A novel mutagenesis strategy identifies distantly spaced amino acid sequences that are required for the phosphorylation of both the oligosaccharides of procathepsin D by N-acetylglucosamine 1-phosphotransferase

Engineering of GlcNAc-1-Phosphotransferase for Production of Highly Phosphorylated Lysosomal Enzymes for Enzyme Replacement Therapy

Several lysosomal enzymes currently used for enzyme replacement therapy in patients with lysosomal storage diseases contain very low levels of mannose 6-phosphate, limiting their uptake via mannose 6-phosphate receptors on the surface of the deficient cells. These enzymes are produced at high levels by mammalian cells and depend on endogenous GlcNAc-1-phosphotransferase α/β precursor to phosphorylate the mannose residues on their glycan chains. We show that co-expression of an engineered truncated GlcNAc-1-phosphotransferase α/β precursor and the lysosomal enzyme of interest in the producing cells resulted in markedly increased phosphorylation and cellular uptake of the secreted lysosomal enzyme. This method also results in the production of highly phosphorylated acid β-glucocerebrosidase, a lysosomal enzyme that normally has just trace amounts of this modification.

Role of spacer-1 in the maturation and function of GlcNAc-1-phosphotransferase

The UDP-GlcNAc:lysosomal enzyme, N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-PT), is an α2 β2 γ2 hexamer that mediates the initial step in the formation of the mannose 6-phosphate targeting signal on newly synthesized lysosomal acid hydrolases. The GNPTAB gene encodes the 1256 amino acid long α/β precursor which is normally cleaved at K928 in the early Golgi by Site-1 protease (S1P). Here, we show that removal of the so-called 'spacer-1' domain (residues 86-322) results in cleavage almost exclusively at a second S1P consensus sequence located upstream of K928. In addition, GlcNAc-1-PT lacking spacer-1 exhibits enhanced phosphorylation of several non-lysosomal glycoproteins, while the phosphorylation of lysosomal acid hydrolases is not altered. In view of these effects on the maturation and function of GlcNAc-1-PT, we suggest renaming `spacer-1' the `regulatory-1' domain.

Multiple Domains of GlcNAc-1-phosphotransferase Mediate Recognition of Lysosomal Enzymes

The Golgi enzyme UDP-GlcNAc:lysosomal enzymeN-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase), an α2β2γ2hexamer, mediates the initial step in the addition of the mannose 6-phosphate targeting signal on newly synthesized lysosomal enzymes. This tag serves to direct the lysosomal enzymes to lysosomes. A key property of GlcNAc-1-phosphotransferase is its unique ability to distinguish the 60 or so lysosomal enzymes from the numerous non-lysosomal glycoproteins with identical Asn-linked glycans. In this study, we demonstrate that the two Notch repeat modules and the DNA methyltransferase-associated protein interaction domain of the α subunit are key components of this recognition process. Importantly, different combinations of these domains are involved in binding to individual lysosomal enzymes. This study also identifies the γ-binding site on the α subunit and demonstrates that in the majority of instances the mannose 6-phosphate receptor homology domain of the γ subunit is required for optimal phosphorylation. These findings serve to explain how GlcNAc-1-phosphotransferase recognizes a large number of proteins that lack a common structural motif.

Latency-associated peptide of transforming growth factor-β1 is not subject to physiological mannose phosphorylation

Latent TGF-β1 was one of the first non-lysosomal glycoproteins reported to bear mannose 6-phosphate (Man-6-P) residues on its N-glycans. Prior studies have suggested that this sugar modification regulates the activation of latent TGF-β1 by allowing it to bind cell surface-localized Man-6-P receptors. Man-6-P has also been proposed as an anti-scarring therapy based on its ability to directly block the activation of latent TGF-β1. A complete understanding of the physiological relevance of latent TGF-β1 mannose phosphorylation, however, is still lacking. Here we investigate the degree of mannose phosphorylation on secreted latent TGF-β1 and examine its Man-6-P-dependent activation in primary human corneal stromal fibroblasts. Contrary to earlier reports, minimal to no Man-6-P modification was found on secreted and cell-associated latent TGF-β1 produced from multiple primary and transformed cell types. Results showed that the inability to detect Man-6-P residues was not due to masking by the latent TGF-β1-binding protein (LTBP). Moreover, the efficient processing of glycans on latent TGF-β1 to complex type structures was consistent with the lack of mannose phosphorylation during biosynthesis. We further demonstrated that the conversion of corneal stromal fibroblast to myofibroblasts, a well known TGF-β1-dependent process, was not altered by Man-6-P addition when latent forms of this growth factor were present. Collectively, these findings indicate that Man-6-P-dependent effects on latent TGF-β1 activation are not mediated by direct modification of its latency-associated peptide.

