Identification of catalytic residues of Ca2+-independent 1,2-alpha-D-mannosidase from Aspergillus saitoi by site-directed mutagenesis

Cellular responses to the expression of unstable secretory proteins in the filamentous fungus Aspergillus oryzae

Filamentous fungi are often used as cell factories for recombinant protein production because of their ability to secrete large quantities of hydrolytic enzymes. However, even using strong transcriptional promoters, yields of nonfungal proteins are generally much lower than those of fungal proteins. Recent analyses revealed that expression of certain nonfungal secretory proteins induced the unfolded protein response (UPR), suggesting that they are recognized as proteins with folding defects in filamentous fungi. More recently, however, even highly expressed endogenous secretory proteins were found to evoke the UPR. These findings raise the question of whether the unfolded or misfolded state of proteins is selectively recognized by quality control mechanisms in filamentous fungi. In this study, a fungal secretory protein (1,2-α-D-mannosidase; MsdS) with a mutation that decreases its thermostability was expressed at different levels in Aspergillus oryzae. We found that, at moderate expression levels, wild-type MsdS was secreted to the medium, while the mutant was not. In the strain with a deletion for the hrdA gene, which is involved in the endoplasmic reticulum-associated degradation pathway, mutant MsdS had specifically increased levels in the intracellular fraction but was not secreted. When overexpressed, the mutant protein was secreted to the medium to a similar extent as the wild-type protein; however, the mutant underwent hyperglycosylation and induced the UPR. Deletion of α-amylase (the most abundant secretory protein in A. oryzae) alleviated the UPR induction by mutant MsdS overexpression. These findings suggest that misfolded MsdS and unfolded species of α-amylase might act synergistically for UPR induction.

Development of enzyme technology for Aspergillus oryzae, A. sojae, and A. luchuensis, the national microorganisms of Japan

This paper describes the modern enzymology in Japanese bioindustries. The invention of Takadiastase by Jokiti Takamine in 1894 has revolutionized the world of industrial enzyme production by fermentation. In 1949, a new γ-amylase (glucan 1,4-α-glucosidase, EC 3.2.1.3) from A. luchuensis (formerly designated as A. awamori), was found by Kitahara. RNase T1 (guanyloribonuclease, EC 3.1.27.3) was discovered by Sato and Egami. Ando discovered Aspergillus nuclease S1 (single-stranded nucleate endonuclease, EC 3.1.30.1). Aspergillopepsin I (EC 3.4.23.18) from A. tubingensis (formerly designated as A. saitoi) activates trypsinogen to trypsin. Shintani et al. demonstrated Asp76 of aspergillopepsin I as the binding site for the basic substrate, trypsinogen. The new oligosaccharide moieties Man10GlcNAc2 and Man11GlcNAc2 were identified with α-1,2-mannosidase (EC 3.2.1.113) from A. tubingensis. A yeast mutant compatible of producing Man5GlcNAc2 human compatible sugar chains on glycoproteins was constructed. The acid activation of protyrosinase from A. oryzae at pH 3.0 was resolved. The hyper-protein production system of glucoamylase was established in a submerged culture.

A Single Free Cysteine Residue and Disulfide Bond Contribute to the Thermostability ofAspergillus saitoi1,2-α-Mannosidase

Aspergillus saitoi 1,2-alpha-mannosidase contains three conserved cysteine residues (Cys334, Cys363, and Cys443). We showed that Cys334 and Cys363 are involved in a disulfide bond, and that Cys443 contains a free thiol group. The cysteines were not essential for the activity analyzed by site-directed mutagenesis and kinetics. The substitution at each cysteine residue greatly destabilized the enzyme. The T(m) values of WT, C443A, C443G, C443S, and C443T were 55.8, 51.9, 50.2, 50.0, and 52.8 degrees C respectively. The specific activity of these mutants was almost equal to that of WT. Introducing Asp, Leu, Met, or Val at position 443 caused partial denaturation, although the enzymes had some activity. C443F, C443I, C443N, and C443Y were not secreted. These results suggest that the hydrophilic and large side chain causes the destabilization. Molecular modelling showed that the Cys443 residue is buried and surrounded by a hydrophobic environment. Cys334 and Cys363 form a disulfide bond, and Cys443 is involved in a hydrophobic interaction to stabilize the enzyme.

