THE LOCALIZATION OF BETA-FRUCTOFURANOSIDASE IN NEUROSPORA

Invertases in Phytophthora infestans Localize to Haustoria and Are Programmed for Infection-Specific Expression

The oomycete Phytophthora infestans, the causal agent of potato and tomato blight, expresses two extracellular invertases. Unlike typical fungal invertases, the P. infestans genes are not sucrose induced or glucose repressed but instead appear to be under developmental control. Transcript levels of both genes were very low in mycelia harvested from artificial medium but high in preinfection stages (sporangia, zoospores, and germinated cysts), high during biotrophic growth in leaves and tubers, and low during necrotrophy. Genome-wide analyses of metabolic enzymes and effectors indicated that this expression profile was fairly unusual, matched only by a few other enzymes, such as carbonic anhydrases and a few RXLR effectors. Genes for other metabolic enzymes were typically downregulated in the preinfection stages. Overall metabolic gene expression during the necrotrophic stage of infection clustered with artificial medium, while the biotrophic phase formed a separate cluster. Confocal microscopy of transformants expressing green fluorescent protein (GFP) fusions indicated that invertase protein resided primarily in haustoria during infection. This localization was not attributable to haustorium-specific promoter activity. Instead, the N-terminal regions of proteins containing signal peptides were sufficient to deliver proteins to haustoria. Invertase expression during leaf infection was linked to a decline in apoplastic sucrose, consistent with a role of the enzymes in plant pathogenesis. This was also suggested by the discovery that invertase genes occur across multiple orders of oomycetes but not in most animal pathogens or a mycoparasite.IMPORTANCE Oomycetes cause hundreds of diseases in economically and environmentally significant plants. How these microbes acquire host nutrients is not well understood. Many oomycetes insert specialized hyphae called haustoria into plant cells, but unlike their fungal counterparts, a role in nutrition has remained unproven. The discovery that Phytophthora invertases localize to haustoria provides the first strong evidence that these structures participate in feeding. Since regions of proteins containing signal peptides targeted proteins to the haustorium-plant interface, haustoria appear to be the primary machinery for secreting proteins during biotrophic pathogenesis. Although oomycete invertases were acquired laterally from fungi, their expression patterns have adapted to the Phytophthora lifestyle by abandoning substrate-level regulation in favor of developmental control, allowing the enzymes to be produced in anticipation of plant colonization. This study highlights how a widely distributed hydrolytic enzyme has evolved new behaviors in oomycetes.

Location of Aryl Sulfatase in Conidia and Young Mycelia of Neurospora crassa

Aryl sulfatase (arylsulfate sulfohydrolase, EC 3.1.6.1) was found to have multiple locations in Neurospora conidia. Some enzyme activity remained in the supernatant when a spore suspension was centrifuged or filtered. Part of the cell-bound activity could be detected by adding the assay ingredients to a suspension of intact spores (patent enzyme), and additional activity was only detectable when the spores were first treated to destroy their permeability barriers (cryptic enzyme). Such treatments include: disruption with an X-press, brief rinsing with chloroform or acetone, incubation at 60 C for 5 min, and incubation with phenethyl alcohol, nystatin, or ascosin. Part of the patent aryl sulfatase was inactivated by briefly acid treating the intact spores (no loss of conidial viability). This enzyme was considered to have a cell surface location. Some enzyme was acid-resistant in intact spores, but all of the enzyme was acid-sensitive in spores whose permeability barriers had been disrupted. The pH dependence, kinetic properties, and p-nitrophenyl sulfate uptake were investigated in acid-treated conidia. No aryl sulfatase was detected in ascospores. Young mycelia contained more aryl sulfatase than did conidia, but little, if any, was secreted into the growth medium. Cryptic activity was demonstrated in young mycelia by brief chloroform treatment or by rinsing the cells with 0.1 m acetate buffer. Enzyme activity in young mycelia was completely labile to acid treatment, as was cell viability.