Analysis of mannose 6-phosphate uncovering enzyme mutations associated with persistent stuttering

GlcNAc-1-phosphodiester-N-acetylglucosaminidase ("uncovering enzyme" (UCE); EC 3.1.4.45) is a Golgi enzyme that mediates the second step in the synthesis of the mannose 6-phosphate lysosomal targeting signal on acid hydrolases. Recently, three mutations (two missense and one deletion/frameshift) in the NAGPA gene that encodes UCE have been identified in individuals with persistent stuttering. We now demonstrate that each mutation leads to lower cellular UCE activity. The p.R328C mutation impairs folding in the endoplasmic reticulum, resulting in degradation of a significant portion by the proteasomal system. The p.H84Q mutation also impairs folding and, in addition, decreases the specific activity of the enzyme that folds sufficiently to traffic to the Golgi. The p.F513SfsX113 frameshift mutation adds 113 amino acids to the C terminus of the cytoplasmic tail of the protein, including a VWLL sequence that causes rapid degradation via the proteasomal system. These biochemical findings extend the genetic data implicating mutations in the NAGPA gene in the persistent stuttering phenotype.

Extensive mannose phosphorylation on leukemia inhibitory factor (LIF) controls its extracellular levels by multiple mechanisms

In addition to soluble acid hydrolases, many nonlysosomal proteins have been shown to bear mannose 6-phosphate (Man-6-P) residues. Quantification of the extent of mannose phosphorylation and the relevance to physiological function, however, remain poorly defined. In this study, we investigated the mannose phosphorylation status of leukemia inhibitory factor (LIF), a previously identified high affinity ligand for the cation-independent mannose 6-phosphate receptor (CI-MPR), and we analyzed the effects of this modification on its secretion and uptake in cultured cells. When media from LIF-overexpressing cells were fractionated using a CI-MPR affinity column, 35-45% of the total LIF molecules were bound and specifically eluted with free Man-6-P thus confirming LIF as a bona fide Man-6-P-modified protein. Surprisingly, mass spectrometric analysis of LIF glycopeptides enriched on the CI-MPR column revealed that all six N-glycan sites could be Man-6-P-modified. The relative utilization of these sites, however, was not uniform. Analysis of glycan-deleted LIF mutants demonstrated that loss of glycans bearing the majority of Man-6-P residues leads to higher steady-state levels of secreted LIF. Using mouse embryonic stem cells, we showed that the mannose phosphorylation of LIF mediates its internalization thereby reducing extracellular levels and stimulating embryonic stem cell differentiation. Finally, immunofluorescence experiments indicate that LIF is targeted directly to lysosomes following its biosynthesis, providing another mechanism whereby mannose phosphorylation serves to control extracellular levels of LIF. Failure to modify LIF in the context of mucolipidosis II and its subsequent accumulation in the extracellular space may have important implications for disease pathogenesis.

Functions of the alpha, beta, and gamma subunits of UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase

UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase is an alpha(2)beta(2)gamma(2) hexamer that mediates the first step in the synthesis of the mannose 6-phosphate recognition marker on lysosomal acid hydrolases. Using a multifaceted approach, including analysis of acid hydrolase phosphorylation in mice and fibroblasts lacking the gamma subunit along with kinetic studies of recombinant alpha(2)beta(2)gamma(2) and alpha(2)beta(2) forms of the transferase, we have explored the function of the alpha/beta and gamma subunits. The findings demonstrate that the alpha/beta subunits recognize the protein determinant of acid hydrolases in addition to mediating the catalytic function of the transferase. In mouse brain, the alpha/beta subunits phosphorylate about one-third of the acid hydrolases at close to wild-type levels but require the gamma subunit for optimal phosphorylation of the rest of the acid hydrolases. In addition to enhancing the activity of the alpha/beta subunits toward a subset of the acid hydrolases, the gamma subunit facilitates the addition of the second GlcNAc-P to high mannose oligosaccharides of these substrates. We postulate that the mannose 6-phosphate receptor homology domain of the gamma subunit binds and presents the high mannose glycans of the acceptor to the alpha/beta catalytic site in a favorable manner.

Compensatory expression of human N-acetylglucosaminyl-1-phosphotransferase subunits in mucolipidosis type III gamma

The N-Acetylglucosaminyl-1-phosphotransferase plays a key role in the generation of mannose 6-phosphate (M6P) recognition markersessential for efficient transport of lysosomal hydrolases to lysosomes. The phosphotransferase is composed of six subunits (alpha2, beta2, gamma2). The alpha- and beta-subunits are catalytically active and encoded by a single gene, GNPTAB, whereas the gamma-subunit encoded by GNPTG is proposed to recognize conformational structures common to lysosomal enzymes. Defects in GNPTG cause mucolipidosis type III gamma, which is characterized by missorting and cellular loss of lysosomal enzymes leading to lysosomal accumulation of storage material. Using plasmon resonance spectrometry, we showed that recombinant gamma-subunit failed to bind the lysosomal enzyme arylsulfatase A. Additionally, the overexpression of the gamma-subunit in COS7 cells did not result in hypersecretion of newly synthesized lysosomal enzymes expected for competition for binding sites of the endogenous phosphotransferase complex. Analysis of fibroblasts exhibiting a novel mutation in GNPTG (c.619insT, p.K207IfsX7) revealed that the expression of GNPTAB was increased whereas in gamma-subunit overexpressing cells the GNPTAB mRNA was reduced. The data suggest that the gamma-subunit is important for the balance of phosphotransferase subunits rather for general binding of lysosomal enzymes.

