Ser72Pro active-site disease mutation in human lysosomal aspartylglucosaminidase: abnormal intracellular processing and evidence for extracellular activation

Development of precision therapies for rare inborn errors of metabolism: Functional investigations in cell culture models

Due to the low number of patients, rare genetic diseases are a special challenge for the development of therapies, especially for diseases that result from numerous, patient-specific pathogenic variants. Precision medicine makes use of various kinds of molecular information about a specific variant, so that the possibilities for an effective therapy based on the molecular features of the variants can be elucidated. The attention to personalized precision therapies has increased among scientists and clinicians, since the "single drug for all patients" approach does not allow the classification of individuals in subgroups according to the differences in the disease genotype or phenotype. This review article summarizes some approaches of personalized precision medicine that can be used for a cost-effective and fast development of therapies, even for single patients. We have focused on specific examples on inborn errors of metabolism, with special attention on drug repurposing. Furthermore, we provide an overview of cell culture models that are suitable for precision medicine approaches.

A new horizon in the phosphorylated sites of AGA: the structural impact of C163S mutation in aspartylglucosaminuria through molecular dynamics simulation

Aspartylglucosaminuria (AGU) is a lysosomal storage disorder caused by insufficient aspartylglucosaminidase (AGA) activity leading to chronic neurodegeneration. We utilized the PhosphoSitePlus tool to identify the AGA protein's phosphorylation sites. The phosphorylation was induced on the specific residue of the three-dimensional AGA protein, and the structural changes upon phosphorylation were studied via molecular dynamics simulation. Furthermore, the structural behaviour of C163S mutation and C163S mutation with adjacent phosphorylation was investigated. We have examined the structural impact of phosphorylated forms and C163S mutation in AGA. Molecular dynamics simulations (200 ns) exposed patterns of deviation, fluctuation, and change in compactness of Y178 phosphorylated AGA protein (Y178-p), T215 phosphorylated AGA protein (T215-p), T324 phosphorylated AGA protein (T324-p), C163S mutant AGA protein (C163S), and C163S mutation with Y178 phosphorylated AGA protein (C163S-Y178-p). Y178-p, T215-p, and C163S mutation demonstrated an increase in intramolecular hydrogen bonds, leading to greater compactness of the AGA forms. Principle component analysis (PCA) and Gibbs free energy of the phosphorylated/C163S mutation structures exhibit transition in motion/orientation than Wild type (WT). T215-p may be more dominant among these than the other studied phosphorylated forms. It might contribute to hydrolyzing L-asparagine functioning as an asparaginase, thereby regulating neurotransmitter activity. This study revealed structural insights into the phosphorylation of Y178, T215, and T324 in AGA protein. Additionally, it exposed the structural changes of the C163S mutation and C163S-Y178-p of AGA protein. This research will shed light on a better understanding of AGA's phosphorylated mechanism.Communicated by Ramaswamy H. Sarma.

Aspartylglucosaminuria: Clinical Presentation and Potential Therapies

Aspartylglucosaminuria (AGU) is a recessively inherited neurodegenerative lysosomal storage disease characterized by progressive intellectual disability, skeletal abnormalities, connective tissue overgrowth, gait disturbance, and seizures followed by premature death. AGU is caused by pathogenic variants in the aspartylglucosaminidase (AGA) gene, leading to glycoasparagine accumulation and cellular dysfunction. Although more prevalent in the Finnish population, more than 30 AGA variants have been identified worldwide. Owing to its rarity, AGU may be largely underdiagnosed. Recognition of the following early clinical features may aid in AGU diagnosis: developmental delays, hyperactivity, early growth spurt, inguinal and abdominal hernias, clumsiness, characteristic facial features, recurring upper respiratory and ear infections, tonsillectomy, multiple sets of tympanostomy tube placement, and sleep problems. Although no curative therapies currently exist, early diagnosis may provide benefit through the provision of anticipatory guidance, management of expectations, early interventions, and prophylaxis; it will also be crucial for increased clinical benefits of future AGU disease-modifying therapies.

