Kouki Shioya

Evaluation of essential and variable residues of nukacin ISK‐1 by NNK scanning

Nukacin ISK‐1, a type‐A(II) lantibiotic, comprises 27 amino acids with a distinct linear N‐terminal and a globular C‐terminal region. To identify the positional importance or redundancy of individual residues responsible for nukacin ISK‐1 antimicrobial activity, we replaced the native codons of the parent peptide with NNK triplet oligonucleotides in order to generate a bank of nukacin ISK‐1 variants. The bioactivity of each peptide variant was evaluated by colony overlay assay, and hence we identified three Lys residues (Lys1, Lys2 and Lys3) that provided electrostatic interactions with the target membrane and were significantly variable. The ring structure of nukacin ISK‐1 was found to be crucially important as replacing the ring‐forming residues caused a complete loss of bioactivity. In addition to the ring‐forming residues, Gly5, His12, Asp13, Met16, Asn17 and Gln20 residues were found to be essential for antimicrobial activity; Val6, Ile7, Val10, Phe19, Phe21, Val22, Phe23 and Thr24 were relatively variable; and Ser4, Pro8, His15 and Ser27 were extensively variable relative to their positions. We obtained two variants, Asp13Glu and Val22Ile, which exhibited a twofold higher specific activity compared with the wild‐type and are the first reported type‐A(II) lantibiotic mutant peptides with increased potency.

A Novel Type of Immunity Protein, NukH, for the Lantibiotic Nukacin ISK-1 Produced by Staphylococcus warneri ISK-1

Staphylococcus warneri ISK-1 produces a lantibiotic, nukacin ISK-1. The nukacin ISK-1 gene cluster consists of at least six genes, nukA, -M, -T, -F, -E, and -G, and two open reading frames, ORF1 and ORF7 (designated nukH). Sequence comparisons suggested that NukF, -E, -G, and -H contribute to immunity to nukacin ISK-1. We investigated the immunity levels of recombinant Lactococcus lactis expressing nukFEG and nukH against nukacin ISK-1. The co-expression of nukFEG and nukH resulted in a high degree of immunity. The expression of either nukFEG or nukH conferred partial immunity against nukacin ISK-1. These results suggest that NukH contributes cooperatively to self-protection with NukFEG. The nukacin ISK-1 immunity system might function against another lantibiotic, lacticin 481. Western blot analysis showed that NukH expressed in Staphylococcus carnosus was localized in the membrane. Peptide release/bind assays indicated that the recombinant L. lactis expressing nukH interacted with nukacin ISK-1 and lacticin 481 but not with nisin A. These findings suggest that NukH contributes cooperatively to host immunity as a novel type of lantibiotic-binding immunity protein with NukFEG.

Localization and Interaction of the Biosynthetic Proteins for the Lantibiotic, Nukacin ISK-1

Nukacin ISK-1 is a type-A(II) lantibiotic produced by Staphylococcus warneri ISK-1. In this study, we characterized NukM and NukT, which are predicted to be involved in modification of prepeptide (NukA) and cleavage of leader peptide and subsequent secretion respectively. Localization analysis of NukM and NukT in the wild-type strain indicated that both proteins were located at the cytoplasm membrane. Interestingly, NukM expressed heterologously in St. carnosus TM300 was also located at the cytoplasm membrane even in the absence of NukT. Yeast two-hybrid assay showed that a complex of at least two each of NukM and NukT was associated with NukA. In vitro interaction analysis by surface plasmon resonance biosensor further suggested that membrane-located NukM interacted with NukA. These results indicate that NukM and NukT form a membrane-located multimeric protein complex and that post-translational modification of nukacin ISK-1 would occur at the cytoplasm membrane.