Murine UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase lacking the gamma-subunit retains substantial activity toward acid hydrolases

UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase) mediates the first step in the synthesis of the mannose 6-phosphate recognition marker on acid hydrolases. The transferase exists as an alpha(2)beta(2)gamma(2) hexameric complex with the alpha- and beta-subunits derived from a single precursor molecule. The catalytic function of the transferase is attributed to the alpha- and beta-subunits, whereas the gamma-subunit is believed to be involved in the recognition of a conformation-dependent protein determinant common to acid hydrolases. Using knock-out mice with mutations in either the alpha/beta gene or the gamma gene, we show that disruption of the alpha/beta gene completely abolishes phosphorylation of high mannose oligosaccharides on acid hydrolases whereas knock-out of the gamma gene results in only a partial loss of phosphorylation. These findings demonstrate that the alpha/beta-subunits, in addition to their catalytic function, have some ability to recognize acid hydrolases as specific substrates. This process is enhanced by the gamma-subunit.

Sialylated core 1 O-glycans influence the sorting of Pmel17/gp100 and determine its capacity to form fibrils

Pmel17 is a melanocyte/melanoma-specific protein that is essential for the maturation of melanosomes to form mature, fibrillar, and pigmented organelles. Recently, we reported that the less glycosylated form of Pmel17 (termed iPmel17) is sorted via the plasma membrane in a manner distinct from mature Pmel17 (termed mPmel17), which is sorted directly to melanosomes. To clarify the mechanism(s) underlying the distinct processing and sorting of Pmel17, we generated a highly specific antibody (termed alphaPEP25h) against an epitope within the repeat domain of Pmel17 that is sensitive to changes in O-glycosylation. alphaPEP25h recognizes only iPmel17 and allows analysis of the processing and sorting of iPmel17 when compared with alphaPEP13h, an antibody that recognizes both iPmel17 and mPmel17. Our novel findings using alphaPEP25h demonstrate that iPmel17 differs from mPmel17 not only in its sensitivity to endoglycosidase H, but also in the content of core 1 O-glycans modified with sialic acid. This evidence reveals that iPmel17 is glycosylated differently in the Golgi and that it is sorted through the secretory pathway. Analysis of Pmel17 processing in glycosylation-deficient mutant cells reveals that Pmel17 lacking the correct addition of sialic acid and galactose loses the ability to form fibrils. Furthermore, we show that addition of sialic acid affects the stability and sorting of Pmel17 and reduces pigmentation. Alterations in sialyltransferase activity and substrates differ between normal and transformed melanocytes and may represent a critical change during malignant transformation.

Folding, activity and targeting of mutated human cathepsin D that cannot be processed into the double-chain form

The precursor of human cathepsin D (CD) is converted into the single-chain and the double-chain active polypeptides by subsequent proteolysis reactions taking place in the endosomal-lysosomal compartment and involving specific aminoacid sequences. We have mutagenized the region of aminoacids (comprising the beta-hairpin loop) involved in the latter proteolytic maturation step and generated a mutant CD that cannot be converted into the mature double-chain form. This mutant CD expressed in rodent cells reaches the lysosome and is stable as single-chain polypeptide, bears high-mannose type sugars, binds to pepstatin A and is enzymatically active, indicating that it is correctly folded. The present work provides new insights on the aminoacid region involved in the terminal processing of human CD and on the function of the processing beta-hairpin loop.

Mechanisms in protein O-glycan biosynthesis and clinical and molecular aspects of protein O-glycan biosynthesis defects: a review

BACKGROUND

Genetic diseases that affect the biosynthesis of protein O-glycans are a rapidly growing group of disorders. Because this group of disorders does not have a collective name, it is difficult to get an overview of O-glycosylation in relation to human health and disease. Many patients with an unsolved defect in N-glycosylation are found to have an abnormal O-glycosylation as well. It is becoming increasingly evident that the primary defect of these disorders is not necessarily localized in one of the glycan-specific transferases, but can likewise be found in the biosynthesis of nucleotide sugars, their transport to the endoplasmic reticulum (ER)/Golgi, and in Golgi trafficking. Already, disorders in O-glycan biosynthesis form a substantial group of genetic diseases. In view of the number of genes involved in O-glycosylation processes and the increasing scientific interest in congenital disorders of glycosylation, it is expected that the number of identified diseases in this group will grow rapidly over the coming years.

CONTENT

We first discuss the biosynthesis of protein O-glycans from their building blocks to their secretion from the Golgi. Subsequently, we review 24 different genetic disorders in O-glycosylation and 10 different genetic disorders that affect both N- and O-glycosylation. The key clinical, metabolic, chemical, diagnostic, and genetic features are described. Additionally, we describe methods that can be used in clinical laboratory screening for protein O-glycosylation biosynthesis defects and their pitfalls. Finally, we introduce existing methods that might be useful for unraveling O-glycosylation defects in the future.

