Molecular cloning of the cDNA which encodes beta-N-acetylhexosaminidase A from Dictyostelium discoideum. Complete amino acid sequence and homology with the human enzyme

Identification and Characterization of a β-N-Acetylhexosaminidase with a Biosynthetic Activity from the Marine Bacterium Paraglaciecola hydrolytica S66T

β-N-Acetylhexosaminidases are glycoside hydrolases (GHs) acting on N-acetylated carbohydrates and glycoproteins with the release of N-acetylhexosamines. Members of the family GH20 have been reported to catalyze the transfer of N-acetylglucosamine (GlcNAc) to an acceptor, i.e., the reverse of hydrolysis, thus representing an alternative to chemical oligosaccharide synthesis. Two putative GH20 β-N-acetylhexosaminidases, PhNah20A and PhNah20B, encoded by the marine bacterium Paraglaciecola hydrolytica S66^T, are distantly related to previously characterized enzymes. Remarkably, PhNah20A was located by phylogenetic analysis outside clusters of other studied β-N-acetylhexosaminidases, in a unique position between bacterial and eukaryotic enzymes. We successfully produced recombinant PhNah20A showing optimum activity at pH 6.0 and 50 °C, hydrolysis of GlcNAc β-1,4 and β-1,3 linkages in chitobiose (GlcNAc)₂ and GlcNAc-1,3-β-Gal-1,4-β-Glc (LNT2), a human milk oligosaccharide core structure. The kinetic parameters of PhNah20A for p-nitrophenyl-GlcNAc and p-nitrophenyl-GalNAc were highly similar: k_cat/K_M being 341 and 344 mM⁻¹·s⁻¹, respectively. PhNah20A was unstable in dilute solution, but retained full activity in the presence of 0.5% bovine serum albumin (BSA). PhNah20A catalyzed the formation of LNT2, the non-reducing trisaccharide β-Gal-1,4-β-Glc-1,1-β-GlcNAc, and in low amounts the β-1,2- or β-1,3-linked trisaccharide β-Gal-1,4(β-GlcNAc)-1,x-Glc by a transglycosylation of lactose using 2-methyl-(1,2-dideoxy-α-d-glucopyrano)-oxazoline (NAG-oxazoline) as the donor. PhNah20A is the first characterized member of a distinct subgroup within GH20 β-N-acetylhexosaminidases.

Dictyostelium, a microbial model for brain disease

BACKGROUND

Most neurodegenerative diseases are associated with mitochondrial dysfunction. In humans, mutations in mitochondrial genes result in a range of phenotypic outcomes which do not correlate well with the underlying genetic cause. Other neurodegenerative diseases are caused by mutations that affect the function and trafficking of lysosomes, endosomes and autophagosomes. Many of the complexities of these human diseases can be avoided by studying them in the simple eukaryotic model Dictyostelium discoideum.

SCOPE OF REVIEW

This review describes research using Dictyostelium to study cytopathological pathways underlying a variety of neurodegenerative diseases including mitochondrial, lysosomal and vesicle trafficking disorders.

MAJOR CONCLUSIONS

Generalised mitochondrial respiratory deficiencies in Dictyostelium produce a consistent pattern of defective phenotypes that are caused by chronic activation of a cellular energy sensor AMPK (AMP-activated protein kinase) and not ATP deficiency per se. Surprisingly, when individual subunits of Complex I are knocked out, both AMPK-dependent and AMPK-independent, subunit-specific phenotypes are observed. Many nonmitochondrial proteins associated with neurological disorders have homologues in Dictyostelium and are associated with the function and trafficking of lysosomes and endosomes. Conversely, some genes associated with neurodegenerative disorders do not have homologues in Dictyostelium and this provides a unique avenue for studying these mutated proteins in the absence of endogeneous protein.

GENERAL SIGNIFICANCE

Using the Dictyostelium model we have gained insights into the sublethal cytopathological pathways whose dysregulation contributes to phenotypic outcomes in neurodegenerative disease. This work is beginning to distinguish correlation, cause and effect in the complex network of cross talk between the various organelles involved. This article is part of a Special Issue entitled Frontiers of Mitochondrial Research.

Dictyostelium as a model for human lysosomal and trafficking diseases

Dictyostelium cells are genetically haploid and therefore easily analyzed for mutant phenotypes. In the past, many tools and molecular markers have been developed for a quantitative and qualitative analysis of the endocytic pathway in these amoebae. This review outlines parallels and discrepancies between mutants in Dictyostelium, the corresponding mammalian cells and the symptoms of human patients affected by lysosomal and trafficking defects. Situations where knowledge from Dictyostelium may potentially help understand human disease and vice versa are also addressed.

