic clone. Five introns were found, and putative 5' upstream regulatory sequences were identified. Amino acid sequence comparisons have revealed several highly conserved motifs between Drosophila Acph-1 and vertebrate lysosomal and prostatic acid phosphatases.

Clinical and biological aspects of acid phosphatase

The identity and genetic origins of the nonspecific orthophosphate monoesterases with an acid pH optimum--the acid phosphatases--are now becoming clear. They form a family of genetically distinct isoenzymes, many of which show significant posttranslational modification. Four true isoenzymes exist. The erythrocytic and lysosomal forms show widespread distribution and are expressed in most cells; in contrast, the prostatic and macrophagic forms have a more limited expression. The erythrocytic and macrophagic forms are distinguished from the others in resisting inhibition by dextrorotatory tartrate. The prostatic form has long been used as a marker for prostatic cancer and the macrophagic forms have been linked with miscellaneous disorders, notably increased osteolysis, Gaucher's disease of spleen, and hairy cell leukemia, whereas the normal levels of intravesical lysosomal acid phosphatase in I cell disease pointed the way toward the mechanisms underlying its intracellular processing.

Biogenesis of lysosomal membranes

Lysosomal membrane glycoproteins are highly glycosylated proteins decorating the luminal surface of lysosomal membranes. Their biosynthetic route from the rough endoplasmic reticulum to the lysosomal compartment has been elucidated during recent years. Signals for intracellular sorting have been identified and characterized. The function of these proteins remains to be determined. Besides resident proteins the lysosomal membrane harbours at least one transient passenger, lysosomal acid phosphatase, which is sorted as a membrane-bound precursor like resident lysosomal membrane proteins and liberated from the membrane by limited proteolysis upon arrival in dense lysosomes.

Human gastric adenocarcinoma cathepsin B: isolation and sequencing of full-length cDNAs and polymorphisms of the gene

Four full-length cDNA clones coding for preprocathepsin B were isolated from a human gastric adenocarcinoma cDNA library (AGS 1-6-30-1) and analyzed for possible sequence modifications that might be linked to altered intracellular trafficking and secretion of cathepsin B (CTSB) in malignant tumors. Comparison of AGS 1-6-30-1 cDNAs with human kidney/hepatoma cDNAs revealed: (1) three potential N-glycosylation sites instead of two, (2) a nucleotide (nt) substitution in the coding region for the propeptide from GTG to CTG which would result in a Val26-->Leu change, (3) three silent nt replacements in the coding region for the mature protein, (4) five single-nt differences in the 5'- and 3'-UTR (untranslated regions), (5) heterogeneity in the 5'-UTR, and (6) a 10-bp insertion in the 3'-UTR. The 10-bp insertion in the 3'-UTR may alter the stability of CTSB mRNA transcripts and thereby the expression of CTSB. These clones should be useful for expressing human tumor CTSB and analyzing the function of this enzyme in malignant progression. Two restriction-fragment length polymorphisms (RFLPs), EcoRI and TaqI, were detected by Southern blot analysis of genomic DNA from 36 unrelated Caucasians. Inheritance and distribution of the EcoRI alleles (13.0 and 11.0 kb) and the TaqI alleles (5.7 and 4.6 kb) indicated they were independent polymorphisms. In contrast to the EcoRI alleles of 13.0 and 11.0 kb observed in the population survey, genomic DNA from two AGS gastric adenocarcinoma subclones revealed two EcoRI alleles of 13.0 and 7.8 kb.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification, characterization and molecular cloning of human hepatic lysosomal acid lipase

Lysosomal acid lipase (LAL) is a hydrolase essential for the intracellular degradation of cholesteryl esters and triacylglycerols. This report describes a multi-step procedure for the purification of LAL from human liver. After solubilization with non-ionic detergent, acid hydrolase activity was purified 17000-fold to apparent homogeneity by sequential chromatography on Concanavalin A Sepharose, carboxymethyl-cellulose, phenyl Superose, Mono S cation exchange and Superose 12 gel-filtration columns. This procedure yielded two silver-staining protein bands of 56 kDa and 41 kDa on SDS/PAGE. Size-exclusion chromatography of the 41-kDa protein indicated that the enzyme was catalytically competent as a monomer of approximately 38 kDa. When assayed in the presence of cholesteryl oleate or trioleoylglycerol, purified acid lipase had Vmax values of 4390 nmol fatty acid.min-1.mg protein and 4756 nmol fatty acid.min-1.mg protein-1, and apparent Km values of 0.142 mM and 0.138 mM, respectively. The purified enzyme was most active at low pH (4.5-5.0) and required non-ionic detergent and ethylene glycol for optimal stability. Incubation of the 41-kDa acid lipase with endoglucosaminidase H reduced the molecular mass by 4-6 kDa, demonstrating Asn-linked glycosylation with high-mannose oligosaccharides. Deglycosylation did not affect enzymic activity, indicating that carbohydrates are not required for LAL activity. Based on partial peptide sequence, an oligonucleotide was synthesized and utilized to isolate LAL cDNA clones from a human liver cDNA library. A full-length LAL cDNA contained 2626 nucleotides and coded for a predicted protein of 372 amino acids, preceded by a 27 residue hydrophobic signal peptide. Hepatic LAL differed from fibroblast acid lipase at the N-terminus and revealed extensive similarities with human gastric lipase and rat lingual lipase, confirming a gene family of acid lipases. Northern hybridization using the complete LAL cDNA as a radiolabeled probe indicated striking differences in mRNA expression among human tissues. LAL mRNA was most abundant in brain, lung, kidney and mammary gland. Placenta and HeLa cells expressed intermediate amounts of LAL mRNA, while RNA extracted from liver and heart showed low levels of expression.