Sialylation enhances the secretion of neurotoxic amyloid-beta peptides

Alzheimer's disease (AD) is characterized by amyloid-beta peptide (Abeta) deposition in the brain. Abeta is produced by sequential cleavage of amyloid precursor protein (APP) by beta-secretase (BACE1: beta-site APP-cleaving enzyme 1) and gamma-secretase. Previously, we demonstrated that BACE1 also cleaves beta-galactoside alpha2,6-sialyltransferase (ST6Gal-I) and down-regulates its transferase activity. Here, we report that overexpression of ST6Gal-I in Neuro2a cells enhanced alpha2,6-sialylation of endogenous APP and increased the extracellular levels of its metabolites [Abeta by two-fold, soluble APPbeta (sAPPbeta) by three-fold and sAPPalpha by 2.5-fold). Sialylation-deficient mutant (Lec-2) cells secreted half as much Abeta as wild-type Chinese hamster ovary (CHO) cells. Furthermore, wild-type CHO cells showed enhanced secretion of the APP metabolites upon ST6Gal-I overexpression, whereas Lec-2 cells did not, indicating that the secretion enhancement requires sialylation of cellular protein(s). Secretion of metabolites from a mutant APP (APP-Asn467,496Ala) that lacked N-glycosylation sites was not enhanced upon ST6Gal-I overexpression, suggesting that the N-glycans on APP itself are required for the enhanced secretion. In the mouse brain, the amount of alpha2,6-sialylated APP appeared to be correlated with the sAPPbeta level. These results suggest that sialylation of APP promotes its metabolic turnover and could affect the pathology of AD.

Identification of the Minimal Lysosomal Enzyme Recognition Domain in Cathepsin D

Specific recognition of lysosomal hydrolases by UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, the initial enzyme in the biosynthesis of mannose 6-phosphate residues, is governed by a common protein determinant. Previously, we generated a lysosomal enzyme recognition domain in the secretory protein glycopepsinogen by substituting in two regions (lysine 203 and amino acids 265-293 of the beta loop) from cathepsin D, a highly related lysosomal protease. Here we show that substitution of just two lysines (Lys-203 and Lys-267) stimulates mannose phosphorylation 116-fold. Substitution of additional residues in the beta loop, particularly lysines, increased phosphorylation 4-fold further, approaching the level obtained with intact cathepsin D. All the phosphorylation occurred at the carboxyl lobe glycan, indicating that additional elements are required for phosphorylation of the amino lobe glycan. These data support the proposal that as few as two lysines in the correct orientation to each other and to the glycan can serve as the minimal elements of the lysosomal enzyme recognition domain. However, our findings show that the spacing between lysines is flexible and other residues contribute to the recognition marker.

Specific Amino Acids in the First Fibronectin Type III Repeat of the Neural Cell Adhesion Molecule Play a Role in Its Recognition and Polysialylation by the Polysialyltransferase ST8Sia IV/PST

Polysialic acid is an anti-adhesive protein modification that promotes cell migration and the plasticity of cell interactions. Because so few proteins carry polysialic acid, we hypothesized that polysialylation is a protein-specific event and that a specific polysialyltransferase-substrate interaction is the basis of this specificity. The major substrate for the polysialyltransferases is the neural cell adhesion molecule, NCAM. Previous work demonstrates that the first fibronectin type III repeat of NCAM (FN1) was necessary for the polysialylation of the N-glycans on the adjacent immunoglobulin domain (Ig5) (Close, B. E., Mendiratta, S. S., Geiger, K. M., Broom, L. J., Ho, L. L., and Colley, K. J. (2003) J. Biol. Chem. 278, 30796-30805). This suggested that FN1 may be a recognition site for the polysialyltransferases. In this study, we showed that the second fibronectin type III repeat (FN2) of NCAM cannot replace FN1. Arg substitution of three unique acidic amino acids on the surface of FN1 eliminated polysialylation not only of a minimal Ig5-FN1 substrate but also of full-length NCAM. Ala substitution of these residues eliminated Ig5-FN1 polysialylation but not that of full-length NCAM, suggesting that the two proteins are interacting differently with the enzymes and that multiple residues are involved in the enzyme-NCAM interaction. By using another truncated protein, Ig5-FN1-FN2, we confirmed the importance of enzyme-substrate positioning for optimal recognition and polysialylation. In sum, we have found that acidic residues on the surface of FN1 are part of a larger protein interaction region that is critical for NCAM recognition and polysialylation by the polysialyltransferases.

Expression and characterization of human recombinant and alpha-N-acetylglucosaminidase

Mucopolysaccharidosis type IIIB (MPS-IIIB, Sanfilippo type B Syndrome) is a heterosomal, recessive lysosomal storage disorder resulting from a deficiency of [alpha]-N-acetylglucosaminidase (NAGLU). To characterize this enzyme further and evaluate its potential for enzyme replacement studies we expressed the NAGLU-encoding cDNA in Chinese hamster ovary cells (CHO-K1 cells) and purified the recombinant enzyme from the medium of stably transfected cells by a two-step affinity chromatography. Two isoforms of recombinant NAGLU with apparent molecular weights of 89 and 79 kDa were purified and shown to differ in their glycosylation pattern. The catalytic parameters of both forms of the recombinant enzyme were indistinguishable from each other and similar to those of NAGLU purified from various tissues. However, compared to other recombinant lysosomal enzymes expressed from CHO-K1 cells, the mannose-6-phosphate receptor mediated uptake of the secreted form of recombinant NAGLU into cultured skin fibroblasts was considerably reduced. A small amount of phosphorylated NAGLU present in purified enzyme preparations was shown to be endocytosed by MPS-IIIB fibroblasts via the mannose-6-phosphate receptor-mediated pathway and transported to the lysosomes, where they corrected the storage phenotype. Direct metabolic labeling experiments with Na(2) (32)PO(4) confirmed that the specific phosphorylation of recombinant NAGLU secreted from transfected CHO cells is significantly lower when compared with a control lysosomal enzyme. These results suggest that the use of secreted NAGLU in future enzyme and gene replacement therapy protocols will be severely limited due to its small degree of mannose-6-phosphorylation.