Molecular cloning and functional characterization of beta-N-acetylglucosaminidase genes from Sf9 cells

Sf9, a cell line derived from the lepidopteran insect, Spodoptera frugiperda, is widely used as a host for recombinant glycoprotein expression and purification by baculovirus vectors. Previous studies have shown that this cell line has one or more beta-N-acetylglucosaminidase activities that may be involved in the degradation and/or processing of N-glycoprotein glycans. However, these enzymes and their functions remain poorly characterized. Therefore, the goal of this study was to isolate beta-N-acetylglucosaminidase genes from Sf9 cells, over-express the gene products, and characterize their enzymatic activities. A degenerate PCR approach yielded three Sf9 cDNAs, which appeared to encode two distinct beta-N-acetylglucosaminidases, according to bioinformatic analyses. Baculovirus-mediated expression of these two cDNA products induced membrane-associated beta-N-acetylglucosaminidase activities in Sf9 cells, which cleaved terminal N-acetylglucosamine residues from the alpha-3 and -6 branches of a biantennary N-glycan substrate with acidic pH optima and completely hydrolyzed chitotriose to its constituent N-acetylglucosamine monomers. GFP-tagged forms of both enzymes exhibited punctate cytoplasmic fluorescence, which did not overlap with either lysosomal or Golgi-specific dyes. Together, these results indicated that the two new Sf9 genes identified in this study encode broad-spectrum beta-N-acetylglucosaminidases that appear to have unusual intracellular distributions. Their relative lack of substrate specificity and acidic pH optima are consistent with a functional role for these enzymes in glycoprotein glycan and chitin degradation, but not with a role in N-glycoprotein glycan processing.

Detachment of Actinobacillus actinomycetemcomitans biofilm cells by an endogenous beta-hexosaminidase activity

When cultured in broth, fresh clinical isolates of the gram-negative periodontal pathogen Actinobacillus actinomycetemcomitans form tenaciously adherent biofilm colonies on surfaces such as plastic and glass. These biofilm colonies release adherent cells into the medium, and the released cells can attach to the surface of the culture vessel and form new colonies, enabling the biofilm to spread. We mutagenized A. actinomycetemcomitans clinical strain CU1000 with transposon IS903phikan and isolated a transposon insertion mutant that formed biofilm colonies which were tightly adherent to surfaces but which lacked the ability to release cells into the medium and disperse. The transposon insertion in the mutant strain mapped to a gene, designated dspB, that was predicted to encode a secreted protein homologous to the catalytic domain of the family 20 glycosyl hydrolases. A plasmid carrying a wild-type dspB gene restored the ability of biofilm colonies of the mutant strain to disperse. We expressed A. actinomycetemcomitans DspB protein engineered to contain a hexahistidine metal-binding site at its C terminus in Escherichia coli and purified the protein by using Ni affinity chromatography. Substrate specificity studies performed with monosaccharides labeled with 4-nitrophenyl groups showed that DspB hydrolyzed the 1-->4 glycosidic bond of beta-substituted N-acetylglucosamine, which is consistent with the known functions of other family 20 glycosyl hydrolases. When added to culture medium, purified DspB protein, but not heat-inactivated DspB, restored the ability of the mutant strain to release cells and disperse. DspB protein also caused the detachment of cells from preformed biofilm colonies of strain CU1000 grown attached to plastic and the disaggregation of highly autoaggregated clumps of CU1000 cells in solution. We concluded that dspB encodes a soluble beta-N-acetylglucosaminidase that causes detachment and dispersion of A. actinomycetemcomitans biofilm cells.

Molecular cloning and sequence analysis of an ascidian egg beta-N-acetylhexosaminidase with a potential role in fertilization

Beta-N-acetylhexosaminidase, which is found almost ubiquitously in sperm of invertebrates and vertebrates, supposedly mediates a carbohydrate-based transient sperm-egg coat binding. In ascidians and mammals, beta-hexosaminidase released at fertilization from eggs has been proposed to modify sperm receptor glycoproteins of the egg envelope, thus setting up a block to polyspermy. Previously, it was shown that in potential sperm receptor glycoproteins of the ascidian Phallusia mammillata, N-acetylglucosamine is the prevailing glycoside residue and that the egg harbors three active molecular forms of beta-hexosaminidase. In the present study, P. mammillata beta-hexosaminidase cDNA was isolated from an ovarian cDNA library and characterized. The deduced amino acid sequence showed a high similarity with other known beta-hexosaminidases; however, P. mammillata beta-hexosaminidase had a unique potential N-glycosylation site. A phylogenetic analysis suggested that P. mammillata beta-hexosaminidase developed independently after having branched off from the common ancestor gene of the chordate enzyme before two isoforms of the mammalian enzyme appeared. In situ hybridization revealed stage-specific expression of beta-hexosaminidase mRNA during oogenesis in the oocyte and in the accessory test and follicle cells. This suggests that the three egg beta-hexosaminidase forms are specific for the oocyte, test cells and follicle cells.