Amlexanox provides a potential therapy for nonsense mutations in the lysosomal storage disorder Aspartylglucosaminuria

Aspartylglucosaminuria (AGU) is a lysosomal storage disorder caused by mutations in the gene for aspartylglucosaminidase (AGA). This enzyme participates in glycoprotein degradation in lysosomes. AGU results in progressive mental retardation, and no curative therapy is currently available. We have here characterized the consequences of AGA gene mutations in a compound heterozygous patient who exhibits a missense mutation producing a Ser72Pro substitution in one allele, and a nonsense mutation Trp168X in the other. Ser72 is not a catalytic residue, but is required for the stabilization of the active site conformation. Thus, Ser72Pro exchange impairs the autocatalytic activation of the AGA precursor, and results in a considerable reduction of the enzyme activity and in altered AGA precursor processing. Betaine, which can partially rescue the AGA activity in AGU patients carrying certain missense mutations, turned out to be ineffective in the case of Ser72Pro substitution. The Trp168X nonsense allele results in complete lack of AGA polypeptide due to nonsense-mediated decay (NMD) of the mRNA. Amlexanox, which inhibits NMD and causes a translational read-through, facilitated the synthesis of a full-length, functional AGA protein from the nonsense allele. This could be demonstrated as presence of the AGA polypeptide and increased enzyme activity upon Amlexanox treatment. Furthermore, in the Ser72Pro/Trp168X expressing cells, Amlexanox induced a synergistic increase in AGA activity and polypeptide processing due to enhanced processing of the Ser72Pro polypeptide. Our data show for the first time that Amlexanox might provide a valid therapy for AGU.

Biochemical characterization and comparison of aspartylglucosaminidases secreted in venom of the parasitoid wasps Asobara tabida and Leptopilina heterotoma

Aspartylglucosaminidase (AGA) is a low-abundance intracellular enzyme that plays a key role in the last stage of glycoproteins degradation, and whose deficiency leads to human aspartylglucosaminuria, a lysosomal storage disease. Surprisingly, high amounts of AGA-like proteins are secreted in the venom of two phylogenetically distant hymenopteran parasitoid wasp species, Asobara tabida (Braconidae) and Leptopilina heterotoma (Cynipidae). These venom AGAs have a similar domain organization as mammalian AGAs. They share with them key residues for autocatalysis and activity, and the mature α- and β-subunits also form an (αβ)2 structure in solution. Interestingly, only one of these AGAs subunits (α for AtAGA and β for LhAGA) is glycosylated instead of the two subunits for lysosomal human AGA (hAGA), and these glycosylations are partially resistant to PGNase F treatment. The two venom AGAs are secreted as fully activated enzymes, they have a similar aspartylglucosaminidase activity and are both also efficient asparaginases. Once AGAs are injected into the larvae of the Drosophila melanogaster host, the asparaginase activity may play a role in modulating their physiology. Altogether, our data provide new elements for a better understanding of the secretion and the role of venom AGAs as virulence factors in the parasitoid wasps' success.

Functional Analysis of the Ser149/Thr149 Variants of Human Aspartylglucosaminidase and Optimization of the Coding Sequence for Protein Production

Aspartylglucosaminidase (AGA) is a lysosomal hydrolase that participates in the breakdown of glycoproteins. Defects in the AGA gene result in a lysosomal storage disorder, aspartylglucosaminuria (AGU), that manifests mainly as progressive mental retardation. A number of AGU missense mutations have been identified that result in reduced AGA activity. Human variants that contain either Ser or Thr in position 149 have been described, but it is unknown if this affects AGA processing or activity. Here, we have directly compared the Ser149/Thr149 variants of AGA and show that they do not differ in terms of relative specific activity or processing. Therefore, Thr149 AGA, which is the rare variant, can be considered as a neutral or benign variant. Furthermore, we have here produced codon-optimized versions of these two variants and show that they are expressed at significantly higher levels than AGA with the natural codon-usage. Since optimal AGA expression is of vital importance for both gene therapy and enzyme replacement, our data suggest that use of codon-optimized AGA may be beneficial for these therapy options.