Identification of amino acids that modulate mannose phosphorylation of mouse DNase I, a secretory glycoprotein

We have reported that bovine DNase I, a secretory glycoprotein, acquires mannose 6-phosphate residues on 12.6% of its Asn-linked oligosaccharides when expressed in COS-1 cells and that the extent of phosphorylation increases to 79.2% when lysines are placed at positions 27 and 74 of the mature protein (Nishikawa, A., Gregory, W. , Frenz, J., Cacia, J., and Kornfeld, S. (1997) J. Biol. Chem. 272, 19408-19412). We now demonstrate that murine DNase I, which contains Lys27 and Lys74, is phosphorylated only 20.9% when expressed in the same COS-1 cell system. This difference is mostly due to the absence of three residues present in bovine DNase I (Tyr54, Lys124, and Ser190) along with the presence of a valine at position 23 that is absent in the bovine species. We show that Val23 inhibits phosphorylation at the Asn18 glycosylation site, whereas Tyr54, Lys124, and Ser190 enhance phosphorylation at the Asn106 glycosylation site. Tyr54 and Ser190 are widely separated from each other and from Asn106 on the surface of DNase I, indicating that residues present over a broad area influence the interaction with UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, which is responsible for the formation of mannose 6-phosphate residues on lysosomal enzymes.

Interaction of arylsulfatase A with UDP-N-acetylglucosamine:Lysosomal enzyme-N-acetylglucosamine-1-phosphotransferase

The critical step in lysosomal targeting of soluble lysosomal enzymes is the recognition by an UDP-N-acetylglucosamine:lysosomal enzyme-N-acetylglucosamine-1-phosphotransferase. The structure of the determinant common to all lysosomal enzymes for proper recognition by the phosphotransferase is not completely understood. Our current knowledge is largely based on the introduction of targeted amino acid substitutions into lysosomal enzymes and analysis of their effects on phosphotransferase recognition. We have investigated the effect of eight anti-arylsulfatase A monoclonal antibodies on the interaction of arylsulfatase A with the lysosomal enzyme phosphotransferase in vitro. We also show that a lysine-rich surface area of arylsulfatases A and B is essential for proper recognition by the phosphotransferase. Monoclonal antibodies bind to at least six different epitopes at different locations on the surface of arylsulfatase A. All antibodies bind outside the lysine-rich recognition area, but nevertheless Fab fragments of these antibodies prevent interaction of arylsulfatase A with the phosphotransferase. Our data support a model in which binding of arylsulfatase A to the phosphotransferase is not restricted to a limited surface area but involves the simultaneous recognition of large parts of arylsulfatase A.

Identification of UDP-N-acetylglucosamine-phosphotransferase-binding sites on the lysosomal proteases, cathepsins A, B, and D

A key step in the targeting of soluble lysosomal enzymes is their recognition and phosphorylation by a 540 kDa multisubunit enzyme, UDP-N-acetylglucosamine-phosphotransferase (phosphotransferase). The molecular mechanism of recognition is still unknown, but previous experiments suggested that the phosphotransferase-binding sites on lysosomal proteins are represented by structurally conserved surface patches of amino acids. We identified four such regions on nonhomologous lysosomal enzymes, cathepsins A, B, and D, which were superimposed by rotating their structures around the Calpha atom of the glycosylated Asn residue. We proposed that these regions represent putative phosphotransferase-binding sites and tested synthetic peptides, derived from these regions on the basis of surface accessibility, for their ability to inhibit in vitro phosphorylation of purified cathepsins A, B, and D. Our results indicate that cathepsin A and cathepsin D have one closely related phosphotransferase recognition site represented by a structurally and topologically conserved beta-hairpin loop, similar to that previously identified in lysosomal beta-glucuronidase. The most potent inhibition of phosphorylation was demonstrated by homologous peptides derived from the regions located on cathepsin molecules opposite the oligosaccharide chains which are phosphorylated by the phosphotransferase. We propose that recognition and catalytic sites of the phosphotransferase are located on different subunits, therefore, providing an effective mechanism for binding and phosphorylation of lysosomal proteins of different molecular size.

Lysine-based structure responsible for selective mannose phosphorylation of cathepsin D and cathepsin L defines a common structural motif for lysosomal enzyme targeting

Previous studies have shown that lysine residues on the surface of cathepsins and other lysosomal proteins are a shared component of the recognition structure involved in mannose phosphorylation. In this study, the involvement of specific lysine residues in mannose phosphorylation of cathepsin D was explored by site-directed mutagenesis. Mutation of two lysine residues in the mature portion of the protein, Lys-203 and Lys-293, cooperated to inhibit mannose phosphorylation by 70%. Other positively charged residues could not substitute for lysine at these positions, and comparison of thermal denaturation curves for the wild type and mutant proteins indicated that the inhibition could not be explained by alterations in protein folding. Structural comparisons of the two lysine residues with those required for phosphorylation of cathepsin L, using models generated from recently acquired crystal structures, revealed several relevant similarities. On both molecules, the lysine residues were positioned approximately 34 A apart (34.06 A for cathepsin D and 33.80 A for cathepsin L). When the lysine pairs were superimposed, N-linked glycosylation sites on the two proteins were found to be oriented so that oligosaccharides extending out from the sites could share a common region of space. Further similarities in the local environments of the critical lysines were also observed. These results provide details for a common lysosomal targeting structure based on a specific arrangement of lysine residues with respect to each other and to glycosylation sites on the surface of lysosomal proteins.