W474C amino acid substitution affects early processing of the alpha-subunit of beta-hexosaminidase A and is associated with subacute G(M2) gangliosidosis

Mutations in the HEXA gene, encoding the alpha-subunit of beta-hexosaminidase A (Hex A), that abolish Hex A enzyme activity cause Tay-Sachs disease (TSD), the fatal infantile form of G(M2) gangliosidosis, Type 1. Less severe, subacute (juvenile-onset) and chronic (adult-onset) variants are characterized by a broad spectrum of clinical manifestations and are associated with residual levels of Hex A enzyme activity. We identified a 1422 G-->C (amino acid W474C) substitution in the first position of exon 13 of HEXA of a non-Jewish proband who manifested a subacute variant of G(M2) gangliosidosis. On the second maternally inherited allele, we identified the common infantile disease-causing 4-bp insertion, +TATC 1278, in exon 11. Pulse-chase analysis using proband fibroblasts revealed that the W474C-containing alpha-subunit precursor was normally synthesized, but not phosphorylated or secreted, and the mature lysosomal alpha-subunit was not detected. When the W474C-containing alpha-subunit was transiently co-expressed with the beta-subunit to produce Hex A (alphabeta) in COS-7 cells, the mature alpha-subunit was present, but its level was much lower than that from normal alpha-subunit transfections, although higher than in those cells transfected with an alpha-subunit associated with infantile TSD. Furthermore, the precursor level of the W474C alpha-subunit was found to accumulate in comparison to the normal alpha-subunit precursor levels. We conclude that the 1422 G-->C mutation is the cause of Hex A enzyme deficiency in the proband. The resulting W474C substitution clearly interferes with alpha-subunit processing, but because the base substitution falls at the first position of exon 13, aberrant splicing may also contribute to Hex A deficiency in this proband.

Structural and functional characterization of Streptomyces plicatus beta-N-acetylhexosaminidase by comparative molecular modeling and site-directed mutagenesis

We have sequenced the Streptomyces plicatus beta-N-acetylhexosaminidase (SpHex) gene and identified the encoded protein as a member of family 20 glycosyl hydrolases. This family includes human beta-N-acetylhexosaminidases whose deficiency results in various forms of GM2 gangliosidosis. Based upon the x-ray structure of Serratia marcescens chitobiase (SmChb), we generated a three-dimensional model of SpHex by comparative molecular modeling. The overall structure of the enzyme is very similar to homology modeling-derived structures of human beta-N-acetylhexosaminidases, with differences being confined mainly to loop regions. From previous studies of the human enzymes, sequence alignments of family 20 enzymes, and analysis of the SmChb x-ray structure, we selected and mutated putative SpHex active site residues. Arg162 --> His mutation increased Km 40-fold and reduced Vmax 5-fold, providing the first biochemical evidence for this conserved Arg residue (Arg178 in human beta-N-acetylhexosaminidase A (HexA) and Arg349 in SmChb) as a substrate-binding residue in a family 20 enzyme, a finding consistent with our three-dimensional model of SpHex. Glu314 --> Gln reduced Vmax 296-fold, reduced Km 7-fold, and altered the pH profile, consistent with it being the catalytic acid residue as suggested by our model and other studies. Asp246 --> Asn reduced Vmax 2-fold and increased Km only 1.2-fold, suggesting that Asp246 may play a lesser role in the catalytic mechanism of this enzyme. Taken together with the x-ray structure of SmChb, these studies suggest a common catalytic mechanism for family 20 glycosyl hydrolases.