Intralysosomal glycerophospholipid catabolism in liver: hydrolysis of amino alcohol-containing phospholipids and their metabolites

Liver lysosomes were isolated from untreated rats and rats pretreated with Triton WR-1339. Purified lysosomes were also separated into lysosomal matrix and membrane fractions. With freshly prepared and frozen biological material, the lysosomal catabolism of various stereospecifically radiolabeled amino alcohol-containing glycerophospholipids and their potential metabolites was studied. Basically there was no qualitative difference in the formation of phospholipid metabolites in both preparations: after long-term incubation, free fatty acids, lysophospholipids, acyl-free phosphodiesters were detected, and to a far lesser extent, amino alcohol-containing phosphomonoesters and only traces of free amino alcohols. These findings indicate the presence of lysosomal phospholipases A as well as C and lysophospholipase(s), with pH optima of about 4.5, and they clearly exclude phospholipase D activity. Unfractionated lysosomes and their soluble as well as particulate subfractions were not capable of hydrolysing the acyl-free amino alcohol-containing phosphodiesters. These compounds must therefore be considered one of the end products of the intralysosomal catabolism of amino alcohol-containing phosphoglycerides. They are presumably cleared from the lysosomal compartment by an as yet unknown transport system in the lysosomal membrane. In liver, the extralysosomal site of their (Mg(2+)-dependent) hydrolysis seems to be the plasma membrane. By contrast, hydrolysis of glycero-3-phosphate and the amino alcohol-containing phosphomonoesters was catalysed in the lysosomal compartment, with a pH optimum of about 5.0, although at considerably lower rates than that of glycero-2-phosphate, a model substrate for lysosomal acid phosphatase.

Hydrolytic specificity of the barley grain aspartic proteinase

We recently published the primary structure and inhibition data of the barley grain aspartic proteinase (HvAP, Hordeum vulgare aspartic proteinase) which revealed similarity to mammalian cathepsin D and yeast aspartic proteinase A. Here we present evidence, based on Km and kcat values for the enzyme as well as on its cleavage sites in haemoglobin, the insulin B-chain, glucagon and melittin, that the similarity extends to its hydrolytic specificity. Like the animal and microbial aspartic proteinases, HvAP preferentially cleaves peptide bonds between amino acid residues with large hydrophobic side chains. The narrow hydrolytic specificity of HvAP suggests that plant aspartic proteinases may perform regulatory functions by limited proteolysis.

The early and late processing of lysosomal enzymes: proteolysis and compartmentation

Lysosomal enzymes are subjected to a number of modifications including carbohydrate restructuring and proteolytic maturation. Some of these reactions support lysosomal targeting, others are necessary for activation or keeping the enzyme inactive before being segregated, while still others may be adventitious. The non-segregated fraction of the enzyme is secreted and can be isolated from the medium. It is considered that the secreted lysosomal enzymes fulfill certain physiological and pathophysiological roles. By comparing the secreted and the intracellular enzymes it is possible to distinguish between the reactions that occur before and after the segregation. In this review the reactions that may influence the segregation are referred to as the early processing and those characteristic for the enzymes isolated from lysosomal compartments as the late processing. The early processing is characterized mainly by modifications of carbohydrate side chains. In the late processing, proteolytic fragmentation represents the most conspicuous changes. The review focuses on the compartmentation of the reactions and the proteolytic fragmentation of lysosomal enzyme precursors. While a plethora of proteolytic reactions are involved, our knowledge of the proteinases responsible for the particular maturation reactions remains very limited. The review points also to work with cells from patients affected with lysosomal storage disorders, which contributed to our understanding of the lysosomal apparatus.