Purification and partial characterization of the high and low molecular weight form (S- and F-form) of invertase secreted by Aspergillus nidulans

Two forms of secreted invertase have been purified from Aspergillus nidulans by ion-exchange and gel-filtration chromatography. S-invertase gave a single, broad, glycoprotein band on PAGE and SDS-PAGE corresponding in size to 185 and 78 kDa, respectively, compared with 94 and 110 kDa for F-invertase. The carbohydrate of S-invertase contained mainly mannose (14%) and less galactose (5%) whereas the F-form yielded mainly galactose (29%) and less mannose (12%). Three sharp bands of enzymically active glycoprotein for both the S-form (pI 4.9-5.2) and the F-form (pI 3-4.2) were observed after isoelectric focusing. Deglycosylation with Endo H simplified this pattern to one enzymically active protein band (pI 5.2). The aglycoenzymes gave narrow bands on PAGE and SDS-PAGE corresponding to 115 kDa and 60 kDa respectively for both S- and F-forms. The specific activity of S-invertase was three-fold higher than that of F-invertase both before and after deglycosylation. The Km values of the two forms of invertase were very similar. Significant homology existed between the N-terminal amino-acid sequences of S-invertase (and of internal peptides derived from it) and sequences of invertase from other species. It is suggested that the higher carbohydrate content in F-invertase results in the native enzyme existing as a monomer and having a greater negative charge and lower specific enzyme activity compared with the dimeric S-enzyme.

Conidial alkaline phosphatase from Neurospora crassa

An alkaline phosphatase was purified from conidia of a Neurospora crassa wild type strain. The M(r) of the purified native enzyme was estimated as ca 145,000 and 110,000 by gel filtration, in the presence and absence of magnesium ions, respectively. A single polypeptide band of M(r) 36,000 was detected by SDS-PAGE, suggesting that the native enzyme was a tetramer of apparently identical subunits. Conidial alkaline phosphatase was an acidic protein (pl = 4.0 +/- 0.1), with 40% carbohydrate content. Optimal pH was affected by substrate concentration and magnesium ions. Low concentrations of calcium ions (0.1 mM) had slight stimulatory effects, but in excess (5 mM) caused protein aggregates with decreased activity. The enzyme specificity against different substrates was compared with those reported for constitutive or Pi-repressible alkaline phosphatases produced by N. crassa. The results suggested that the conidial alkaline phosphatase represented a different class among other such enzymes synthesized by this organism.

Purification and characterisation of an Aspergillus niger invertase and its DNA sequence

A secreted invertase was purified 23-fold by ultrafiltration, ion-exchange, and gel filtration chromatography from the culture supernatant of 18 h sucrose-grown cultures of Aspergillus niger. The purified enzyme hydrolysed sucrose and raffinose but there was no detectable hydrolysis of inulin, melezitose or PNPG. Invertase activity was optimal at pH 5.5 and 50 degrees C. The molecular mass of reduced invertase was 115 kDa, as determined by SDS gel electrophoresis. The native molecular weight of between 225 kDa and 250 kDa, estimated by electrophoresis under non-denaturing conditions, suggests that the protein is a dimer of identical subunits. The suc1 gene encoding this protein was completely-sequenced. The translated sequence yields a protein of 566 amino acids with a calculated molecular mass of 61 kDa, suggesting that carbohydrates represent about 50% of the mass of the protein.

Purification and properties of an extracellular conidial trehalase from Humicola grisea var. thermoidea

An extracellular trehalase (alpha, alpha-trehalose glucohydrolase, EC 3.2.1.28) was purified from conidia of Humicola grisea var. thermoidea. The purified enzyme is a glycoprotein and migrates as a single polypeptide band during polyacrylamide gel electrophoresis under non-denaturing conditions. The apparent molecular weight of the enzyme was estimated as 580,000 by gel filtration chromatography. The enzyme is separable into three polypeptide bands of 105,000, 98,000 and 84,000 daltons on SDS-PAGE. It is specific for trehalose and its activity is not inhibited by other disaccharides. It has a Km of 2.3 mM, an optimum pH of 5.6 in sodium acetate buffer and a temperature optimum of 60 degrees C. The enzyme is activated by Ca2+, Co2+ and Mn2+ and inhibited by inorganic phosphate, AMP, ADP or ATP. The inhibitory effect of phosphate, AMP and ADP, but not that of ATP, was abolished in the presence of Ca2+.