Dictyostelium discoideum fatty-acyl amidase II has deacylase activity on Rhizobium nodulation factors

Dictyostelium discoideum (Amoebidae) secretes cell-lysing enzymes: esterases, amidases, and glycosylases, many of which degrade soil bacteria to provide a source of nutrients. Two of these enzymes, fatty-acyl amidases FAA I and FAA II, act sequentially on the N-linked long chain acyl groups of lipid A, the lipid anchor of Gram-negative bacterial lipopolysaccharide. FAA I selectively hydrolyzes the 3-hydroxymyristoyl group N-linked to the proximal glucosamine residue of de-O-acylated lipid A. Substrate specificity for FAA II is less selective, but does require prior de-N-acylation of the proximal sugar, i.e. bis-N-acylated lipid A is not a substrate. We have synthesized a 14C-labeled substrate analog for FAA II and used this in a novel assay to monitor its purification. Inhibitory studies indicate that FAA II is not a serine protease, but may have a catalytic mechanism similar to metalloprotein de-N-acetylases such as LpxC. Interestingly, rhizobial Nod factor signal oligosaccharides that induce root nodules on leguminous plants have many of the structural requirements for substrate recognition by FAA II. In vitro evidence indicates that Rhizobium fredii Nod factors are selectively de-N-acylated by purified FAA II, suggesting that the enzyme may reduce the N2-fixing efficiency of Rhizobium-legume symbioses. In contrast, N-methylated Nod factors from transgenic R. fredii carrying the rhizobial nodS gene were resistant to FAA II, suggesting a mechanism by which Nod factors may be protected from enzymatic de-N-acylation. Since FAA II and Nod factors are both secreted, and Nod factors that lack the N-acyl group are unable to induce nodules, dictyostelial FAA II may decrease the efficiency of symbiotic nitrogen fixation in the environment by reducing the available biologically active nodule inducer signal.

Cloning and transcriptional regulation of the gene encoding the vacuolar/H+ ATPase B subunit of Dictyostelium discoideum

The main function of vacuolar H+ ATPases in eukaryotic cells is to generate proton and electrochemical gradients across the membrane of inner compartments. We have isolated the gene encoding the B subunit of Dictyostelium discoideum vacuolar H+ ATPase (vatB) and analyzed its transcriptional regulation. The deduced protein comprises 493 amino acids with a calculated molecular mass of 54874 Da. The predicted protein sequence is highly homologous to previously determined V/H+ ATPase B subunit sequences. The protein is encoded by a single gene in the Dictyostelium genome. The gene is maximally expressed during growth and it decreases during the first hours of development. Gene expression is rapidly enhanced by phagocytosis, but not by fluid-phase endocytosis. Acidic and alkaline conditions affect vatB gene expression differently.

Beta-N-acetylhexosaminidase: a target for the design of antifungal agents

This review provides biochemical, analytical, and biological background information relating to beta-N-acetylhexosaminidase (HexNAc'ase; EC 3.2.1.52) as an emerging target for the design of low-molecular-weight antifungals. The article includes the following: (1) a biochemical description of HexNAc'ase (reaction catalyzed, nomenclature, and mechanism of action) that sets it apart from other, similar enzymes; (2) an overview and a critical evaluation of methods to assay the enzyme, including in crude extracts (photo- and fluorometric procedures with model substrates; HPLC/pulsed amperometric detection of N-acetylglucosamine and chito-oligomers; end-point vs. rate measurements); (3) a summary of some general characteristics of HexNAc'ases from fungi and organisms of other types (Km values, substrate preference, and glycoconjugation); (4) an hypothesis of a specific target function of wall-associated HexNAc'ase (a component of the assembly of surface-located enzymes effecting a continuous turnover and remodelling of the wall fabric through its combined hydrolytic and transglycosylating activities, and a mediator enzyme acting in concert with chitinase and chitin synthase to provide for the controlled lysis and synthesis of chitin during growth); (5) a tabulation of the structural formulae of reaction-based HexNAc'ase inhibitors with Ki values < or = 100 microM (some of them representing transition state mimics that could serve as leads for the development of new antifungals); and (6) an outline of approaches towards the establishment of a three-dimensional model of HexNAc'ase suitable for a truly rational design of antimycotics as well as agricultural fungicides.

The chitin catabolic cascade in the marine bacterium Vibrio furnissii. Molecular cloning, isolation, and characterization of a periplasmic beta-N-acetylglucosaminidase

