Sayuki Nikkuni
Ministry of Agriculture, Forestry and Fisheries
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Structure | 1997
Tatsuki Kashiwagi; Naoki Kunishima; Chise Suzuki; Fumihiko Tsuchiya; Sayuki Nikkuni; Yoji Arata; Kosuke Morikawa
BACKGROUND Several strains of yeasts and fungi produce proteinous substances, termed killer toxins, which kill sensitive strains. The SMK toxin, secreted by the halotolerant yeast Pichia farinosa KK1 strain, uniquely exhibits its maximum killer activity under conditions of acidic pH and high salt concentration. The toxin is composed of two distinct subunits, alpha and beta, which tightly interact with each other under acidic conditions. However, they are easily dissociated under neutral conditions and lose the killer activity. The three-dimensional structure of the SMK toxin will provide a better understanding of the mechanism of toxicity of this protein and the cause of its unique pH-dependent stability. RESULTS Two crystal structures of the SMK toxin have been determined at 1.8 A resolution in different ionic strength conditions. The two subunits, alpha and beta, are jointly folded into an ellipsoidal, single domain structure belonging to the alpha/beta-sandwich family. The folding topology of the SMK toxin is essentially the same as that of the fungal killer toxin, KP4. This shared topology contains two left-handed split betaalphabeta motifs, which are rare in the other proteins. Many acidic residues are clustered at the bottom of the SMK toxin molecule. Some of the carboxyl sidechains interact with each other through hydrogen bonds. The ionic strength difference induces no evident structural change of the SMK toxin except that, in the high ionic strength crystal, a number of sulfate ions are electrostatically bound near the basic residues which are also locally distributed at the bottom of the toxin molecule. CONCLUSIONS The two killer toxins, SMK and KP4, share a unique folding topology which contains a rare structural motif. This observation may suggest that these toxins are evolutionally and/or functionally related. The pH-dependent stability of the SMK toxin is a result of the intensive interactions between the carboxyl groups. This finding is important for protein engineering, for instance, towards stabilization of the toxin molecule in a broader pH range. The present crystallographic study revealed that the structure of the SMK toxin itself is hardly affected by the ionic strength, implying that a high salt concentration affects the sensitivity of the cell against the toxin.
Agricultural and biological chemistry | 1989
Chise Suzuki; Kazufumi Yamada; Noriyuki Okada; Sayuki Nikkuni
Halotolerant killer yeasts which showed killer activity in the presence of NaCl were isolated from fermented foods, such as miso, soy sauce and salted vegetables, and identified as Debaryomyces hansenii, Hansenula anomala, Candida naeodendra and Pichia farinosa. The killer strains of C. naeodendra and P. farinosa were found here for the first time. Seventy-six percent of the salted vegetable samples contained killer yeasts, mainly D. hansenii. On the other hand, killer strains were isolated from 3 of 18 samples of miso and soy sauce. The killer spectra against the standard killer strains, K1 ~ K10, were different from those of other killer strains reported previously.
Journal of Fermentation and Bioengineering | 1996
Sayuki Nikkuni; Tika Bahadur Karki; Toshiro Terao; Chise Suzuki
Abstract A rice koji Mana sample obtained in Nepal contained 1.5 × 106 cfu/g of Mucorales, 3.5 × 107 cfu/g of aspergilli and 1.1 × 105 cfu/g of lactic acid bacteria, while yeast was present at less than 103 cfu/g. All of the isolates of Aspergillus (5 strains) had metulae with length of 7–15 μm and proved to be aflatoxin-negative. The sequence of partial 18S rRNA gene of the isolate was identical to those of Aspergillus oryzae and Aspergillus flavus. The isolates had the same restriction enzyme cleavage patterns of genomic DNA as that of A. oryzae. A. oryzae was found to be a dominant species in the Mana sample prepared with steamed rice.
Journal of Food Science and Technology-mysore | 1990
Noriyuki Okada; Takashi Akimoto; Sayuki Nikkuni; Masaru Manabe
Reversion of protoplasts to the bacillary form has been done either by plating in a soft agar layer onto the surface of hypertonic media or by spreading on the surface of hypertonic media. The spreading-plate method is constituted of two steps: all protoplasts are reversed first then fusants are detected on a minimal medium. A novel mixing-plate method, namely plating directly in hypertonic media, was proposed to make the fusion procedure much more simple than by above methods. In order to apply the mixing-plate method, protoplast reversion and fusant detection should be done by one step. Revised protoplast reversion media that could accomplish this were developed by reducing the tonicity of media less than half as customary. With those media, it was found that not only protoplast reversion could be done by the new method but also reversion frequency increased very high, perhaps because protoplasts were completely surrounded by a hypertonic reagent and a reversion stimulant as well as because the tonicity of media was reduced. It is concluded that the mixing-plate method is not only applicable but also superior to the ordinary methods. (Received Oct.4, 1989)
Journal of General and Applied Microbiology | 1998
Sayuki Nikkuni; Hirofumi Nakajima; Shin-ichi Hoshina; Masahiro Ohno; Chise Suzuki; Yutaka Kashiwagi; Katsumi Mori
Journal of Food Science and Technology-mysore | 1991
Noriyuki Okada; Sayuki Nikkuni; M. Manabe
Food Science and Technology International, Tokyo | 1995
Sayuki Nikkuni; Tika Karki; Kamaljit S. Vilkhu; Tadanao Suzuki; Kumiko Shindoh; Chise Suzuki; Noriyuki Okada
Agricultural and biological chemistry | 1989
Chise Suzuki; Sayuki Nikkuni
Journal of Food Science and Technology-mysore | 1992
Tateo Fujii; Sayuki Nikkuni; Haruka Iida
Journal of General and Applied Microbiology | 1996
Sayuki Nikkuni; Naoharu Kosaka; Chise Suzuki; Katsumi Mori