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 proαβ 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 α and two β 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.     

Lysosomal aspartylglucosaminidase is processed to the active subunit complex in the endoplasmic reticulum.

Aspartylglucosaminidase (AGA) is a lysosomal enzyme, the deficiency of which leads to a human storage disease, aspartylglucosaminuria (AGU). Although numerous mutations have been identified in AGU patients, elucidation of the molecular pathogenesis of the disease has been hampered by the missing information on the cellular events resulting in the maturation and activation of the enzyme. Here we used the expression of in vitro mutagenized constructs of the AGA cDNA to define three specific proteolytic trimming steps resulting in mature AGA. Removal of the signal peptide is immediately followed by proteolytic cleavage of the precursor into two subunits and results in biologically active enzyme already in the endoplasmic reticulum. This early activation has not previously been described for lysosomal enzymes. The subsequent lysosomal trimming does not influence the enzymatic activity of AGA. It consists only of a single proteolytic cleavage which removes 10 amino acids from the C-terminal end of the larger subunit, in contrast to the multistep lysosomal processing observed in several other hydrolases.     

Aspartylglucosaminuria: cDNA encoding human aspartylglucosaminidase and the missense mutation causing the disease.

We have isolated a 2.1 kb cDNA which encodes human aspartylglucosaminidase (AGA, E.C. 3.5.1.26). The activity of this lysosomal enzyme is deficient in aspartylglucosaminuria (AGU), a recessively inherited lysosomal accumulation disease resulting in severe mental retardation. The polypeptide chain deduced from the AGA cDNA consists of 346 amino acids, has two potential N-glycosylation sites and 11 cysteine residues. Transient expression of this cDNA in COS-1 cells resulted in increased expression of immunoprecipitable AGA protein. Direct sequencing of amplified AGA cDNA from an AGU patient revealed a G----C transition resulting in the substitution of cysteine 163 with serine. This mutation was subsequently found in all the 20 analyzed Finnish AGU patients, in the heterozygous form in all 53 carriers and in none of 67 control individuals, suggesting that it represents the major AGU causing mutation enriched in this isolated population. Since the mutation produces a change in the predicted flexibility of the AGA polypeptide chain and removes an intramolecular S-S bridge, it most probably explains the deficient enzyme activity found in cells and tissues of AGU patients.     

Characterization of the nickel-iron periplasmic hydrogenase from Desulfovibrio fructosovorans

The periplasmic hydrogenase from Desulfovibrio fructosovorans grown on fructose/sulfate medium was purified to homogeneity. It exhibits a molecular mass of 88 kDa and is composed of two different subunits of 60 kDa and 28.5 kDa. The absorption spectrum of the enzyme is characteristic of an iron-sulfur protein and its absorption coefficients at 400 and 280 nm are 50 and 180 mM-1 cm-1, respectively. D. fructosovorans hydrogenase contains 11 +/- 1 iron atoms, 0.9 +/- 0.15 nickel atom and 12 +/- 1 acid-labile sulfur atoms/molecule but does not contain selenium. The amino acid composition of the protein and of its subunits, as well as the N-terminal sequences of the small and large subunits, have been determined. The cysteine residues of the protein are distributed between the large (9 residues) and the small subunits (11 residues). Electron spin resonance (ESR) properties of the enzyme are consistent with the presence of nickel(III), [3Fe-4S] and [4Fe-4S] clusters. The hydrogenase of D. fructosovorans isolated under aerobic conditions required an incubation with hydrogen or other reductants in order to express its full catalytic activity. H2 uptake and H2 evolution activities doubled after a 3-h incubation under reducing conditions. Comparison with the (NiFe) hydrogenase from D. gigas shows great structural similarities between the two proteins. However, there are significant differences between the catalytic properties of the two enzymes which can be related to the respective state of their nickel atom. ESR showed a higher proportion of the Ni-B species (g = 2.33, 2.16, 2.01) which can be related to a more facile conversion to the ready state. The periplasmic location of the enzyme and the presence of hydrogenase activity in other cellular compartments are discussed in relation to the ability of D. fructosovorans to participate actively in interspecies hydrogen transfer.     