We have described some steps in chitin catabolism by Vibrio furnissii, and proposed that chitin oligosaccharides are hydrolyzed in the periplasmic space to GlcNAc and (GlcNAc)2. Since (GlcNAc)2 is an important inducer in the cascade, it must resist hydrolysis in the periplasm. Known V. furnissii periplasmic hydrolases comprise an endoenzyme (Keyhani, N. O. and Roseman, S. (1996) J. Biol. Chem. 271, 33414-33424), and the beta-N-acetylglucosaminidase, ExoI, reported here. ExoI was isolated from a recombinant strain of Escherichia coli, and hydrolyzes aryl-beta-GlcNAc, aryl-beta-GalNAc, and chitin oligosaccharides. No other beta-GlcNAc glycosides were cleaved. The pH optimum was 7.0 for (GlcNAc)n, n = 3-6, but 5.8 for (GlcNAc)2. At the pH of sea water (8.0-8.3), the enzymatic activity with (GlcNAc)2 is virtually undetectable. These results explain the stability of (GlcNAc)2 in the periplasmic space. The cloned beta-GlcNAcidase gene, exoI, encodes a 69,377-kDa protein (611 amino acids); the predicted N-terminal 20 amino acid residues matched those of the isolated protein. The protein amino acid sequence displays significant homologies to the alpha- and beta-chains of human hexosaminidase despite their marked differences in substrate specificities and pH optima.

Dictyostelium discoideum contains a family of calmodulin-related EF-hand proteins that are developmentally regulated

A full-length genomic DNA fragment that codes for a novel EF-hand protein Dictyostelium discoideum was cloned and sequenced. The protein is composed of 168 amino acids and contains four consensus sequences that are typical for (Ca2+)-binding EF-hand domains. The protein sequence exhibits only minor similarities to other calmodulin-type proteins from Dictyostelium. The genomic DNA harbors two short introns; their positions suggest that the gene is unrelated to the EF-hand proteins from the calmodulin group. Northern blot analysis showed that the mRNA level was significantly increased during development. Polyclonal antibodies raised against the recombinant protein recognized in Western blots a protein of about 20 kDa. Like the mRNA, also the protein was more abundant in developing cells. Overlay experiments with 45Ca2+ indicated that the EF-hands in fact have (Ca2+)-binding activity. The recent description of CBP1, another calmodulin-type Dictyostelium protein that is upregulated during development [Coukell et al. (1995) FEBS Lett. 362, 342-346], suggests that D. discoideum contains a family of EF-hand proteins that have specific functions during distinct steps of development. We therefore designate the protein described in this report as CBP2.

The primary structure of an Entamoeba histolytica beta-hexosaminidase A subunit

An Entamoeba histolytica gene (hex-A1) that encodes subunit A of the lysosomal enzyme beta-hexosaminidase has been cloned and sequenced. The inferred 59 kDa hex-A1 protein has the same molecular weight and 32% amino acid residue identity with the human and mouse proteins and 28% residue identity with the Dictyostelium protein. Northern blot analysis identified a mRNA of approximately 1.6 kb, which is in agreement with the expected size of a mRNA encoding the 522 amino acid hex-A1 protein. Southern blot analysis indicated the presence of at least two beta-hexosaminidase A subunit genes.

Isolation of N-acetyl-beta-hexosaminidase from Acanthamoeba castellanii

The lysosomal enzyme N-acetyl-beta-hexosaminidase (beta hex) has been purified from Acanthamoeba castellanii growth medium by a three step procedure. The enzyme was precipitated with ammonium sulfate, partially purified on a DE52 column and purified to homogeneity on an affinity column. The purified beta hex appeared to be a monomer with a molecular mass of 58 kDa and a pI of approximately 5.8. The enzyme activity in growth medium at RT was stable for several months. The purified beta hex was enzymatically deglycosylated and injected into two rabbits to make polyclonal antibodies. One antiserum was specific for beta hex, but the other stained many bands on immunoblots of whole cell preparations. Using fluorescently labelled secondary antibodies we have determined that both antisera stain digestive vacuoles in the Acanthamoeba cytoplasm, and do not stain the contractile vacuole. The multi-specific antiserum had high avidity for beta hex, but also stained the carbohydrate portion of other molecules. These other molecules may be lysosomal enzymes as well, since the activity of several other lysosomal enzymes was partially immunoprecipitable with the antiserum. We plan to use these antibodies to study traffic patterns among the variety of vacuolar structures in Acanthamoeba cytoplasm.

Cloning and expression of the beta-N-acetylglucosaminidase gene from Streptococcus pneumoniae. Generation of truncated enzymes with modified aglycon specificity

The gene encoding a beta-N-acetylglucosaminidase from Streptococcus pneumoniae has been obtained by screening an expression library for beta-N-acetylglucosaminidase activity. Clones of different nucleotide sizes each having arylglycoside activity were obtained, and DNA sequencing revealed a gene of 3933 base pairs possessing typical bacterial transcription initiation and termination sequences and terminating in an ochre stop codon. Computer analysis of the translated protein of 1311 amino acids (144,210 Da) identified a tandem repeat within which lies a sequence homologous with six other hexosaminidase gene products from a wide variety of species ranging from bacteria to humans. Also found were an amino-terminal putative secretion signal peptide and a carboxyl-terminal cell sorting/anchorage motif typically found in over 20 other Gram-positive surface proteins. The expression of an almost complete DNA clone in Escherichia coli produced a functional and authentic beta-N-acetylglucosaminidase with aglycon specificity identical to the wild-type enzyme. However, enzymes produced from truncated DNA clones show more restricted aglycon specificity and are unable to hydrolyze terminal beta 1-2GlcNAc residues from N-glycans containing a bisecting N-acetylglucosamine. The availability of these clones allows structural analyses to be made of catalytic and oligosaccharide recognition protein domains that enhance functional activity.

