Sachiko Machida

Cycloamylose as an efficient artificial chaperone for protein refolding

High molecular weight cyclic α‐1,4‐glucan (referred to as cycloamylose) exhibited an artificial chaperone property toward three enzymes in different categories. The inclusion properties of cycloamylose effectively accommodated detergents, which keep the chemically denatured enzymes from aggregation, and promoted proper protein folding. Chemically denatured citrate synthase was refolded and completely recovered its enzymatic activity after dilution with polyoxyethylenesorbitan buffer followed by cycloamylose treatment. The refolding was completed within 2 h, and the activity of the refolded citrate synthase was quite stable. Cycloamylose also promoted the refolding of denatured carbonic anhydrase B and denatured lysozyme of a reduced form.

A rapid identification method for aflatoxin-producing strains of Aspergillus flavus and A. parasiticus by ammonia vapor.

The colony reverse of aflatoxin (AF)-producing strains ofAspergillus flavus andA. parasiticus turned pink when their cultures were exposed to ammonia vapor. The color change was visible for colonies grown on media suitable for AF production such as potato dextrose, coconut, and yeast extract sucrose agars after 2 d incubation at 25°C. Of the 120 strains ofA. flavus, A. parasiticus, and two related species inA. flavus group:A. oryzae andA. sojae tested in this study, only the AF-producing strains ofA. flavus andA. parasiticus showed the pink pigmentation. The color change occurred immediately after the colony was contacted with ammonia vapor. This method was useful for rapid screening the AF-producing strains ofA. flavus andA. parasiticus.

Interaction of SMKT, a killer toxin produced by Pichia farinosa, with the yeast cell membranes

SMKT (salt‐mediated killer toxin), a killer toxin produced by the halotolerant yeast, Pichia farinosa, kills yeasts of several genera, including Saccharomyces cerevisiae. To elucidate the killing mechanism of SMKT, we examined the interaction of SMKT with membranes using liposomes. Leakage of calcein from calcein‐entrapped liposomes was observed in the presence of SMKT. Destruction of liposomes was observed by dark‐field microscopy. Comparison of intact S. cerevisiae cells with SMKT‐treated cells by dark‐field microscopy indicated that the spherical cell membrane is disrupted by SMKT. Using sodium carbonate extraction, we obtained direct evidence for the first time that SMKT is associated with the membrane of sensitive cells. Our results indicate that SMKT kills sensitive S. cerevisiae by interacting with the yeast cell membrane. Copyright

An investigation of the nature and function of module 10 in a family F/10 xylanase FXYN of Streptomyces olivaceoviridis E-86 by module shuffling with the Cex of Cellulomonas fimi and by site-directed mutagenesis

Although the amino acid homology in the catalytic domain of FXYN xylanase from Streptomyces olivaceoviridis E‐86 and Cex xylanase from Cellulomonas fimi is only 50%, an active chimeric enzyme was obtained by replacing module 10 in FXYN with module 10 from Cex. In the family F/10 xylanases, module 10 is an important region as it includes an acid/base catalyst and a substrate binding residue. In FXYN, module 10 consists of 15 amino acid residues, while in Cex it consists of 14 amino acid residues. The K m and k cat values of the chimeric xylanase FCF‐C10 for PNP‐xylobioside (PNP‐X2) were 10‐fold less than those for FXYN. CD spectral data indicated that the structure of the chimeric enzyme was similar to that of FXYN. Based on the comparison of the amino acid sequences of FXYN and Cex in module 10, we constructed four mutants of FXYN. When D133 or S135 of FXYN was deleted, the kinetic properties were not changed from those of FXYN. By deletion of both D133 and S135, the K m value for PNP‐X2 decreased from the 2.0 mM of FXYN to 0.6 mM and the k cat value decreased from the 20 s−1 of FXYN to 8.7 s−1. Insertion of Q140 into the doubly deleted mutant further reduced the K m value to 0.3 mM and the k cat value to 3.8 s−1. These values are close to those for the chimeric enzyme FCF‐C10. These results indicate that module 10 itself is able to accommodate changes in the sequence position of amino acids which are critical for enzyme function. Since changes of the spatial position of these amino acids would be expected to result in enzyme inactivation, module 10 must have some flexibility in its tertiary structure. The structure of module 10 itself also affects the substrate specificity of the enzyme.

Importance of five amino acid residues at C-terminal region for the folding and stability of β-glucosidase of Cellvibrio gilvus

Abstract To determine the role of the C-terminal region of Cellvibrio gilvus β-glucosidase, a deletion mutant was constructed lacking five amino acid residues (RGRAR), three of which were arginine, from the C-terminal end. The mutant, designated ΔRGRAR, could be folded into an active form when expressed with the molecular chaperons GroEL ES . In comparison with the native enzyme, the optimum pH of the mutant ΔRGRAR shifted to the acidic region and the pH stability to the neutral region, while its heat stability decreased. No significant difference in the kinetic parameter Km was observed. It was concluded that the RGRAR residues located at the C-terminal end are quite important for the stability of the enzyme and protein folding.

Design of a Novel Membrane-Destabilizing Peptide Selectively Acting on Acidic Liposomes

The design of amphipathic peptides resulted in a novel peptide with a selective ability to destabilize lipid bilayers of acidic liposomes. The newly synthesized peptide, termed mast 21, is a 21-residue long amino acid chain and can only act effectively on acidic liposomes lacking cholesterol. Moreover, mast 21 killed Gram-positive and Gram-negative bacteria, and it had no hemolytic activity. The antimicrobial and hemolytic activities paralleled the results of membrane destabilizing activity using liposomes. Circular dichroism and Trp-fluorescence emission spectra showed changes in the peptide conformation and circumstances around the peptide during interaction with liposomes. These changes were consistent with an increased α-helical content and a less polar environment for the tryptophan residue of the peptide. Mast 21 was observed under dark-field microscopy in real time attacking liposomes. Acidic liposomes were attacked, which resulted in peeling of the lipid bilayer with its subsequent destruction.