Large-scale purification and preliminary X-ray diffraction studies of human aspartylglucosaminidase

Aspartylglucosaminidase (AGA) is a lysosomal asparaginase that takes part in the ordered degradation of glycoproteins and a deficiency of which results in a lysosomal accumulation disease aspartylglucosaminuria in human. The mature enzyme consists of 24-kDa and 17-kDa subunits, which are both heterogeneously glycosylated. Activation of the enzyme from a single precursor polypeptide into two subunits is accomplished in the endoplasmic reticulum (ER). The relative lack of this proteolytic capacity in several tested high-producing expression systems has complicated the production of active recombinant enzyme in high quantities, which would be an alternative for purification of this molecule for crystallization. Consequently, the AGA enzyme has to be purified directly from cellular or tissue sources for crystallographic analysis. Here we describe a large-scale purification method to produce milligram amounts of homogeneous AGA from human leukocytes. The purified AGA enzyme represents a heterogeneous pool of molecules not only due to glycosylation, but also heterogeneity at the polypeptide level, as demonstrated here. We were able to isolate a homogeneous polypeptide pool that was successfully crystallized and preliminary X-ray data collected from the crystals. The crystals diffract well to 2.0 Å and are thus suitable for determination of the crystal structure of AGA.     

Alanyl-tRNA synthetase from Escherichia coli, Bombyx mori and Ratus ratus. Existence of common structural features

Alanyl-tRNA synthetase from Escherichia coli, Bombyx mori and rat were examined with respect to the following functional and structural properties: the effect of substrates on sensitivity to proteolysis, secondary structure as determined by circular dichroism, amino acid composition and, in the case of the rat and insect enzymes, partial amino acid sequence determination on a 60-kDa C-terminal tryptic fragment. Digestion of the enzyme from all three sources with trypsin resulted in significant decline in aminoacyl-tRNA synthetase activity with little effect on pyrophosphate-exchange activity. In each case the presence of alanine and ATP together, but not separately, reduced the rate of digestion by trypsin; the largest effect was observed with the enzyme from rat liver. Trypsin digestion generated fragments of 47 kDa and 40 kDa with all three enzymes, but detection of significant quantities of the 47-kDa fragment from the rat enzyme required the presence of ATP and alanine. Trypsin digestion produced a fragment of 60 kDa with all three enzymes, but detection of significant quantities of this fragment with the bacterial enzyme required the presence of ATP and alanine. Limited sequence analysis of the 60-kDa fragment from the insect and rat enzymes indicated that trypsin cleaved both proteins at the same site to generate this species. Similar effects of substrates were observed when the enzymes were digested with chymotrypsin suggesting that the effects of substrates on protease sensitivity were not unique to trypsin. Circular dichroism spectra obtained for the three enzymes were qualitatively and quantitatively similar. There is some similarity in amino acid composition between the rat and insect enzymes.     

A gamma-glutamyl transpeptidase of Aphidius ervi venom induces apoptosis in the ovaries of host aphids

Parasitism by the endophagous braconid Aphidius ervi (Hymenoptera, Braconidae) has a negative impact on the reproductive activity of its host, Acyrthosiphon pisum (Homoptera, Aphididae). The host castration is induced by the parasitoid venom and is reproduced by the injection of chromatographic fractions highly enriched with two proteins, of 18 (p18) and 36 kDa (p36) in size, respectively. Here we demonstrate that these bioactive proteins trigger apoptosis of the cells in the germaria and ovariole sheath of the host aphid. Both p18 and p36 were internally sequenced and the gathered information was matched against the deduced amino acid sequence of the putative proteins encoded by cDNA clones, randomly selected from a cDNA library, which was raised using mRNA extracted from A. ervi venom glands. The identified cDNA clones contained an insert corresponding to the RNA product of an interrupted gene, made of six exons and five introns, which was found to be transcribed at higher levels in adult females of A. ervi than in males. This gene codes for a putative protein composed of 541 amino acids, with a calculated molecular mass of 56.9 kDa, which contained the amino acid sequences experimentally determined for both p18 and p36. This putative protein showed a significant level of sequence identity with gamma-glutamyl transpeptidases (gamma-GT), and it was named Ae-gamma-GT. The gamma-GTs are enzymes which play a key role in the metabolism of glutathione (GSH) and, as observed in most organisms, they are membrane-bound heterodimers formed by a large and a small subunit, which originate by post-translational processing of a single-chain precursor. The expression in insect cells of Ae-gamma-GT confirmed the occurrence of the expected post-translational processing, and demonstrated that, unlike other gamma-GTs, this protein is secreted in the extracellular environment. A measurable gamma-GT activity was detected in the venom of A. ervi and in the chromatographic fractions containing Ae-gamma-GT. Thus, we suggest that this venom protein may induce apoptosis in the host ovarioles by generating an alteration of the GSH metabolism and a consequent oxidative stress.     