Molecular and biochemical analysis of an aspartylglucosaminidase from the venom of the parasitoid wasp Asobara tabida (Hymenoptera: Braconidae)

The most abundant venom protein of the parasitoid wasp Asobara tabida was identified to be an aspartylglucosaminidase (hereafter named AtAGA). The aim of the present work is the identification of: 1) its cDNA and deduced amino acid sequences, 2) its subunits organization and 3) its activity. The cDNA of AtAGA coded for a proalphabeta precursor molecule preceded by a signal peptide of 19 amino acids. The gene products were detected specifically in the wasp venom gland (in which it could be found) under two forms: an (active) heterotetramer composed of two alpha and two beta subunits of 30 and 18 kDa respectively and a homodimer of 44 kDa precursor. The activity of AtAGA enzyme showed a limited tolerance toward variations of pH and temperatures. Since the enzyme failed to exhibit any glycopeptide N-glycosidase activity toward entire glycoproteins, its activity seemed to be restricted to the deglycosylation of free glycosylasparagines like human AGA, indicating AtAGA did not evolve a broader function in the course of evolution. The study of this enzyme may allow a better understanding of the functional evolution of venom enzymes in hymenopteran parasitoids.

Glycosylation, transport, and complex formation of palmitoyl protein thioesterase 1 (PPT1)--distinct characteristics in neurons

BACKGROUND

Neuronal ceroid lipofuscinoses (NCLs) are collectively the most common type of recessively inherited childhood encephalopathies. The most severe form of NCL, infantile neuronal ceroid lipofuscinosis (INCL), is caused by mutations in the CLN1 gene, resulting in a deficiency of the lysosomal enzyme, palmitoyl protein thioesterase 1 (PPT1). The deficiency of PPT1 causes a specific death of neocortical neurons by a mechanism, which is currently unclear. To understand the function of PPT1 in more detail, we have further analyzed the basic properties of the protein, especially focusing on possible differences in non-neuronal and neuronal cells.

RESULTS

Our study shows that the N-glycosylation of N197 and N232, but not N212, is essential for PPT1's activity and intracellular transport. Deglycosylation of overexpressed PPT1 produced in neurons and fibroblasts demonstrates differentially modified PPT1 in different cell types. Furthermore, antibody internalization assays showed differences in PPT1 transport when compared with a thoroughly characterized lysosomal enzyme aspartylglucosaminidase (AGA), an important observation potentially influencing therapeutic strategies. PPT1 was also demonstrated to form oligomers by size-exclusion chromatography and co-immunoprecipitation assays. Finally, the consequences of disease mutations were analyzed in the perspective of our new results, suggesting that the mutations increase both the degree of glycosylation of PPT1 and its ability to form complexes.

CONCLUSION

Our current study describes novel properties for PPT1. We observe differences in PPT1 processing and trafficking in neuronal and non-neuronal cells, and describe for the first time the ability of PPT1 to form complexes. Understanding the basic characteristics of PPT1 is fundamental in order to clarify the molecular pathogenesis behind neurodegeneration in INCL.