The cellular trafficking and zinc dependence of secretory and lysosomal sphingomyelinase, two products of the acid sphingomyelinase gene

The acid sphingomyelinase (ASM) gene, which has been implicated in ceramide-mediated cell signaling and atherogenesis, gives rise to both lysosomal SMase (L-SMase), which is reportedly cation-independent, and secretory SMase (S-SMase), which is fully or partially dependent on Zn2+ for enzymatic activity. Herein we present evidence for a model to explain how a single mRNA gives rise to two forms of SMase with different cellular trafficking and apparent differences in Zn2+ dependence. First, we show that both S-SMase and L-SMase, which contain several highly conserved zinc-binding motifs, are directly activated by zinc. In addition, SMase assayed from a lysosome-rich fraction of Chinese hamster ovary cells was found to be partially zinc-dependent, suggesting that intact lysosomes from these cells contain subsaturating levels of Zn2+. Analysis of Asn-linked oligosaccharides and of N-terminal amino acid sequence indicated that S-SMase arises by trafficking through the Golgi secretory pathway, not by cellular release of L-SMase during trafficking to lysosomes or after delivery to lysosomes. Most importantly, when Zn2+-dependent S-SMase was incubated with SMase-negative cells, the enzyme was internalized, trafficked to lysosomes, and became zinc-independent. We conclude that L-SMase is exposed to cellular Zn2+ during trafficking to lysosomes, in lysosomes, and/or during cell homogenization. In contrast, the pathway targeting S-SMase to secretion appears to be relatively sequestered from cellular pools of Zn2+; thus S-SMase requires exogeneous Zn2+ for full activity. This model provides important information for understanding the enzymology and regulation of L- and S-SMase and for exploring possible roles of ASM gene products in cell signaling and atherogenesis.

Factors controlling the glycosylation potential of the Golgi apparatus

Most of the biosynthetic reactions that generate the oligosaccharide structures of eukaryotic cells occur in compartments of the Golgi apparatus. This article provides a brief outline of the major glycosylation pathways of the Golgi, and discusses current understanding of the many factors that can control the glycosylation potential of this organelle. Old and new approaches towards elucidating the organization of glycosylation machinery in the Golgi are also considered.

Several cooperating binding sites mediate the interaction of a lysosomal enzyme with phosphotransferase

Lysosomal targeting of soluble lysosomal hydrolases is mediated by mannose 6-phosphate receptors, which recognize and bind mannose 6-phosphate residues in the oligosaccharide chains of proteins destined for delivery to lysosomes. This recognition marker is generated by the sequential action of two enzymes, the first of which, UDP-N-acetylglucosamine phosphotransferase, recognizes lysosomal enzymes on the basis of a structural determinant in their polypeptide chains. This recognition event is a key step in lysosomal targeting of soluble proteins, but the exact nature of the recognition determinant is not well understood. In this study we have characterized the phosphotransferase recognition signals of human lysosomal aspartylglucosaminidase (AGA) using transient expression of polypeptides carrying targeted amino acid substitutions. We found that three lysine residues and a tyrosine residing in three spatially distinct regions of the AGA polypeptide are necessary for phosphorylation of the oligosaccharides. Two of the lysines are especially important for the lysosomal targeting efficiency of AGA, which seems to be mostly dictated by the degree of phosphorylation of the alpha subunit oligosaccharide. On the basis of the results of this and previous studies we suggest a general model for recognition of lysosomal enzymes by the phosphotransferase.

The phosphorylation of bovine DNase I Asn-linked oligosaccharides is dependent on specific lysine and arginine residues

The secretory glycoprotein DNase I acquires mannose 6-phosphate moieties on its Asn-linked oligosaccharides, indicating that it is a substrate for UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (phosphotransferase) (Cacia, J., Quan, C., and Frenz, J. (1995) Glycobiology 4, 99). Phosphotransferase recognizes a conformation-dependent protein determinant that is present in lysosomal hydrolases, but absent in most secretory glycoproteins. To identify the amino acid residues of DNase I that are required for interaction with phosphotransferase, wild-type and mutant forms of bovine DNase I were expressed in COS-1 cells and the extent of oligosaccharide phosphorylation determined. Phosphorylation of DNase I oligosaccharides decreased from 12.6% to 2.3% when Lys-50, Lys-124, and Arg-27 were mutated to alanines, indicating that these residues are required for the basal level of phosphorylation. Mutation of lysines at other positions did not impair phosphorylation, demonstrating the selectivity of this process. When Arg-27 was replaced with a lysine, phosphorylation increased to 54%, showing that phosphotransferase prefers lysine residues to arginines. Mutation of Asn-74 to a lysine also increased phosphorylation to 50.3%, and the double mutant (R27K/N74K) was phosphorylated 79%, equivalent to the values obtained with lysosomal hydrolases. Interestingly, Lys-27 and Lys-74 caused selective phosphorylation of the neighboring Asn-linked oligosaccharide. Finally, mutation of Lys-117 to an alanine stimulated phosphorylation, demonstrating that some residues may be negative regulators of this process. We conclude that selected lysine and arginine residues on the surface of DNase I constitute the major elements of the phosphotransferase recognition domain present on this secretory glycoprotein.