A study of the heterogenous structure of guinea pig lysosomal beta-mannosidase using a polyclonal antibody

Lysosomal beta-mannosidase (EC 3.2.11.25) has a functional size of 120-150 kDa, but the enzyme purified from guinea pig liver (GPL) reportedly gave a single band corresponding to a molecular mass of 110 kDa. In order to investigate the subunit structure and tissue-specific expression of beta-mannosidase, we prepared a polyclonal antibody against GPL beta-mannosidase in rabbits which immunoprecipitated beta-mannosidase activity, free from other lysosomal hydrolase activity. Following storage at -20 degrees C and SDS polyacrylamide gel electrophoresis in the presence of 2-mercaptoethanol, a sample of purified GPL beta-mannosidase gave a major Coomassie blue staining band at 97 kDa. This was confirmed by Western blot analysis, which also revealed a faster moving 37 kDa protein. In contrast, Western blot analysis of fresh GPL homogenate prepared in the presence of proteinase inhibitors showed a major band at 150 kDa. Upon freezing and thawing, we observed immunoreactive bands at 120 and 20 kDa and finally, immunoreactive bands at 97, 37 and 20 kDa. The formation of the 97, 37 and 20 kDa forms from the 150 kDa species was accelerated by an n-butanol/ether extraction of the associated lipids, suggesting some tight hydrophobic association of these subunits. In contrast to liver, both fresh and freeze-thawed preparations of guinea pig kidney (GPK) yielded only the 97, 37 and 20 kDa subunit forms confirming that these are the major beta-mannosidase subunits. Endo-F treatment converted both the liver and kidney 97 kDa into a 91 kDa form and the 37 kDa form into a 35 kDa form, whereas the 20 kDa form was unaffected. Total beta-mannosidase activity, as measured with the synthetic substrate 4MU-beta-mannoside was unaffected by dissociation of the 150 form into the 97, 37 and 20 kDa subunits, suggesting that these are the functional forms of the enzyme rather than proteolytic degradation products.     

Lysosomal enzyme binding receptor protein from monkey brain and its phosphorylation.

The lysosomal enzyme binding receptor protein isolated from monkey brain by phosphomannan-Sepharose affinity chromatography was phosphorylated by [gamma-32P] ATP by protein kinases tightly associated with the receptor protein. A greater than 200 kDa protein was phosphorylated on both serine and tyrosine residues and a approximately 45 kDa protein was phosphorylated on only serine residues as evidenced by SDS-gel electrophoresis, autoradiography and phosphoamino acid analysis [(Panneerselvam, Ramamoorthy & Balasubramanian (1987) Biochem Biophys Res Commun, 147, 927-935)]. 125I-labelled lysosomal enzymes could be cross-linked to the receptor protein in the presence of disuccinimidyl suberate. Phosphorylation of the receptor on both serine and tyrosine residues was inhibited by quercetin, polylysine and polymyxin B. Catalytic subunit of cyclic AMP-dependent protein kinase preferentially phosphorylated the approximately 45 kDa protein. In the presence of Triton X-100, phosphorylation of a few additional protein bands on non-tyrosine residues was observed. There was a marked reduction in the efficiency of binding lysosomal enzymes by the phosphorylated receptor protein in comparison to the unphosphorylated receptor protein.     

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.     

Gene cloning and characterization of the very large NAD-dependent l-glutamate dehydrogenase from the psychrophile Janthinobacterium lividum, isolated from cold soil