Lantibiotic Engineering: Molecular Characterization and Exploitation of Lantibiotic-Synthesizing Enzymes for Peptide Engineering

Lanthionine-containing peptide antibiotics called lantibiotics are produced by a large number of Gram-positive bacteria. Nukacin ISK-1 produced by Staphylococcus warneri ISK-1 is type-A(II) lantibiotic. Ribosomally synthesized nukacin ISK-1 prepeptide (NukA) consists of an N-terminal leader peptide followed by a C-terminal propeptide moiety that undergoes several post-translational modification events including unusual amino acid formation by the modification enzyme NukM, cleavage of leader peptide and export by the dual functional ABC transporter NukT, finally yielding a biologically active peptide. Unusual amino acids in lantibiotics contribute to biological activity and also structural stability against proteases. Thus, lantibiotic-synthesizing enzymes have a high potentiality for peptide engineering by introduction of unusual amino acids into desired peptides with altering biological and physicochemical properties, e.g., activity and stability, termed lantibiotic engineering. We report the establishment of a heterologous expression of nukacin ISK-1 biosynthetic gene cluster by the nisin-controlled expression system and discuss our recent progress in understanding of the biosynthetic enzymes for nukacin ISK-1 such as localization, molecular interaction in biophysical and biochemical aspects. Substrate specificity of the lantibiotic-synthesizing enzymes was evaluated by complementation of the biosynthetic enzymes (LctM and LctT) of closely related lantibiotic lacticin 481 for nukacin ISK-1 biosynthesis. We further explored a rapid and powerful tool for introduction of unusual amino acids by co-expression of hexa-histidine-tagged NukA and NukM in Escherichia coli.

ATP-dependent leader peptide cleavage by NukT, a bifunctional ABC transporter, during lantibiotic biosynthesis

NukT, a possible ABC transporter maturation and secretion (AMS) protein, may contribute to the cleavage of the leader peptide of NukA, which is the prepeptide of the lantibiotic nukacin ISK-1, and to nukacin ISK-1 transport. In this study, we reconstituted in vitro peptidase activity of the full-length NukT overexpressed in inside-out membrane vesicles of Staphylococcus carnosus TM300. We found that the presence of unusual amino acids in NukA is required for leader peptide cleavage. Furthermore, NukT peptidase activity was inhibited by phenylmethylsulfonyl fluoride, a serine/cysteine protease inhibitor; this finding strongly suggests that NukT, like other AMS proteins, is a cysteine protease. Interestingly, NukT peptidase activity depended on ATP hydrolysis. These results suggest that the N-terminal peptidase domain of NukT may cooperatively function with the C-terminal ATP-binding domain. This is the first in vitro study on lantibiotics that reports the processing mechanism of a full-length bifunctional ABC transporter.

Characterization of modification enzyme NukM and engineering of a novel thioether bridge in lantibiotic nukacin ISK-1

The lantibiotic nukacin ISK-1 is an antimicrobial peptide containing unusual amino acids such as lanthionine and dehydrobutyrine. The nukacin ISK-1 prepeptide (NukA) undergoes posttranslational modifications, such as the dehydration and cyclization reactions required to form the unusual amino acids by the modification enzyme NukM. We have previously constructed a system for the introduction of unusual amino acids into NukA by coexpression of NukM in Escherichia coli. Using this system, we describe the substrate specificity of NukM by the coexpression of a series of NukA mutants. Our results revealed the following characteristics of NukM: (1) its dehydration activity is not coupled to its cyclization activity; (2) its dehydration activity is site-specific; (3) the length of the substrate is important for its dehydration activity. Furthermore, we succeeded in introducing a novel thioether bridge in NukA by replacing an unmodified Ser at position 27 with a Cys residue.

Effect of a Negatively Charged Lipid on Membrane-Lacticin Q Interaction and Resulting Pore Formation

Lacticin Q is an antimicrobial peptide that forms pores on membranes. We investigated effects of negatively charged lipids on the binding and pore formation of lacticin Q with liposomes by surface plasmon resonance analysis and fluorescence dye leakage experiments respectively. Negatively charged lipids accelerated the binding of lacticin Q on the membranes and the resulting pore formation. However, the acceleration was not an essential factor in the killing activity of lacticin Q, since pore-forming activities against electrically neutral and negatively charged liposomes occurred similarly.