Crystal Structure of Isoaspartyl Aminopeptidase in Complex with l-Aspartate

The crystal structure of Escherichia coli isoaspartyl aminopeptidase/asparaginase (EcAIII), an enzyme belonging to the N-terminal nucleophile (Ntn)-hydrolases family, has been determined at 1.9-A resolution for a complex obtained by cocrystallization with l-aspartate, which is a product of both enzymatic reactions catalyzed by EcAIII. The enzyme is a dimer of heterodimers, (alphabeta)(2). The (alphabeta) heterodimer, which arises by autoproteolytic cleavage of the immature protein, exhibits an alphabetabetaalpha-sandwich fold, typical for Ntn-hydrolases. The asymmetric unit contains one copy of the EcAIII.Asp complex, with clearly visible l-aspartate ligands, one bound in each of the two active sites of the enzyme. The l-aspartate ligand is located near Thr(179), the N-terminal residue of subunit beta liberated in the autoproteolytic event. Structural comparisons with the free form of EcAIII reveal that there are no major rearrangements of the active site upon aspartate binding. Although the ligand binding mode is similar to that observed in an l-aspartate complex of the related enzyme human aspartylglucosaminidase, the architecture of the EcAIII active site sheds light on the question of substrate specificity and explains why EcAIII is not able to hydrolyze glycosylated asparagine substrates.

Autoproteolytic activation of human aspartylglucosaminidase

Aspartylglucosaminidase (AGA) belongs to the N-terminal nucleophile (Ntn) hydrolase superfamily characterized by an N-terminal nucleophile as the catalytic residue. Three-dimensional structures of the Ntn hydrolases reveal a common folding pattern and equivalent stereochemistry at the active site. The activation of the precursor polypeptide occurs autocatalytically, and for some amidohydrolases of prokaryotes, the precursor structure is known and activation mechanisms are suggested. In humans, the deficient AGA activity results in a lysosomal storage disease, aspartylglucosaminuria (AGU) resulting in progressive neurodegeneration. Most of the disease-causing mutations lead to defective molecular maturation of AGA, and, to understand the structure-function relationship better, in the present study, we have analysed the effects of targeted amino acid substitutions on the activation process of human AGA. We have evaluated the effect of the previously published mutations and, in addition, nine novel mutations were generated. We could identify one novel amino acid, Gly258, with an important structural role on the autocatalytic activation of human AGA, and present the molecular mechanism for the autoproteolytic activation of the eukaryotic enzyme. Based on the results of the present study, and by comparing the available information on the activation of the Ntn-hydrolases, the autocatalytic processes of the prokaryotic and eukaryotic enzymes share common features. First, the critical nucleophile functions both as the catalytic and autocatalytic residue; secondly, the side chain of this nucleophile is oriented towards the scissile peptide bond; thirdly, conformational strain exists in the precursor at the cleavage site; finally, water molecules are utilized in the activation process.

Identification of an aspartylglucosaminidase-like protein in the venom of the parasitic wasp Asobara tabida (Hymenoptera: Braconidae)

This study was designed to identify one of the main components of venomous secretions of the endoparasitic wasp Asobara tabida. By using electrophoretic methods, partial amino acid sequencing and immunostaining, we demonstrated the presence of an aspartylglucosaminidase (AGA)-like protein in the venom of this insect. The enzyme had a polymeric conformation and was formed of 30 and 18 kDa subunits. The relative positions of several amino acids involved in substrate binding and catalytic activity of known AGA-proteins, which are usually lysosomal enzymes, were conserved in the NH(2)-terminal ends of these subunits. Antibodies raised against human AGA recognized the two subunits of the protein and a 44 kDa protein, suggesting the presence of a precursor molecule of the enzyme in the venom. However, no reliable measurement of the AGA activity could be performed on the venom extracts, which could be explained by the fact the enzyme would be stored in the reservoir of the venom apparatus under an inactive form. These results constitute the first description of an AGA-like protein in an insect venom and are discussed with respect to the knowledge acquired on lysosomal and venom enzymes.

Correction of peripheral lysosomal accumulation in mice with aspartylglucosaminuria by bone marrow transplantation

OBJECTIVE

Bone marrow transplantation has been shown to alleviate symptoms outside the CNS in many lysosomal storage diseases depending on the type and stage of the disease, but the effect on neurological symptoms is variable or still unclear. Aspartylglucosaminuria (AGU) is a lysosomal storage disease characterized by mental retardation, recurrent infections in childhood, hepatosplenomegaly and coarse facial features. Vacuolized storage lysosomes are found in all tissues of patients and uncleaved enzyme substrate is excreted in the urine. The recently generated AGU mouse model closely mimicks the human disease and serves as a good model to study the efficiency of bone marrow transplantation in this disease.