NAD-dependent l-glutamate dehydrogenase (NAD-GDH) activity was detected in cell extract from the psychrophile Janthinobacterium lividum UTB1302, which was isolated from cold soil and purified to homogeneity. The native enzyme (1,065 kDa, determined by gel filtration) is a homohexamer composed of 170-kDa subunits (determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis). Consistent with these findings, gene cloning and sequencing enabled deduction of the amino acid sequence of the subunit, which proved to be comprised of 1,575 amino acids with a combined molecular mass of 169,360 Da. The enzyme from this psychrophile thus appears to belong to the GDH family characterized by very large subunits, like those expressed by Streptomyces clavuligerus and Pseudomonas aeruginosa (about 180 kDa). The entire amino acid sequence of the J. lividum enzyme showed about 40% identity with the sequences from S. clavuligerus and P. aeruginosa enzymes, but the central domains showed higher homology (about 65%). Within the central domain, the residues related to substrate and NAD binding were highly conserved, suggesting that this is the enzyme's catalytic domain. In the presence of NAD, but not in the presence of NADP, this GDH exclusively catalyzed the oxidative deamination of l-glutamate. The stereospecificity of the hydride transfer to NAD was pro-S, which is the same as that of the other known GDHs. Surprisingly, NAD-GDH activity was markedly enhanced by the addition of various amino acids, such as l-aspartate (1,735%) and l-arginine (936%), which strongly suggests that the N- and/or C-terminal domains play regulatory roles and are involved in the activation of the enzyme by these amino acids.     

A molybdenum and a tungsten isoenzyme of formylmethanofuran dehydrogenase in the thermophilic archaeon Methanobacterium wolfei.

We have recently reported that the thermophilic archaeon Methanobacterium wolfei contains two formylmethanofuran dehydrogenases, I and II. Formylmethanofuran dehydrogenase II, which is preferentially expressed in tungsten-grown cells, has been purified and shown to be a tungsten-iron-sulfur protein. We have now purified and characterized formylmethanofuran dehydrogenase I from molybdenum-grown cells and shown that it is a molybdenum-iron-sulfur protein. The purified enzyme, with a specific activity of 27 U/mg protein, was found to be composed of three subunits of apparent molecular mass 64 kDa, 51 kDa, and 31 kDa and to contain per mol 146-kDa molecule approximately 0.23 mol molybdenum, 0.46 mol molybdopterin guanine dinucleotide, and 6.6 mol non-heme iron but no tungsten (< 0.01 mol). The molybdenum enzyme differed from the tungsten enzyme (8 U/mg) in that it catalyzed the oxidation of N-furfurylformamide and formate and was inactivated by cyanide. The two enzymes also differed significantly in the pH optimum, in the apparent Km for the electron acceptor, and in the chromatographic behaviour. The molybdenum enzyme and the tungsten enzyme were similar, however, in that the N-terminal amino acid sequences determined for the alpha and beta subunits were identical up to residue 23, indicating that the two proteins are isoenzymes. The molybdenum enzyme, as isolated, was found to display an EPR signal derived from molybdenum as evidenced by isotope substitution.     

Purification and characterization of cAMP dependent protein kinase from Microsporum gypseum

A cyclic AMP dependent protein kinase (PKA), its regulatory (R) and catalytic (C) subunits were purified to homogeneity from soluble extract of Microsporum gypseum. Purified enzyme showed a final specific activity of 277.9 nmol phosphate transferred min(-1) mg protein(-1) with kemptide as substrate. The enzyme preparation showed two bands with molecular masses of 76 kDa and 45 kDa on sodium dodecyl polyacrylamide gel electrophoresis. The 76 kDa subunit was found to be the regulatory (R) subunit of PKA holoenzyme as determined by its immunoreactivity and the isoelectric point of this subunit was 3.98. The 45 kDa subunit was found to be the catalytic (C) subunit by its immunoreactivity and phosphotransferase activity. Gel filtration using Sepharose CL-6B revealed the molecular mass of PKA holoenzyme to be 240 kDa, compatible with its tetrameric structure, consisting of two regulatory subunits (76 kDa) and two catalytic subunits (45 kDa). The specificity of enzyme towards protein acceptors in decreasing order of phosphorylation was found to be kemptide, casein, syntide and histone IIs. Purified enzyme had apparent K(m) values of 71 microM and 25 microM for ATP and kemptide, respectively. Phosphorylation was strongly inhibited by mammalian PKA inhibitor (PKI) but not by inhibitors of other protein kinases. The PKA showed maximum activity at pH 7.0 and enzyme activity was inhibited in the presence of N-ethylmaleimide (NEM) which shows the involvement of sulfhydryl groups for the activity of PKA. PKA phosphorylated a number of endogenous proteins suggesting the multifunctional role of cAMP dependent protein kinase in M. gypseum. Further work is under progress to identify the natural substrates of this enzyme through which it may regulate the enzymes involved in phospholipid metabolism.     

Plant mitochondrial F0F1 ATP synthase. Identification of the individual subunits and properties of the purified spinach leaf mitochondrial ATP synthase.

