Biosynthetic Studies on a D-tryptophan-containing Lasso Peptide, MS-271 D-トリプトファン含有ラッソペプチド天然物MS-271の生合成研究

MS-271, originally isolated from Streptomyces sp. M-271 as a potent inhibitor of calmodulin-activated myosin light-chain kinase, is a lasso peptide natural productcomprising 21 amino acids (aa) with a D-tryptophan (Trp) at its C-terminus. Lasso peptides are peptide natural products that have a characteristic isopeptide-bonded slipknot structure. In terms of their biosynthesis, lasso peptides belong to a group of ribosomally synthesized and post-translationally modified peptides (RiPPs). Thus, the biosynthesis of MS-271, especially the mechanism of D-Trp introduction, is of great interest. In this study, I investigated the biosynthesis of MS-271.In chapter 2, I first identified MS-271 biosynthetic gene cluster (msl) spanning a ca. 11-kbp region from mslR1 to mslH by draft genome sequencing followed by searching for DNA region encoding the amino acid sequence of MS-271. Sequence analysis revealed that precursor peptide gene (mslA) encoded 42-residue peptide with a leader peptide at its N-terminus, and most importantly, the C-terminal core region contained all 21 amino acid residues of MS-271 including the C-terminal Trp. This suggested that the D-Trp residue is introduced via epimerization into a ribosomal peptide as a post translational modification. The cluster also contained genes for modification enzymes such as a macrolactam synthetase (mslC), precursor peptide recognition element (mslB1), cysteine protease (mslB2), disulfide oxidoreductases (mslE, mslF). Although obvious epimerase genes were absent in the cluster, the cluster encoded a protein of unknown function (mslH). Hence, I next carried out heterologous expression of the mslcluster in Streptomyces lividans. As the results, the production of MS-271 was confirmed by LC-MS and chiral amino acid analysis, indicating that the cluster contains all the necessary genes for MS-271 production including a novel peptide epimerase gene. I also showed that MslB1, B2, C and H were indispensable for MS-271 production by gene knockout experiments. Overall, these results suggested that MslH is responsible for the epimerization of the C-terminal Trp.In chapter 3, I characterized the function of MslH in vivo and in vitro. Considering that many modification enzymes involved in RiPP biosynthesis require leader peptides for their substrate recognition, I speculated that the epimerization occurs on the nascent full-length MslA in MS-271 biosynthesis. As expected, in vivo experiments revealed the formation of epi-MslA when mslA was expressed with mslH in Escherichia coli. Additional coexpression of precursor peptide recognition element (mslB1) enhanced the formation of MslA. Furthermore, in vitro experiments revealed that MslH catalyzed epimerization of C-terminal Trp of MslA in metal- and cofactor-independent mannerand that the leader peptide in MslA is indispensable for the substrate recognition by MslH. I also examine substrate specificity of MslH by heterologous expression of the msl cluster to produce MS-271 derivatives by altering the core peptide sequences of the mslA gene, and demonstrated that MslH exhibited broad substrate specificities toward the N-terminal region of core peptides while the C-terminal “CFW” sequence is important for substrate recognition. Overall, I fully characterized MslH as a novelpeptide epimerase. This is the first example epimerase that catalyzes epimerization at the C⍺ center adjacent to a carboxylic acid in a cofactor-independent manner. Taken together, I identified MslH, previously annotated as a hypothetical protein, as a novel epimerase involved in the post-translational epimerization of the C-terminal Trp residue of the precursor peptide MslA. I also demonstrated that MslH exhibitedbroad substrate specificity toward the N-terminal region of the core peptide, showing that MslH-type epimerases offer opportunities in peptide bioengineering.

(主査) 教授 松本 謙一郎, 特任教授 髙木 睦, 教授 大利 徹, 准教授 南 篤志, 准教授 小笠原 泰志

総合化学院（総合化学専攻）

MS-271, originally isolated from Streptomyces sp. M-271 as a potent inhibitor of calmodulin-activated myosin light-chain kinase, is a lasso peptide natural product comprising 21 amino acids (aa) with a D-tryptophan (Trp) at its C-terminus. Lasso peptides are peptide natural products that have a characteristic isopeptide-bonded slipknot structure. In terms of their biosynthesis, lasso peptides belong to a group of ribosomally synthesized and post-translationally modified peptides (RiPPs). Thus, the biosynthesis of MS-271, especially the mechanism of D-Trp introduction, is of great interest. In this study, I investigated the biosynthesis of MS-271. In chapter 2, I first identified MS-271 biosynthetic gene cluster (msl) spanning a ca. 11-kbp region from mslR1 to mslH by draft genome sequencing followed by searching for DNA region encoding the amino acid sequence of MS-271. Sequence analysis revealed that precursor peptide gene (mslA) encoded 42-residue peptide with a leader peptide at its N-terminus, and most importantly, the C-terminal core region contained all 21 amino acid residues of MS-271 including the C-terminal Trp. This suggested that the D-Trp residue is introduced via epimerization into a ribosomal peptide as a post translational modification. The cluster also contained genes for modification enzymes such as a macrolactam synthetase (mslC), precursor peptide recognition element (mslB1), cysteine protease (mslB2), disulfide oxidoreductases (mslE, mslF). Although obvious epimerase genes were absent in the cluster, the cluster encoded a protein of unknown function (mslH). Hence, I next carried out heterologous expression of the msl cluster in Streptomyces lividans. As the results, the production of MS-271 was confirmed by LC-MS and chiral amino acid analysis, indicating that the cluster contains all the necessary genes for MS-271 production including a novel peptide epimerase gene. I also showed that MslB1, B2, C and H were indispensable for MS-271 production by gene knockout experiments. Overall, these results suggested that MslH is responsible for the epimerization of the C-terminal Trp. In chapter 3, I characterized the function of MslH in vivo and in vitro. Considering that many modification enzymes involved in RiPP biosynthesis require leader peptides for their substrate recognition, I speculated that the epimerization occurs on the nascent full-length MslA in MS-271 biosynthesis. As expected, in vivo experiments revealed the formation of epi-MslA when mslA was expressed with mslH in Escherichia coli. Additional coexpression of precursor peptide recognition element (mslB1) enhanced the formation of MslA. Furthermore, in vitro experiments revealed that MslH catalyzed epimerization of C-terminal Trp of MslA in metal- and cofactor-independent manner and that the leader peptide in MslA is indispensable for the substrate recognition by MslH. I also examine substrate specificity of MslH by heterologous expression of the msl cluster to produce MS-271 derivatives by altering the core peptide sequences of the mslA gene, and demonstrated that MslH exhibited broad substrate specificities toward the N-terminal region of core peptides while the C-terminal “CFW” sequence is important for substrate recognition. Overall, I fully characterized MslH as a novel peptide epimerase. This is the first example epimerase that catalyzes epimerization at the C⍺ center adjacent to a carboxylic acid in a cofactor-independent manner. Taken together, I identified MslH, previously annotated as a hypothetical protein, as a novel epimerase involved in the post-translational epimerization of the C-terminal Trp residue of the precursor peptide MslA. I also demonstrated that MslH exhibited broad substrate specificity toward the N-terminal region of the core peptide, showing that MslH-type epimerases offer opportunities in peptide bioengineering.