Sedimentation properties of chitosomal chitin synthetase from the wild-type strain and the 'slime' variant of Neurospora crassa

Marked differences in the pattern of sedimentation of cellular structures were observed after isopycnic centrifugation of crude cell-free preparations from the Neurospora crassa wall-less 'slime' variant and mycelial wild-type strain. Kinetic studies of particle sedimentation showed that the various types of subcellular components, as revealed by turbidity, UV absorption, polypeptide patterns, and chitin synthetase activity determinations, sediment independently of one another. An important feature was the finding that chitin synthetase from 'slime' peaked at a median specific gravity of 1.1201 +/- 0.0036, whereas that from wild-type strain sedimented at a higher buoyant density (specific gravity 1.1349 +/- 0.0024). Different cultivation conditions or cell breakage procedures (osmotic lysis or ballistic disruption) did not seem to affect this sedimentation behavior. Electron microscopy revealed the presence of chitosomes (microvesicles containing chitin synthetase) in the chitin synthetase activity peaks obtained after isopycnic centrifugation of cell-free extracts from 'slime' and wild-type strains. The discrepancy in buoyant density of chitin synthetases from both N. crassa strains might point to inherent differences in chemical composition of the chitosomal microvesicles. In any case, the lower buoyant density of 'slime' chitosomes appears to be one of several major alterations in sedimentation behavior of subcellular structures. These alterations might be related to the inability of 'slime' to make a cell wall.

Localization of glycosidases in the wall of living cells from cultured Convolvulus arvensis tissue

Nine glycosidase activities were detected in isolated cell walls of cultured Convolvulus arvensis L. cells. Seven were also found in the cytoplasmic fraction. Optimal pH values were all in the acid region. The in vivo localization of some of these glycosidases was studied by assaying whole cells. Suspended cells hydrolysed the externally supplied substrates for α-galactosidase (EC 3.2.1.22), β-galactosidase (EC 3.2.1.23), β-glucosidase (EC 3.2.1.21), α-mannosidase (EC 3.2.1.24) and β-xylosidase (EC 3.2.1.37). Since intracellular enzymes were latent or showed little leakage, the cells were regarded to be relatively intact. In the cases investigated, the apparent glycosidase activity values of whole cells corresponded to those of isolated cell walls. The pH-activity profiles were similar. The Km values were identical indicating equal accessibility to substrate. The enzymes could be partially removed from the cells by strong salt solutions. The data are consistent with an in vivo cell wall localization of a number of glycosidases.

Lytic enzymes in the autolysis of filamentous fungi

The degrees of autolysis attained by five different genera of filamentous fungi during an incubation period of 60 days, under the same culture conditions were: 87.3% for Penicillium oxalicum; 65.9% for Neurospora crassa; 62.7% for Polystictus versicolor; 51.7% for Aspergillus niger and 23.5% for Nectria galligena. N. crassa, A. niger and P. versicolor reached the end of the autolysis during this incubation period (60 days), whereas P. oxalicum and N. galligena did not. The excretion of the lytic enzymes beta-N-acetylglucosaminidase, beta -1-3 glucanase, chitinase, invertase and acid phosphatase into the culture medium during growth and autolysis was investigated. The excretion of these enzymes was consistent with the degree of autolysis reached, the maximum excretion belonging to P. oxalicum and the minimum to N. galligena. The N. crassa invertase was excreted into the culture liquid at levels very much higher than the other enzymes studied, and at levels very much higher than the invertases excreted by the other fungi.