Spinach leaf mitochondrial F0F1 ATPase has been purified and is shown to consist of twelve polypeptides. Five of the polypeptides constitute the F1 part of the enzyme. The remaining polypeptides, with molecular masses of 28 kDa, 23 kDa, 18.5 kDa, 15 kDa, 10.5 kDa, 9.5 kDa and 8.5 kDa, belong to the F0 part of the enzyme. This is the first report concerning identification of the subunits of the plant mitochondrial F0. The identification of the components is achieved on the basis of the N-terminal amino acid sequence analysis and Western blot technique using monospecific antibodies against proteins characterized in other sources. The 28-kDa protein crossreacts with antibodies against the subunit of bovine heart ATPase with N-terminal Pro-Val-Pro- which corresponds to subunit F0b of Escherichia coli F0F1. Sequence analysis of the N-terminal 32 amino acids of the 23-kDa protein reveals that this protein is similar to mammalian oligomycin-sensitivity-conferring protein and corresponds to the F1 delta subunit of the chloroplast and E. coli ATPases. The 18.5-kDa protein crossreacts with antibodies against subunit 6 of the beef heart F0 and its N-terminal sequence of 14 amino acids shows a high degree of sequence similarity to the conserved regions at N-terminus of the ATPase subunits 6 from different sources. ATPase subunit 6 corresponds to subunit F0a of the E. coli enzyme. The 15-kDa protein and the 10.5-kDa protein crossreact with antibodies against F6 and the endogenous ATPase inhibitor protein of beef heart F0F1-ATPase, respectively. The 9.5-kDa protein is an N,N'-dicyclohexylcarbodiimide-binding protein corresponding to subunit F0c of the E. coli enzyme. The 8.5-kDa protein is of unknown identity. The isolated spinach mitochondrial F0F1 ATPase catalyzes oligomycin-sensitive ATPase activity of 3.5 mumol.mg-1.min-1. The enzyme catalyzes also hydrolysis of GTP (7.5 mumol.mg-1.min-1) and ITP (4.4 mumol.mg-1.min-1). Hydrolysis of ATP was stimulated fivefold in the presence of amphiphilic detergents, however the hydrolysis of other nucleotides could not be stimulated by these agents. These results show that the plant mitochondrial F0F1 ATPase complex differs in composition from the other mitochondrial, chloroplast and bacterial ATPases. The enzyme is, however, more closely related to the yeast mitochondrial ATPase and to the animal mitochondrial ATPase than to the chloroplast enzyme. The plant mitochondrial enzyme, however, exhibits catalytic properties which are characteristic for the chloroplast enzyme.     

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.     

Identification of residues essential for carbohydrate recognition and cation dependence of the 46-kDa mannose 6-phosphate receptor

The 46 kDa cation-dependent mannose 6-phosphate receptor (CD-MPR) plays an essential role in the biogenesis of lysosomes by diverting newly synthesized mannose 6-phosphate (Man-6-P)-containing lysosomal enzymes from the secretory pathway to acidified endosomes. Previous crystallographic studies of the CD-MPR have identified 11 amino acids within its carbohydrate binding pocket. These residues were evaluated quantitatively by assaying the binding affinity of mutant receptors containing a single amino acid substitution toward a lysosomal enzyme. The results show that substitution of Gln-66, Arg-111, Glu-133, or Tyr-143 results in a >800-fold decrease in affinity, demonstrating these four amino acids are essential for carbohydrate recognition by the CD-MPR. Solution binding and surface plasmon resonance analyses demonstrated that the presence of Mn2+ enhanced the affinity of the CD-MPR for a lysosomal enzyme by 2- to 4-fold and increased the stoichiometry of the interaction between a heterogeneous population of a lysosomal enzyme and the receptor by approximately 3-fold. In contrast, substitution of Asp-103 results in a protein that no longer exhibits enhanced binding affinities or altered stoichiometry in the presence of cations, and electron spin resonance demonstrated that the D103S mutant exhibits a 6-fold lower affinity for Mn2+ than the wild-type receptor (Kd = 3.7 6 1.4 mM versus 0.6 6 0.1 mM). Chemical cross-linking revealed that Mn2+ influences the stoichiometry of interaction between the CD-MPR and lysosomal enzymes by increasing the oligomeric state of the receptor from dimer to higher order oligomers. Taken together, these studies provide the molecular basis for high affinity carbohydrate recognition by the CD-MPR. Furthermore, Asp-103 has been identified as the key residue which mediates the effects of divalent cations on the binding properties of the CD-MPR.     