Abnormal glycosylation of procathepsin L due to N-terminal point mutations correlates with failure to sort to lysosomes

A single point mutation in the lysosomal proenzyme receptor-inhibiting sequence near the N terminus of mouse procathepsin L can result in glycosylation of a normally cryptic site near its C terminus. When alanine replaced His36, Arg38, or Tyr40, the nascent chain of the mutant protein cotranslationally acquired a high mannose oligosaccharide chain at Asn268. In contrast, when alanine replaced Ser34, Arg37, or Leu39, this second carbohydrate chain was not added. This alternating pattern of abnormal glycosylation suggested that propeptide residues 36-40 normally assume an extended conformation having the side chains of residues 36, 38, and 40 facing in the same direction. When tyrosine conservatively replaced His36 or lysine replaced Arg38, Asn268 was not glycosylated. But the procathepsin L mutant having phenylalanine in place of Tyr40 was glycosylated at Asn268, which indicates that the hydrogen bond between the hydroxyl group of Tyr40 and the carboxylate group of Asp82 is necessary for normal folding of the nascent proenzyme chain. Mutation of the adjacent alpha2p (ERININ) helix of the propeptide or addition of a C-terminal epitope tag sequence to procathepsin L also induced misfolding of the proenzyme, as indicated by addition of the second oligosaccharide chain. In contrast, the propeptide mutation KAKK99-102AAAA had no effect on carbohydrate modification even though it reduced the positive charge of the proenzyme. Misfolded mutant mouse procathepsin L was not efficiently targeted to lysosomes on expression in human HeLa cells, even though it acquired phosphate on mannose residues. The majority of the mutant protein was secreted after undergoing modification with complex sugars. Similarly, epitope-tagged mouse procathepsin L was not targeted to lysosomes in homologous mouse cells but was efficiently secreted. Since production of mature endogenous protease was not reduced in cells expressing the tagged protein, the tagged protein did not compete with endogenous procathepsin L for targeting to lysosomes.

Expression of human H-type alpha1,2-fucosyltransferase encoding for blood group H(O) antigen in Chinese hamster ovary cells. Evidence for preferential fucosylation and truncation of polylactosamine sequences

The human H(O) blood group is specified by the structure Fucalpha1-2Galbeta1-R, but the factors regulating expression of this determinant on cell surface glycoconjugates are not well understood. To learn more about the regulation of H blood group expression, cDNA encoding the human H-type GDPFuc:beta-D-galactoside alpha1, 2-fucosyltransferase (alpha1,2FT) was stably transfected into Chinese hamster ovary (CHO) cells. The new cell line, designated CHO(alpha1,2)FT, expressed surface neoglycans containing the H antigen. The structures of the fucosylated neoglycans in CHO(alpha1, 2)FT cells and the distribution of these glycans on glycoproteins were characterized. Seventeen percent of the [3H]Gal-labeled glycopeptides from CHO(alpha1,2)FT cells bound to the immobilized H blood group-specific lectin Ulex europaeus agglutinin-I (UEA-I), whereas none from parental CHO cells bound to the lectin. The glycopeptides from CHO(alpha1,2)FT cells binding to UEA-I contained polylactosamine [3Galbeta1-4GlcNAcbeta1-]n with the terminal sequence Fucalpha1-2Galbeta1- 4GlcNAc-R. Fucosylation of the polylactosamine sequences on complex-type N-glycans in CHO(alpha1, 2)FT cells caused a decrease in both sialylation and length of polylactosamine. Unexpectedly, only small amounts of terminal fucosylation was found in diantennary complex-type N-glycans. The O-glycans and glycolipids were not fucosylated by the H-type alpha1, 2FT. Two major high molecular weight glycoproteins, one of which was shown to be the lysosome-associated membrane glycoprotein LAMP-1, preferentially contained the H-type structure and were bound by immobilized UEA-I. These results demonstrate that in CHO cells the expressed H-type alpha1,2FT does not indiscriminately fucosylate terminal galactosyl residues in complex-type N-glycans, but it favors glycans containing polylactosamine and dramatically alters their length and sialylation.