Molecular cloning and expression of the Candida albicans beta-N-acetylglucosaminidase (HEX1) gene

beta-N-Acetylglucosaminidase was purified from the spent culture medium of Candida albicans A72 grown in the presence of N-acetylglucosamine (GlcNAc). The N-terminal amino acid sequence of the protein was determined, two degenerate oligonucleotide probes were constructed, and a 3.9-kb BamHI fragment of DNA that hybridized to both probes was subcloned from a lambda EMBL4 library of C. albicans A72 genomic DNA. This fragment of DNA contained the entire beta-N-acetylglucosaminidase (HEX1) gene, which consisted of an open reading frame coding for a polypeptide precursor of 562 amino acids with a putative 22-amino-acid leader sequence. The deduced HEX1 amino acid sequence showed similarity to hexosaminidases from a variety of organisms. Growth of C. albicans on GlcNAc induced transcription of HEX1, resulting in increased specific beta-N-acetylglucosaminidase activity. HEX1 mRNA (2.35 kb) from GlcNAc-grown cells was approximately 200 bp larger than HEX1 mRNA from cells grown on glucose. This size difference was suggested to result from the use of alternative transcription termination sites. The cloned HEX1 gene introduced into C. albicans SGY-243 on a plasmid also responded to GlcNAc induction.

Molecular epidemiology of Tay-Sachs disease in Europe

The abnormalities in the gene coding for the beta-hexosaminidase alpha subunit were analysed from fibroblast's RNAs of 42 Tay-Sachs patients (seven with adult or late onset of Tay-Sachs disease and 35 with infantile Tay-Sachs disease). After first strand synthesis by random priming, PCR was used to amplify in two overlapping fragments (868 and 949 bp) the entire coding region. These amplified products were first studied for changes in size by agarose gel electrophoresis to screen for splicing mutations leading to exon skipping or cryptic splice site activation. For each patient, the two overlapping cDNA fragments were subjected to chemical mismatch cleavage analysis using hydroxylamine to modify C-containing mismatches and osmium tetroxide to modify T-containing mismatches. DGGE was used to screen for mutations in the coding region spanning exon 2 to exon 6, a region putatively encompassing the active site and therefore a potential hot spot of mutations associated with Tay-Sachs disease. To increase the sensitivity of the technique, a 30 bp GC-clamp has been added at the 5' end of the sense oligonucleotide to amplify a fragment of 629 bp. The computerized analysis found that single base changes in domain spanning from nt 313 to nt 693 can be distinguished. Fragments displaying an altered melting behavior or a cleaved product were further analysed by direct sequencing of the amplified material. These methods as a whole allowed us to identify 30/38 alleles studied (79%) with 15 point mutations and one 4 bp insertion detected.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular basis of an adult form of Sandhoff disease: substitution of glutamine for arginine at position 505 of the beta-chain of beta-hexosaminidase results in a labile enzyme

Sandhoff disease is a lysosomal storage disorder characterized by accumulation of GM2 ganglioside due to mutations in the beta-chain of beta-hexosaminidase. Hexosaminidase activity is negligible in infantile Sandhoff disease whereas residual activity is present in juvenile and adult forms. Here we report the molecular basis of the first described adult form of Sandhoff disease. Southern analysis of chromosomal DNA indicated the absence of chromosomal deletions in the gene encoding the beta-chain. Northern analysis of RNA from cultured fibroblasts demonstrated that at least one of the beta-chain alleles was transcribed into normal-length mRNA. Sequence analysis of the entire cDNA prepared from poly-adenylated RNA showed that only one point mutation was present, consisting of a G-->A transition at nucleotide position 1514. This mutation changes the electric charge at amino acid position 505 by substitution of glutamine for arginine in a highly conserved part of the beta-chain, present even in the slime mold Dictyostelium discoideum. The nucleotide transition generated a new restriction site for DdeI, which was present in only one of the alleles of the patient. Reverse transcription of mRNA followed by restriction with DdeI resulted in complete digestion at the mutation site, demonstrating that the second allele was of an mRNA-negative type. Transfection of COS cells with a cDNA construct containing the mutation but otherwise the normal sequence resulted in the expression of a labile form of beta-hexosaminidase. These results show that the patient's is a genetic compound, and that the lability of beta-hexosaminidase found in this form of Sandhoff disease is based on a single nucleotide transition.