Activity and heat stability of trehalase from the mycelium and ascospores of Neurospora

Trehalases from the ascospores of Neurospora tetrasperma and the mycelium of N. crassa were compared. Enzymes from both sources have identical electrophoretic mobilities, K(m)'s, responses to pH, immunological reactions, and activities in low-molarity buffers. Because both enzymes are so similar, conclusions about the properties of the ascospore enzyme may, be made by studying mycelial trehalase. Mycelial trehalase is most active and stable in low-molarity buffers. The enzyme exists in at least three species; the smallest has a molecular weight between 105,000 and 125,000 and is predominant in low-molarity buffers at 37 C. The stability of trehalase to heating at 65 C can be increased by increasing enzyme concentration and by the addition of polyols. Ascospores contain large amounts of trehalose, which protects trehalase from heat inactivation at 65 C. The importance of this phenomenon in vivo and its relationship to the localization of trehalase in ascospores is discussed.

Regeneration of invertase in Neurospora crassa

In Neurospora, invertase is predominately an extracellular enzyme, and acid phosphatase is partially external in location. Both extracellular invertase and acid phosphatase were rapidly and quantitatively inactivated by acid treatment (pH 1.3). When such acid-treated cells were incubated with a suitable carbon source, a substantial regeneration of invertase activity occurred, but no restoration of acid phosphatase could be detected. The regeneration of invertase does not occur by renaturation of the inactivated enzyme, nor by secretion of a preexisting intracellular pool of invertase, but instead requires de novo enzyme synthesis. Invertase synthesis was partially repressed by glucose and mannose and was completely inhibited by 2-deoxyglucose. Acetate was found to inhibit invertase regeneration and the transport and incorporation of uracil and leucine. Several potential inhibitors of transcription, including alpha-amanitin, 5-fluorouracil, actinomycin D, and three derivatives of rifamycin, were ineffective in preventing invertase regeneration and in inhibiting the synthesis of ribonucleic acid. Conidia appeared to be very poorly permeable to these compounds.

Cyanide-resistant respiration in Neurospora crassa

Cell respiration in wild type and poky was studied as part of a long-term investigation of cyanide-resistant respiration in Neurospora. Respiration in wild type proceeds via a cytochrome chain which is similar to that of higher organisms; it is sensitive to antimycin A or cyanide. Poky, on the other hand, respires by means of two alternative oxidase systems. One of these is analogous to the wild-type cytochrome chain in that it can be inhibited by antimycin A or cyanide; this system accounts for as much as 15% of the respiration of poky f(-) and 34% of the respiration of poky f(+). The second oxidase system is unaffected by antimycin A or cyanide at concentrations which inhibit the cytochrome chain maximally. It can, however, be specifically inhibited by salicyl hydroxamic acid. The cyanide-resistant oxidase is not exclusive to poky, but is also present in small quantities in wild type grown under ordinary circumstances. These quantities may be greatly increased (as much as 20-fold) by growing wild type in the presence of antimycin A, cyanide, or chloramphenicol.

Presence of binding site for alpha-amylase and of masking protein for this site on mycelial cell wall of Aspergillus oryzae

Mycelial cell wall of Aspergillus oryzae M-13 grown in an alpha-amylase-forming medium could not bind alpha-amylase (Taka-amylase A, EC 3.2.1.1). However, by treatment with 1.0 n NaOH at 100 C for 30 min, the wall gained the ability to bind alpha-amylase. This phenomenon was caused by removal of a factor (designated as masking factor) which masked the binding site for alpha-amylase. The masking factor was purified as a preparation giving a single peak in both ultracentrifugation (1.6S) and by gel electrophoresis (M(BPB), 1.0). Approximately 20 mug of the purified factor, bound to 10 mg of the alkali-treated mycelial cell wall, prevented the binding of approximately 100 mug of alpha-amylase or released approximately 100 mug of alpha-amylase which previously was bound to the alkali-treated wall. These findings indicate that the factor has much higher affinity than alpha-amylase for the binding site on the mycelial wall. The masking factor was inducibly formed accompanying the secretion of alpha-amylase.