Isolation of a crotalase-like protease with alpha-fibrinogenase activity from the western diamondback rattlesnake, Crotalus atrox.

Venom toxins were isolated from rattlesnake (Crotalus atrox) venom by cation-exchange chromatography. Seven major fractions could be obtained by single-step ion-exchange chromatography with two fractions showing essentially apparent homogeneity by SDS-gel electrophoresis. All fractions showed various extents of specific proteolytic activity against alpha- or beta-chains of fibrinogen molecules. Further characterization of one of the purified fractions with alpha-fribrinogenase activity indicated that it is a single-chain thrombin-like protease with a molecular mass of about 30 kDa. It is relatively heat stable, inhibited by phenylmethanesulfonyl fluoride, N alpha-p-tosyl-L-phenylalanine chloromethyl ketone and N alpha-p-tosyl-L-lysine chloromethyl ketone but not by soybean trypsin inhibitor and beta-mercaptoethanol. Amino acid analysis showed that the enzyme possesses an amino acid composition very similar to thrombin and crotalase characterized before from the closely related snake venoms. N-Terminal sequence analysis of the enzyme corroborated the close similarity between this enzyme and those sequences of crotalase and kallikrein-like enzymes characterized from the same Crotalidae snake family. This study is in contrast to the previous reports which indicated a lack of thrombin- and crotalase-like enzyme in the venom of Western diamondback rattlesnake.     

Purification and biochemical characterization of a lysosomal α-fucosidase from the deuterostomia Asterias rubens

In vertebrates, mannose 6-phosphate receptors [MPR300 (Mr 300 kDa) and MPR46 (Mr 46 kDa)] are highly conserved transmembrane glycoproteins that mediate transport of lysosomal enzymes to lysosomes. Our studies have revealed the appearance of these putative receptors in invertebrates such as the molluscs and deuterostomes. Starfish tissue extracts contain several lysosomal enzyme activities and here we describe the affinity purification of α-fucosidase. The purified enzyme is a glycoprotein that exhibited a molecular mass of ∼56 kDa in SDS-PAGE under reducing conditions. It has also cross-reacted with an antiserum to the mollusc enzyme suggesting antigenic similarities among the two invertebrate enzymes. LC–MS/MS analysis of the proteolytic peptides of the purified enzyme in combination with de novo sequencing allowed us to do partial amino acid sequence determination of the enzyme. These data suggest that this invertebrate enzyme is homologous to the known mammalian enzyme. The purified enzyme exhibited a mannose 6-phosphate dependent interaction with the immobilized starfish MPR300 protein. Our results demonstrate that the lysosomal enzyme targeting pathway is conserved even among the invertebrates.     

Reconstitution of DNA topoisomerase VI of the thermophilic archaeon Sulfolobus shibatae from subunits separately overexpressed in Escherichia coli.

DNA topoisomerase VI from the hyperthermophilic archaeon Sulfolobus shibatae is the prototype of a novel family of type II DNA topoisomerases that share little sequence similarity with other type II enzymes, including bacterial and eukaryal type II DNA topoisomerases and archaeal DNA gyrases. DNA topoisomerase VI relaxes both negatively and positively supercoiled DNA in the presence of ATP and has no DNA supercoiling activity. The native enzyme is a heterotetramer composed of two subunits, A and B, with apparent molecular masses of 47 and 60 kDa, respectively. Here wereport the overexpression in Escherichia coli and the purification of each subunit. The A subunit exhibits clusters of arginines encoded by rare codons in E.coli . The expression of this protein thus requires the co-expression of the minor E.coli arginyl tRNA which reads AGG and AGA codons. The A subunit expressed in E.coli was obtained from inclusion bodies after denaturation and renaturation. The B subunit was overexpressed in E.coli and purified in soluble form. When purified B subunit was added to the renatured A subunit, ATP-dependent relaxation and decatenation activities of the hyperthermophilic DNA topoisomerase were reconstituted. The reconstituted recombinant enzyme exhibits a specific activity similar to the enzyme purified from S.shibatae . It catalyzes transient double-strand cleavage of DNA and becomes covalently attached to the ends of the cleaved DNA. This cleavage is detected only in the presence of both subunits and in the presence of ATP or its non-hydrolyzable analog AMPPNP.