Lysosomal targeting of palmitoyl-protein thioesterase

Palmitoyl-protein thioesterase is a newly described long chain fatty-acid hydrolase that removes fatty acyl groups from modified cysteines in proteins. We have recently identified palmitoyl-protein thioesterase as the defective enzyme in the recessive hereditary neurological degenerative disorder infantile neuronal ceroid lipofuscinosis (Vesa, J., Hellsten, E., Verkruyse, L. A., Camp, L. A. , Rapola, J., Santavuori, P., Hofmann, S. L., and Peltonen, L. (1995) Nature 376, 584-587). A defect in a lysosomal enzyme had been postulated for the disease, but until recently, the relevant defective lysosomal enzyme had not been identified. In this paper, we present evidence for the lysosomal localization of palmitoyl-protein thioesterase. We show that COS cells take up exogenously supplied palmitoyl-protein thioesterase intracellularly and that the cellular uptake is blocked by mannose 6-phosphate, a hallmark of lysosomal enzyme trafficking. The enzyme contains endoglycosidase H-sensitive oligosaccharides that contain phosphate groups. Furthermore, palmitoyl-protein thioesterase cosediments with lysosomal enzyme markers by Percoll density gradient centrifugation. Interestingly, the pH optimum for the enzyme is in the neutral range, a property shared by two other lysosomal enzymes that remove post-translational protein modifications. These findings suggest that palmitoyl-protein thioesterase is a lysosomal enzyme and that infantile neuronal ceroid lipofuscinosis is properly classified as a lysosomal storage disorder.

Repetitive Ser-Gly sequences enhance heparan sulfate assembly in proteoglycans

We showed previously that the synthesis of heparan sulfate on betaglycan occurs at a Ser-Gly dipeptide flanked by a cluster of acidic residues and an adjacent tryptophan (Zhang, L., and Esko, J.D. (1994) J. Biol. Chem. 269, 19295-19299). A survey of the protein data base revealed that most heparan sulfate proteoglycans contain repetitive (Ser-Gly)n segments (n = 2) and a nearby cluster of acidic residues. To study the role of these amino acid sequences in controlling heparan sulfate synthesis, we have examined the assembly of glycosaminoglycans on Chinese hamster ovary (CHO) cell syndecan-1. The glycosylation sites were mapped by making chimeric proteoglycans containing segments of CHO syndecan-1 cDNA fused to Protein A. Two sites near the transmembrane domain (-EGS205GEQ- and -ETS215GEN-) were used solely for chondroitin sulfate synthesis, whereas three sites near the N terminus (-DGS35GDDSDNFS45GS47GTG-) supported both heparan sulfate and chondroitin sulfate synthesis. The strongest sites for heparan sulfate synthesis consisted of the repeat unit, -S45GS47G-. An unusual coupling phenomenon occurred across the adjacent SG dipeptides, leading to a greater proportion of heparan sulfate than predicted by the behavior of each site acting independently. The clusters of acidic residues adjacent to the heparan sulfate sites play important roles as well. These sequence motifs suggest a set of rules for predicting whether heparan sulfate assembles at glycosylation sites in proteoglycan core proteins.

Lysine-based structure in the proregion of procathepsin L is the recognition site for mannose phosphorylation

The recognition of lysosomal enzymes by UDP-GlcNAc: lysosomal-enzyme GlcNAc-1-phosphotransferase (phosphotransferase) is mediated by a protein structure on lysosomal enzymes. It has been previously demonstrated that lysine residues are required for phosphorylation of procathepsin L and are a common feature of the site on many lysosomal proteins. In this work, the procathepsin L recognition structure was further defined by identification of the region of the protein containing the structure and the critical lysine residues involved. Removal of the cathepsin L propeptide by low pH-induced autocatalytic processing abolished phosphorylation. The addition of either the purified propeptide or a glutathione S-transferase-propeptide fusion protein to the processed protein restored phosphorylation. Mutagenesis of individual lysine residues demonstrated that two propeptide lysine residues (Lys-54 and Lys-99) were required for efficient phosphorylation of procathepsin L. By comparison of the phosphorylation rates of procathepsin L, lysine-modified procathepsin L, and the procathepsin L oligosaccharide, lysine residues were shown to account for most, if not all, of the protein-dependent interaction. On this basis, it is concluded that the proregion lysine residues are the major elements of the procathepsin L recognition site. In addition, lysine residues in cathepsin D were shown to be as important for phosphorylation as those in procathepsin L, supporting a general model of the recognition site as a specific three-dimensional arrangement of lysine residues exposed on the surface of lysosomal proteins.

Identification and structural characterization of a mannose-6-phosphate containing oligomannosidic N-glycan from human erythropoietin secreted by recombinant BHK-21 cells

A sialidase resistant mono-charged N-glycan was isolated from glycosylation site I (Asn-24) of recombinant human erythropoietin expressed from baby hamster kidney (BHK-21) cells and constituted approximately 2-4% of the oligosaccharide material at this glycosylation site. Mass spectrometry and both 1- and 2-dimensional NMR techniques revealed a high mannose type structure (Man6) with a phospho-diesterbridged N-acetylglucosamine as follows: [formula: see text]

Lysine residues in the C-terminal lobe and lysosomal targeting of procathepsin D

A major pathway to the lysosome for soluble hydrolases involves the 6-phosphorylation of mannose residues. The initial step in this reaction is catalyzed by a phosphotransferase which recognizes lysosomal precursors. We constructed mutants of human procathepsin D whose targeting to the lysosome could be assayed directly in intact cells. Eight lysine residues were individually converted to glutamic acid on the surface of the carboxyl terminal lobe of the protein. Mutants with as many as four Lys to Glu mutations were normally targeted to the lysosome and processed to the mature form of the enzyme in transfected cells. We conclude that the C-terminal lobe of procathepsin D may not carry a determinant essential for lysosomal targeting in intact fibroblasts.