Biochemical and histochemical localization of invertase in Neurospora crassa during conidial germination and hyphal growth

The intracellular localization of Neurospora invertase, an enzyme partially secreted and partially retained by Neurospora at the cell periphery, was investigated. A cell wall fraction was isolated, to which 24% of the cell-bound invertase was firmly attached. A sensitive osmiophilic stain for invertase was developed and used in conjunction with the technique of indirect immunofluorescence to follow the pattern of invertase localization during the development of Neurospora from the germination of conidia to the mature hypha. These studies revealed that: (i) conidial invertase was uniformly distributed along the cell periphery; (ii) growing hyphal tips of germinating conidia showed pronounced invertase activity as the rest of the conidial cell wall lost its peripheral activity; (iii) hyphae in early log-phase growth had strong enzyme activity associated with the cell wall, and in late log phase the activity became associated with the plasma membrane and points where new hyphal branches were being formed; and (iv) hyphae in early stationary phase had strong fluorescence at incipient branching points, in "dots" close to the plasma membrane, and in the cytoplasm.

Localization of the beta-glucosidases in Neurospora crassa

The beta-glucosidases (EC 3.2.1.21) of Neurospora crassa were studied with respect to their location in conidia and young mycelia. Aryl-beta-glucosidase of conidia was nearly equally divided between extracellular and bound activity. Bound aryl-beta-glucosidase was almost all available to substrate. An induction procedure was used to maximize both beta-glucosidases in 4 to 6-hr cells. Aryl-beta-glucosidase was entirely bound but still mostly (90%) detectable, whereas cellobiase was mostly internal and cryptic. A freeze-thaw cycle or treatment with phenethyl alcohol or deoxycholic acid made the cellobiase detectable without releasing it from the cell. A 10 to 20% increase in cell-bound aryl-beta-glucosidase could be obtained by this treatment. Dilute HCl (0.1 n) destroyed the patent aryl-beta-glucosidase but not the cryptic aryl-beta-glucosidase or the cryptic cellobiase activity in intact cells. This suggested that most aryl-beta-glucosidase activity was exterior to the cell membrane but still within the mural space. The thermal stability of patent aryl-beta-glucosidase and released cellobiase was found to be higher than in corresponding cell-free extracts. Measurements of K(m) suggested a slightly lower affinity for substrate p-nitrophenyl-beta-d-glucopyranoside by the enzymes in intact cells compared to enzymes in extracts.

Genetic determinants of circadian rhythmicity in Neurospora

Timex, a strain of Neurospora crassa which exhibits a circadian rhythm of conidia formation in growth-tube cultures, has been found to differ from wild-type strains by two genes. One gene, inv, is responsible for an invertase deficiency, whereas the second gene, bd, is of unknown function. Both genes map independently from other genes known to induce Neurospora rhythmicity. The inv gene is not essential for the timex phenotype because bd strains express that phenotype on certain media. Although inv strains do exhibit some rhythmicity of their own, the rhythmicity apparently is not a direct result of the invertase deficiency, since there is no correlation between invertase level and rhymicity in 29 strains tested. Of the 29 strains tested, 20 exhibited some rhythmicity in growth-tube cultures, suggesting that morphological manifestations of rhythmicity in Neurospora may result from the function or the loss of function of numerous genes, or both. There was no correlation in these strains between rhythmicity and (i) genetic background; (ii) geographical origin; or (iii) nutritional requirements.

Gene-enzyme relationships in Neurospora invertase

A spontaneous, single-gene mutation responsible for a total lack of invertase activity in Neurospora crassa is described. The mutation is believed to lie in the structural gene for invertase, since an immunologically cross-reacting protein is made by the mutant strain. In addition, there was no evidence for a defect in regulation of invertase activity or synthesis by the following criteria. (i) The invertaseless condition was recessive in heterokaryons; (ii) no invertase inhibitor was found in mutant extracts by mixing experiments; and (iii) none of the several sugars able to induce activity in wild-type strains was able to induce activity in the mutant strain. It was also discovered that most of the wild-type enzyme (55 to 75%) cannot be washed free from the rapidly sedimenting cell debris. This finding provided additional support for the hypothesis that Neurospora invertase is located within or about the cell wall.