METHODS

Eight-week-old AGU mice were lethally irradiated and transplanted with bone marrow from normal donors. The AGA enzyme activity was measured in the liver and the brain and the degree of correction of tissue pathology was analyzed by light and electron microscopy. Reverse bone marrow transplantation (AGU bone marrow to wild-type mice) was also performed.

RESULTS

Six months after transplantation the AGA enzyme activity was 13% of normal in the liver, but only 3% in the brain. Tissue pathology was reversed in the liver and the spleen, but not in the brain and the kidney. The urinary excretion of enzyme substrate was diminished but still detectable. No storage vacuoles were found in the tissues after reverse transplantation, but subtle excretion of uncleaved substrate was detected in the urine.

CONCLUSION

Liver and spleen pathology of AGU was corrected by bone marrow transplantation, but there was no effect on lysosomal accumulation in the CNS and in the kidneys.

Toward understanding the neuronal pathogenesis of aspartylglucosaminuria: expression of aspartylglucosaminidase in brain during development

The deficiency of a lysosomal enzyme, aspartylglucosaminidase, results in a lysosomal storage disorder, aspartylglucosaminuria, manifesting as progressive mental retardation. To understand tissue pathogenesis and disease progression we analyzed the developmental expression of the enzyme, especially in brain, which is the major source of the pathological symptoms. Highest mRNA levels in brain were detected during embryogenesis, the levels decreased neonatally and started to increase again from Day 7 on. In Western analyses, a defective processing of aspartylglucosaminidase was observed in brain as compared to other tissues, resulting in very low levels of the mature, active form of the enzyme. Interestingly immunohistochemical analyses of mouse brain revealed that aspartylglucosaminidase immunoreactivity closely mimicked the myelin basic protein immunostaining pattern. The only evident neuronal staining was observed in the developing Purkinje cells of the cerebellum from Days 3 to 10, reflecting well the mRNA expression. In human infant brain, the immunostaining was also present in myelinated fibers as well as in the Purkinje cells and, additionally, in the soma and extensions of other neurons. In the adult human brain neurons and oligodendrocytes displayed immunoreactivity whereas myelinated fibers were not stained. Our results of aspartylglucosaminidase immunostaining in myelinated fibers of infant brain might imply the involvement of aspartylglucosaminidase in the early myelination process. This is consistent with previous magnetic resonance imaging findings in the brains of aspartylglucosaminuria patients, revealing delayed myelination in childhood.

A Gln747→Pro Substitution in the IIb Subunit Is Responsible for a Moderate IIbβ3Deficiency in Glanzmann Thrombasthenia

To clarify a molecular defect responsible for moderate IIbβ3 deficiency, we examined two unrelated patients, MT and MS, suffering from type II and type I Glanzmann thrombasthenia (GT), respectively. Sequence analysis of polymerase chain reaction (PCR) fragments derived from platelet mRNA showed a single A→C substitution at nucleotide (nt) 2334 leading to a Gln747→ Pro in IIb in both patients. Allele-specific restriction enzyme analysis (ASRA) of genomic DNA demonstrated that patient MT was homozygous for the Gln747→Pro substitution and patient MS was compound heterozygous for this substitution and for an RNA splice mutation at the consensus sequence of the splice acceptor site of exon 18 (AG→AA). Furthermore, ASRA showed that, among 17 unrelated Japanese GT patients, this Gln747→Pro substitution was detected in 4 patients, including MT and MS (homozygous, 2 patients; heterozygous, 2 patients). Cotransfection of Pro747IIb and β3 constructs into 293 cells resulted in moderate reduction in the amount of IIbβ3 within the transfected cells as well as on the cell surface. However, Pro747IIbβ3 bound the ligand mimetic monoclonal antibody (MoAb) PAC-1 after activation of IIbβ3 by the MoAb PT25-2, suggesting that the mutant IIbβ3 possesses the ligand-binding function. The association between the mutant proIIb and β3 was not disturbed. Surface labeling and pulse chase study showed that the Gln747→Pro substitution moderately impaired both intracellular transport of the IIbβ3 heterodimers to the Golgi apparatus and endoproteolytic cleavage of proIIb into heavy and light chains. By contrast, replacement of Gln747 with Ala by mutagenesis did not impair IIbβ3expression on the cell surface. These results suggest that the presence of Pro, rather than the absence of Gln, at amino acid residue 747 on IIb is responsible for moderate IIbβ3 deficiency.