Binding Specificity of the Lantibiotic-Binding Immunity Protein NukH

ABSTRACT NukH is a lantibiotic-binding immunity protein that shows strong binding activity against type A(II) lantibiotics. In this study, the binding specificity of NukH was analyzed by using derivatives of nukacin ISK-1, which is a type A(II) lantibiotic produced by Staphylococcus warneri ISK-1. Interactions between cells of Lactococcus lactis transformants expressing nukH and nukacin ISK-1 derivatives were analyzed by using a quantitative peptide-binding assay. Differences in the cell-binding rates of each nukacin ISK-1 derivative suggested that three lysine residues at positions 1 to 3 of nukacin ISK-1 contribute to the effective binding of nukacin ISK-1 to nukH-expressing cells. The binding levels of mutants with lanthionine and dehydrobutyrine substitutions (S11A nukacin4-27 and T24A nukacin4-27, respectively) to nukH-expressing cells were considerably lower than those of nukacin4-27, suggesting that unusual amino acids play a decisive role in NukH recognition. Additionally, it was suggested that T9A nukacin4-27, a mutant with a 3-methyllanthionine substitution, binds to NukH via an intermolecular disulfide bond after it is weakly recognized by NukH. We succeeded in the detection of specific type A(II) lantibiotics from the culture supernatants of various bacteriocin producers by using the binding specificity of nukH-expressing cells.

Engineering unusual amino acids into peptides using lantibiotic synthetase.

Alteration of protein structure and function by introducing unusual amino acids has great potential to develop new biological tool and to produce novel therapeutic agents. Lantibiotics produced by Gram-positive bacteria are ribosomally synthesized and post-translationally modified antimicrobial peptides. The modification enzyme involved in lantibiotic biosynthesis can catalyze the formation of unusual amino acids in the nascent lantibiotic prepeptide. Here, a novel methodology on the lantibiotic nukacin ISK-1 is described for engineering unusual amino acid residues into hexa-histidine-tagged (His-tagged) prepeptide NukA by the modification enzyme NukM in Escherichia coli. Co-expression of His-tagged NukA and NukM, purification of the resulting His-tagged prepeptide by affinity chromatography, and subsequent mass spectrometry analysis show that the prepeptide is converted into a postulated peptide with decrease in mass which results from the formation of unusual amino acids such as dehydrated amino acid, lanthionine, or 3-methyl lanthionine at the expected positions. The modified prepeptide can be readily obtained by one-step purification. This strategy will thus be a simple and powerful tool for introducing unusual amino acid residues aimed at peptide engineering.

In vitro catalytic activity of N-terminal and C-terminal domains in NukM, the post-translational modification enzyme of nukacin ISK-1

Lantibiotics are antibacterial peptides containing unique thioether cross-links termed lanthionine and methyllanthionine. NukM, the modifying enzyme of nukacin ISK-1, which is produced by Staphylococcus warneri ISK-1, catalyzes the dehydration of specific Ser/Thr residues in a precursor peptide, followed by conjugative addition of intramolecular Cys to dehydrated residues to generate a cyclic structure. By contrast, the precursor peptide of nisin is modified by 2 enzymes, NisB and NisC, which mediate dehydration and cyclization, respectively. While the C-terminal domain of NukM is homologous to NisC, the N-terminal domain has no homology with other known proteins. We expressed and characterized the N- and C-terminal domains of NukM, NukMN, and NukMC, separately. In vitro reconstitution revealed that full-length NukM fully modified the substrate peptide NukA. NukMN partially phosphorylated, dehydrated, and cyclized NukA. By contrast, NukMC did not catalyze dehydration, phosphorylation, or cyclization reactions. Interaction studies using surface plasmon resonance analysis indicated that NukM and NukMN can bind NukA with high affinity, whereas NukMC has low substrate-recognition activity. These results suggest that NukMN is mainly responsible for substrate recognition and dehydration and that the whole NukM structure, including the C-terminal domain, is required for the complete modification of NukA. To the best of our knowledge, this is the first report providing insights into the in vitro catalytic activity of individual domains of a LanM-type modification enzyme.