Two new mutations in a late infantile Tay-Sachs patient are both in exon 1 of the beta-hexosaminidase alpha subunit gene

We have identified two new point mutations in the beta-hexosaminidase alpha subunit (HEX A) gene in a non-Jewish Tay-Sachs disease patient with an unusual late infantile onset disease phenotype. The patient was a compound heterozygote with each allele of the HEX A gene containing a different mutation in exon 1. One of these is a T to C transition in the initiation codon, expected to produce no alpha subunit and therefore a classical infantile phenotype. The unusual clinical aspects and later onset in the patient must therefore be a result of residual hexosaminidase A activity associated with a mutant alpha subunit containing the second mutation, substitution of serine for proline at amino acid 25 owing to a C to T change at nucleotide 73. Western blotting and DE-52 ion exchange chromatography have been used to examine the behaviour of this mutant alpha subunit.

Coactosin, a 17 kDa F-actin binding protein from Dictyostelium discoideum

A 17 kDa protein, designated as coactosin, has been purified from an actin-myosin complex reconstituted in vitro from a soluble fraction of Dictyostelium discoideum cells. The protein binds to F-actin in vitro without significantly altering its viscosity. Immunoblots labeled with monoclonal antibodies indicate that part of the protein is associated with the detergent-insoluble cytoskeleton. cDNA clones comprising the entire coding region of coactosin have been isolated from an expression library. The cDNA-derived amino-acid sequence reveals similarities of coactosin to the drebrins identified in neurons and to actin-binding proteins from other organisms, including yeast ABP1p, and yeast and vertebrate cofilins.

Physical mapping of genes to specific chromosomes in Dictyostelium discoideum

Cloned genes were used to probe a highly redundant library of large cloned fragments of the Dictyostelium discoideum genome carried in yeast artificial chromosomes (YACs). Each gene recognized several independent YAC clones, thereby grouping them into a contig. Individual YACs were arranged within the contig by positioning genes relative to rare restriction sites and the YAC ends. Genes that had been previously assigned to one of the six linkage groups by parasexual genetics were used to establish physically mapped regions on specific chromosomes. Previously unmapped genes were assigned to specific chromosomes when they recognized members of a mapped contig. Linkage was confirmed by congruence of large-scale restriction maps centered on either the previously mapped or the newly mapped genes. At present, the chromosome-assigned map segments comprise approximately 50% of the genome. About half of each map segment is covered by overlapping YACs.

Expression of early developmental genes in Dictyostelium discoideum is initiated during exponential growth by an autocrine-dependent mechanism

Throughout growth, Dictyostelium cells continuously produce an autocrine factor, PSF, that accumulates in proportion to cell density. Production of PSF declines rapidly when cells are shifted to starvation conditions, and the properties of PSF are distinct from those of regulatory factors produced by starving cells. During late exponential growth, PSF induces expression of several early developmental genes, including those for proteins important in cAMP signaling and cell aggregation. Examples are the aggregation stage cAMP receptor (cAR1), the aggregation-specific form of cyclic nucleotide phosphodiesterase, and gp24 (contact sites B). Through PSF, growing cells detect environmental conditions (cell number high, food approaching depletion) that are appropriate for production of the gene products needed to initiate aggregation and development.

Characterization of a cDNA encoding subunit VI of cytochrome c oxidase from the slime mold Dictyostelium discoideum

The primary structure of subunit VI of cytochrome c oxidase from the slime mold Dictyostelium discoideum has been determined by sequencing cDNA and N-terminus of the protein. The 92 amino acid residues long polypeptide (Mr = 10,535) shows homology with subunit IV of mammalian and subunit V of yeast cytochrome c oxidase. Though smaller and synthesized without a cleavable presequence, the slime mold oxidase subunit maintains the presence of a putative membrane spanning region.