© 1998 by The American Society of Hematology.

A Gln747→Pro Substitution in the IIb Subunit Is Responsible for a Moderate IIbβ3Deficiency in Glanzmann Thrombasthenia

To clarify a molecular defect responsible for moderate IIbβ3 deficiency, we examined two unrelated patients, MT and MS, suffering from type II and type I Glanzmann thrombasthenia (GT), respectively. Sequence analysis of polymerase chain reaction (PCR) fragments derived from platelet mRNA showed a single A→C substitution at nucleotide (nt) 2334 leading to a Gln747→ Pro in IIb in both patients. Allele-specific restriction enzyme analysis (ASRA) of genomic DNA demonstrated that patient MT was homozygous for the Gln747→Pro substitution and patient MS was compound heterozygous for this substitution and for an RNA splice mutation at the consensus sequence of the splice acceptor site of exon 18 (AG→AA). Furthermore, ASRA showed that, among 17 unrelated Japanese GT patients, this Gln747→Pro substitution was detected in 4 patients, including MT and MS (homozygous, 2 patients; heterozygous, 2 patients). Cotransfection of Pro747IIb and β3 constructs into 293 cells resulted in moderate reduction in the amount of IIbβ3 within the transfected cells as well as on the cell surface. However, Pro747IIbβ3 bound the ligand mimetic monoclonal antibody (MoAb) PAC-1 after activation of IIbβ3 by the MoAb PT25-2, suggesting that the mutant IIbβ3 possesses the ligand-binding function. The association between the mutant proIIb and β3 was not disturbed. Surface labeling and pulse chase study showed that the Gln747→Pro substitution moderately impaired both intracellular transport of the IIbβ3 heterodimers to the Golgi apparatus and endoproteolytic cleavage of proIIb into heavy and light chains. By contrast, replacement of Gln747 with Ala by mutagenesis did not impair IIbβ3expression on the cell surface. These results suggest that the presence of Pro, rather than the absence of Gln, at amino acid residue 747 on IIb is responsible for moderate IIbβ3 deficiency.

© 1998 by The American Society of Hematology.

Expression and endocytosis of lysosomal aspartylglucosaminidase in mouse primary neurons

Aspartylglucosaminuria (AGU) is a neurodegenerative lysosomal storage disease that is caused by mutations in the gene encoding for a soluble hydrolase, aspartylglucosaminidase (AGA). In this study, we have used our recently developed mouse model for AGU and analyzed processing, intracellular localization, and endocytosis of recombinant AGA in telencephalic AGU mouse neurons in vitro. The processing steps of AGA were found to be similar to the peripheral cells, but both the accumulation of the inactive precursor molecule and delayed lysosomal processing of the enzyme were detected. AGA was distributed to the cell soma and neuronal processes but was not found in the nerve terminals. Endocytotic capability of cultured telencephalic neurons was comparable to that of fibroblasts, and endocytosis of AGA was blocked by free mannose-6-phosphate (M6P), indicating that uptake of the enzyme was mediated by M6P receptors (M6PRs). Uptake of extracellular AGA was also studied in the tumor-derived cell lines rat pheochromocytoma (PC12) and mouse neuroblastoma cells (N18), which both endocytosed AGA poorly as compared with cultured primary neurons. Expression of cation-independent M6PRs (CI-M6PRs) in different cell lines correlated well with the endocytotic capability of these cells. Although a punctate expression pattern of CI-M6PRs was found in fibroblasts and cultured primary neurons, the expression was beyond the detection limit in PC12 and N18 cells. This indicates that PC12 and N18 are not feasible cell lines to describe neuronal uptake of mannose-6-phosphate-tagged proteins. This in vitro data will form an important basis for the brain-targeted therapy of AGU.