Modulation of activity by Arg407: structure of a fungal alpha-1,2-mannosidase in complex with a substrate analogue

Class I alpha-mannosidases (glycoside hydrolase family GH47) play key roles in the maturation of N-glycans and the ER-associated degradation of unfolded glycoproteins. The 1.95 A resolution structure of a fungal alpha-1,2-mannosidase in complex with the substrate analogue methyl-alpha-D-lyxopyranosyl-(1',2)-alpha-D-mannopyranoside (LM) shows the intact disaccharide spanning the -1/+1 subsites, with the D-lyxoside ring in the -1 subsite in the 1C4 chair conformation, and provides insight into the mechanism of catalysis. The absence of the C5' hydroxymethyl group on the D-lyxoside moiety results in the side chain of Arg407 adopting two alternative conformations: the minor one interacting with Asp375 and the major one interacting with both the D-lyxoside and the catalytic base Glu409, thus disrupting its function. Chemical modification of Asp375 has previously been shown to inactivate the enzyme. Taken together, the data suggest that Arg407, which belongs to the conserved sequence motif RPExxE, may act to modulate the activity of the enzyme. The proposed mechanism for modulating the activity is potentially a general mechanism for this superfamily.

Fluorescence Quenching and Time-resolved Fluorescence studies of α-Mannosidase from Aspergillus fischeri (NCIM 508)

Apart from the vital role in glycoprotein biosynthesis and degradation, alpha-mannosidase is currently an important therapeutic target for the development of anticancer agents. Fluorescence quenching and time-resolved fluorescence of alpha-mannosidase, a multitryptophan protein from Aspergillus fischeri were carried out to investigate the tryptophan environment. The tryptophans were found to be differentially exposed to the solvent and were not fully accessible to the neutral quencher indicating heterogeneity in the environment. Quenching of the fluorescence by acrylamide was collisional. Surface tryptophans were found to have predominantly positively charged amino acids around them and differentially accessible to the ionic quenchers. Denaturation led to more exposure of tryptophans to the solvent and consequently in the significant increase in quenching with all the quenchers. The native enzyme showed two different lifetimes, tau (1) (1.51 ns) and tau (2) (5.99 ns). The average lifetime of the native protein (tau) (3.187 ns) was not affected much after denaturation (tau) (3.219 ns), while average lifetime of the quenched protein samples was drastically reduced (1.995 ns for acrylamide and 1.537 ns for iodide). This is an attempt towards the conformational studies of alpha-mannosidase.

Molecular cloning and expression of a cDNA encoding a Coccidioides posadasii 1,2-alpha-mannosidase identified in the coccidioidal T27K vaccine by immunoproteomic methods

The coccidioidal T27K vaccine is protective in mice against respiratory challenge with Coccidioides posadasii (C. posadasii) arthroconidia. The vaccine is a subcellular multicomponent preparation that has not been fully characterized. To identify potential protective antigens in the heterogeneous mixture, the vaccine has been separated by two-dimensional gel electrophoresis and then analyzed for seroreactive proteins using immunoblot analysis with pooled sera from patients with coccidioidomycosis. Two seroreactive spots of identical apparent molecular weight were identified and sequenced using tandem mass spectrometry. Three peptides were generated, two of which matched a tentative consensus sequence in the TIGR C. posadasii 2.0 gene index database that is similar to fungal 1,2-alpha-mannosidases. The 5' and 3' ends of the mannosidase cDNA were mapped using rapid amplification of cDNA ends (RACE) polymerase chain reaction (PCR), and a full-length cDNA was then obtained using reverse-transcription (RT) PCR. The cDNA was cloned and sequenced and expressed as a recombinant protein. The predicted protein consists of 519 amino acids, has a theoretical molecular weight and pI of 56,918 Da and 4.84, respectively, and is very similar (>60%) to other fungal 1,2-alpha-mannosidases. Class I 1,2-alpha-mannosidase enzyme activity was also detected in the T27K vaccine using the substrate, Man-alpha-1,2-Man-alpha-OCH(3) in a spectrophotometric assay.