A difference in the enzyme contents of resorption lacunae and secondary lysosomes of osteoclasts

The distributions of tartrate sensitive lysosomal acid phosphatase (LAP) and cathepsin L in osteoclasts and the effect of parathyroid hormone (PTH) stimulation on them were investigated by using the protein A-gold method on ultracryosections of rat trabecular bone. LAP was located in association with the ruffled border membrane, in the resorption lacuna, on the mineral phase surrounding the lacuna, and in some primary lysosomes. After PTH treatment, the extracellular and ruffled border membrane associated LAP apparently increased. Heavy gold labelling for cathepsin L was confined exclusively to secondary lysosomes. No labelling was seen in the extracellular resorption lacuna or at the ruffled border. Acceleration of bone resorption by PTH-treatment did not change detectably the distribution or intensity of labelling. This study shows that the enzyme contents of secondary lysosomes and resorption lacunae are different and suggests that LAP is directly involved in extracellular bone degradation whereas the role of cathepsin L is restricted to lysosomes.

The essential tyrosine of the internalization signal in lysosomal acid phosphatase is part of a beta turn

For rapid endocytosis lysosomal acid phosphatase requires a Tyr-containing signal in its cytoplasmic domain, as do cell surface receptors mediating endocytosis and clustering in coated pits. To determine the structure of the internalization signal an 18 amino acid peptide representing the cytoplasmic tail of lysosomal acid phosphatase was analyzed by two-dimensional nuclear magnetic resonance spectroscopy. Part of the peptide, 5-PPGY-8, forms a well-ordered beta turn of type I in solution. Our result and data on the structure of the endocytosis signal of the low density lipoprotein receptor reported by Bansal and Gierasch in the accompanying paper represent experimental determinations of the three-dimensional structure of protein transport signals and suggest that the essential aromatic amino acid of internalization signals is recognized by a putative cytoplasmic receptor in the structural context of a tight turn.

Primary structure of a barley-grain aspartic proteinase. A plant aspartic proteinase resembling mammalian cathepsin D

Two enzymatically active heterodimeric forms of an aspartic proteinase, a putative 32 kDa + 16 kDa precursor form and a putative 29 kDa + 11 kDa mature form, are present in resting barley grains (Sarkkinen, P., Kalkkinen, N., Tilgmann, C., Siuro, J., Kervinen, J. & Mikola, L., 1990, in the press). The cDNA corresponding to this enzyme has been cloned and sequenced. The full-length 1863-bp cDNA sequence codes for an open reading frame of 508 amino acids. The open reading frame consists of a 66-amino acid preprosequence and a 442-amino acid mature protein. Comparison of the N-terminal amino acid sequences of the enzyme subunits with the sequence of the cDNA clone indicates that the heterodimeric enzyme is translated as a proenzyme which is processed into two subunits. The localisation of the experimentally determined N-terminal amino acid sequences of all four subunits (32 kDa + 16 kDa and 29 kDa + 11 kDa) in the same transcript, as well as the detection of only one 2.0-kb mRNA on Northern blots from resting seeds, clearly indicates that the larger (32 kDa + 16 kDa) enzyme is an intermediate precursor form of the smaller (29 kDa + 11 kDa) enzyme. The processing pattern of the barley enzyme, which is the first sequenced plant aspartic proteinase, differs from that of all other known aspartic proteinases. The barley enzyme is highly similar to mammalian and yeast aspartic proteinases, especially to human and porcine cathepsin D. This similarity is clearly dispersed over two regions, separated by a dissimilar, barley-specific region of 104 amino acids.

The endo/lysosomal protease cathepsin B is able to process conalbumin fragments for presentation to T cells

The protein antigens conalbumin (CA) and ovalbumin (OVA) are known to require uptake into antigen-presenting cells (APC) for their presentation to major histocompatibility complex (MHC) class II-restricted T cells. In both cases proteolytic cleavage is thought to be a necessary step for the generation of the respective antigenic peptides. A specific inhibitor of the endosomal protease cathepsin B, Cbz-Phe-Ala-CHN2, blocks the presentation of both CA and OVA, whereas this inhibitor has no effect on the presentation of a processing-independent OVA peptide. Furthermore, the presentation of insulin, an antigen that needs processing but no proteolytic cleavage, is enhanced when cathepsin B is inhibited during antigen pulsing. When the APC were treated with an inhibitor of acid proteases, the CA response was not affected, while the presentation of OVA was diminished under these conditions. To estimate the relevance of these findings for the generation of the antigenic CA peptide, extracellular digestions of CA by cathepsin B were carried out. The fragment(s) present in these digests was recognized by T cells without further processing. Furthermore, the time-course of intra- and extracellular CA processing with respect to the capacity to stimulate T cells was similar. Taken together these data suggest that degradation by cathepsin B may be sufficient in vivo to generate the antigenic CA fragment. On the other hand, the blocking of cathepsin B does not appear to have an adverse effect on the general mechanisms of antigen presentation.