Activation and oligomerization of aspartylglucosaminidase

Secretory, membrane, and lysosomal proteins undergo covalent modifications and acquire their secondary and tertiary structure in the lumen of the endoplasmic reticulum (ER). In order to pass the ER quality control system and become transported to their final destinations, many of them are also assembled into oligomers. We have recently determined the three-dimensional structure of lysosomal aspartylglucosaminidase (AGA), which belongs to a newly discovered family of homologous amidohydrolases, the N-terminal nucleophile hydrolases. Members of this protein family are activated from an inactive precursor molecule by an autocatalytic proteolytic processing event whose exact mechanism has not been thoroughly determined. Here we have characterized in more detail the initial events in the ER required for the formation of active AGA enzyme using transient expression of polypeptides carrying targeted amino acid substitutions. We show that His124 at an interface between two heterodimers of AGA is crucial for the thermodynamically stable oligomeric structure of AGA. Furthermore, the side chain of Thr206 is essential both for the proteolytic activation and enzymatic activity of AGA. Finally, the proper geometry of the residues His204-Asp205 seems to be crucial for the activation of AGA precursor polypeptides. We propose here a reaction mechanism for the activation of AGA which could be valid for homologous enzymes as well.

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.

Aspartylglucosaminuria among Palestinian Arabs

Aspartylglucosaminuria (AGU) is a rare disorder of glycoprotein metabolism caused by the deficiency of the lysosomal enzyme aspartylglucosaminidase (AGA). AGU is inherited as an autosomal recessive trait and occurs with a high frequency in Finland because of a founder effect. While very few patients with AGU have been reported from non-Finnish origin, we diagnosed the disorder in 8 patients originating from 3 unrelated families, all Palestinian Arabs from the region of Jerusalem. The clinical diagnosis of AGU is often difficult, in particular early in the course of the disease, and most of the patients are diagnosed after the age of 5 years. However, since these patients excrete early large amounts of aspartylglucosamine in urine, biochemical screening is easy by urine chromatography.

Primary folding of aspartylglucosaminidase. Significance of disulfide bridges and evidence of early multimerization

Aspartylglucosaminidase (AGA) is a lysosomal enzyme involved in the degradation of N-linked glycoproteins in lysosomes. AGA is synthesized as an inactive precursor molecule, which is rapidly activated in the endoplasmic reticulum by a proteolytic cleavage into alpha- and beta-subunits. We have recently determined the three-dimensional structure of AGA and shown that it is a globular molecule with a heterotetrameric (alphabeta)2 structure. On the basis of structural and functional analyses, AGA seems to be the first mammalian protein belonging to a newly described protein family, the N-terminal nucleophile hydrolases. Because the activation of the prokaryotic members of the N-terminal nucleophile hydrolase family seems to be triggered by the assembly of the subunits, we have studied the initial folding and oligomerization of AGA and provide evidence that dimerization of two precursor molecules in the endoplasmic reticulum is a prerequisite for the activation of AGA. To gain further information on the structural determinants influencing the early folding of AGA, we used site-specific mutagenesis of cysteine residues to define the role of intrachain disulfide bridges in the folding and activation of the enzyme. The N-terminal disulfide bridges in both the alpha- and beta-subunits seem to have only a stabilizing role, whereas the C-terminal disulfide bridge in both subunits evidently plays an important role in the early folding and activation of AGA.