Cloning and expression of 1,2-alpha-mannosidase gene (fmanIB) from filamentous fungus Aspergillus oryzae: in vivo visualization of the FmanIBp-GFP fusion protein

1,2-alpha-Mannosidase catalyzes the specific cleavage of 1,2-alpha-mannose residues from protein-linked N-glycan. In this study, a novel DNA sequence homologous to the authentic 1,2-alpha-mannosidase was cloned from a cDNA library prepared from solid-state cultured Aspergillus oryzae. The fmanIB cDNA consisted of 1530 nucleotides and encoded a protein of 510 amino acids in which all consensus motifs of the class I alpha-mannosidase were conserved. Expression of the full length of 1,2-alpha-mannosidase cDNA by the Aspergillus host, though it has rarely been done with other filamentous-fungal mannosidase, was successful with fmanIB and caused an increase in both intracellular and extracellular mannosidase activity. The expressed protein (FmanIBp) specifically hydrolyzed 1,2-alpha-mannobiose with maximal activity at a pH of 5.5 and a temperature of 45 degrees C. With Man(9)GlcNAc(2) as the substrate, Man(5)GlcNAc(2) finally accumulated while hydrolysis of the 1,2-alpha-mannose residue of the middle branch was rate-limiting. To examine the intracellular localization of the enzyme, a chimeric protein of FmanIBp with green fluorescent protein was constructed. It showed a dotted fluorescence pattern in the mycelia of Aspergillus, indicative of the localization in intracellular vesicles. Based on these enzymatic and microscopic results, we estimated that FmanIBp is a fungal substitute for the mammalian Golgi 1,2-alpha-mannosidase isozyme IB. This and our previous report on the presence of another ER-type mannosidase in A. oryzae (Yoshida et al., 2000) support the notion that the filamentous fungus has similar steps of N-linked glycochain trimming to those in mammalian cells.

The production, purification and characterisation of two novel α-d-mannosidases from Aspergillus phoenicis

1,6-alpha-D-Mannosidase from Aspergillus phoenicis was purified by anion-exchange chromatography, chromatofocussing and size-exclusion chromatography. The apparent molecular weight was 74 kDa by SDS-PAGE and 81 kDa by native-PAGE. The isoelectric point was 4.6. 1,6-alpha-D-Mannosidase had a temperature optimum of 60 degrees C, a pH optimum of 4.0-4.5, a K(m) of 14 mM with alpha-D-Manp-(1-->6)-D-Manp as substrate. It was strongly inhibited by Mn(2+) and did not need Ca(2+) or any other metal cofactor of those tested. The enzyme cleaves specifically (1-->6)-linked mannobiose and has no activity towards any other linkages, p-nitrophenyl-alpha-D-mannopyranoside or baker's yeast mannan. 1,3(1,6)-alpha-D-Mannosidase from A. phoenicis was purified by anion-exchange chromatography, chromatofocussing and size-exclusion chromatography. The apparent molecular weight was 97 kDa by SDS-PAGE and 110 kDa by native-PAGE. The 1,3(1,6)-alpha-D-mannosidase enzyme existed as two charge isomers or isoforms. The isoelectric points of these were 4.3 and 4.8 by isoelectric focussing. It cleaves alpha-D-Manp-(1-->3)-D-Manp 10 times faster than alpha-D-Manp-(1-->6)-D-Manp, has very low activity towards p-nitrophenyl-alpha-D-mannopyranoside and baker's yeast mannan, and no activity towards alpha-D-Manp-(1-->2)-D-Manp. The activity towards (1-->3)-linked mannobiose is strongly activated by 1mM Ca(2+) and inhibited by 10mM EDTA, while (1-->6)-activity is unaffected, indicating that the two activities may be associated with different polypeptides. It is also possible that one polypeptide may have two active sites catalysing distinct activities.