A new point mutation within exon 5 of beta-hexosaminidase alpha gene in a Japanese infant with Tay-Sachs disease

A new point mutation within exon 5 of beta-hexosaminidase alpha subunit gene (guanine509----adenine; arginine170----glutamine) has been identified as being responsible for the typical clinical and enzymological phenotype of infantile Tay-Sachs disease in a Japanese infant. Expression of the mutant enzyme protein in the COS I cell system indicated that it is catalytically inactive and also is unstable. The patient is a compound heterozygote, and the exact abnormality in the other allele could not be identified except that it is not any of the other nine known mutations of the beta-hexosaminidase alpha. The data collectively suggest that the other allele is not producing stable messenger RNA (mRNA). The rapidly increasing number of mutations responsible for clinical and enzymological phenotypes and the very large number of statistically possible combinations among them for compound heterozygosity pose a serious pragmatic problem for classification and nomenclature of this group of rare genetic disorders.

Biochemical and genetic analysis of an antigenic determinant found on N-linked oligosaccharides in Dictyostelium

Dictyostelium discoideum synthesizes many highly immunogenic carbohydrates of unknown structure and function. We have used monoclonal antibodies prepared against one of these called CA1 to investigate its structure and the consequences of its loss. CA1 is preferentially expressed on lysosomal enzymes as a specific arrangement of mannose-6-SO4 residues on N-linked oligosaccharides. Mutant strains HL241 and HL243 do not express CA1, and synthesize a truncated lipid-linked oligosaccharide (LLO) precursor that lacks the critical mannose residues needed for expression. The lesion appears to result from the loss of mannosyl transferase activity involved in LLO biosynthesis. The truncated LLO is poorly transferred to an artificial peptide acceptor in a cell-free N-glycosylation assay, and this appears to result from improper topological localization of the LLO or to a lower affinity of the LLO for the oligosaccharyl transferase. Although both mutants share these lesions, they are biochemically and genetically distinct. Only HL243 is lower in N-glycosylation in intact cells, and this is not a result of an altered structure of the LLO. There are other differences between the strains. HL241 can form fruiting bodies at a slower rate than normal while HL243 cannot aggregate. Genetic analysis of defects shows that the CA1 lesion in HL241 is recessive, while the lesion in both CA1 and in development are dominant and co-segregate in HL243 and are, therefore, likely to be in the same gene. Lysosomal enzyme targeting is normal but enzyme processing proceeds at a 2-3 fold slower rate in HL241 and HL243 compared to wild-type. Strain HL244 does not express CA1 since it completely lacks protein sulfation, but lysosomal enzyme targeting and processing proceeds at a normal rate, showing that sulfate is not essential for these processes. Alterations in oligosaccharide structure can have individualized effects on the biosynthesis of lysosomal enzymes. The results presented here illustrate how this approach can be used to study both the structure and function of carbohydrate epitopes.

Characteristics of the sulfation of N-linked oligosaccharides in vesicles from Dictyostelium discoideum: in vitro sulfation of lysosomal enzymes

Lysosomal enzymes from Dictyostelium discoideum contain unusual sulfated N-linked oligosaccharides, whose synthesis has been well studied in vivo. However, little is known about the properties of the pertinent sulfotransferases. To study these transferases, we have prepared a cell-free system which transfers 35SO4 from 3'-phosphoadenosine 5'-phosphosulfate to either endogenous or exogenous acceptors. We found that the 35SO4 was released from macromolecules by protein N-glycanase F to yield a mixture of anionic oligosaccharides with 1-6 negative charges. Some of the labeled molecules contained acid-stable methyl phosphodiesters but none contained phosphomoesters or acid-labile diesters. The sulfate was found in molecules with the acid stability characteristic of esters of primary alcohols. In all these ways, the products resembled those generated in vivo. We also demonstrated that a membrane-associated form of beta-hexosaminidase and the precursor of alpha-mannosidase were among the products. In addition, glycoproteins prepared from a sulfation-deficient mutant strain could act as exogenous acceptors in permeabilized vesicles.

The mutations in Ashkenazi Jews with adult GM2 gangliosidosis, the adult form of Tay-Sachs disease

The adult form of Tay-Sachs disease, adult GM2 gangliosidosis, is an autosomal recessive disorder that results from mutations in the alpha chain of beta-hexosaminidase A. This disorder, like infantile Tay-Sachs disease, is more frequent in the Ashkenazi Jewish population. A point mutation in the alpha-chain gene was identified that results in the substitution of Gly with Ser in eight Ashkenazi adult GM2 gangliosidosis patients from five different families. This amino acid substitution was shown to depress drastically the catalytic activity of the alpha chain after expression in COS-1 cells. All of these patients proved to be compound heterozygotes of the allele with the Gly to Ser change and one of the two Ashkenazi infantile Tay-Sachs alleles. These findings will aid in the diagnosis and understanding of beta-hexosaminidase A deficiency disorders.