Model studies of lantibiotic biogenesis

The Minimum Protein Staple? - Towards 'bio'-Baldwin's rules via inter-phosphosite linking in the MEK1 activation loop

In small molecule organic chemistry, the heuristic insight into ring-forming processes that was enabled by Baldwin's rules some 50 years ago proved a step-change in the role of mechanistically guided synthesis. It created a lens upon and marker of fundamental stereoelectronic and conformation-guided chemical processes. However, despite the widespread role of stereoelectronics and conformational control in Biology, no equivalent coherent exploitation of trapped, ring-forming processes yet exists in biomolecules. In the development of a minimal ring-closing process in intact proteins that might prove suitable in a coherent rule-set, we have tested endo-trig ring-closing conjugate thioether lanthionine (Lan) -CH2-S-CH2- formation as a limiting cyclization. Spontaneous Lan formation in proteins is rare if not non-existent and when found in natural product cyclic peptides it requires the mediation of corresponding biosynthetic enzymes as well as productive reactive conformations to guide it. Here, we show that within a conformationally flexible and functionally important protein loop - the MAPK kinase phosphorylation-targeted activation loop - Lan ring-closing is possible. Ring-closing proves to be critically dependent on the location of a trig electrophilic site in just one of two regioisomeric potential precursors to allow phosphosite-to-phosphosite 'stapling'. This first example of spontaneous protein thioether ring-closing/'stapling' and its accessibility from just one precursor (despite the potential for both to form an identical 'staple') now reveals the potential for Lan formation not only as an accessible form of minimal stapling in proteins but also as an exquisitely sensitive probe of associated protein geometries. We suggest that the use of this (as well as the development of other such, intramolecular protein traps that are dependent on inherent protein-controlled reactivity rather than forced crosslinking) may allow the broader trapping and mapping of relevant, even minor, protein states. In this way, protein ring formation may enable a form of extended 'bio-Baldwin's rules' that help to delineate relevant protein conformational space.

Class V Lanthipeptide Cyclase Directs the Biosynthesis of a Stapled Peptide Natural Product

Lanthipeptides are a class of cyclic peptides characterized by the presence of one or more lanthionine (Lan) or methyllanthionine (MeLan) thioether rings. These cross-links are produced by α,β-unsaturation of Ser or Thr residues in peptide substrates by dehydration, followed by a Michael-type conjugate addition of Cys residues onto the dehydroamino acids. Lanthipeptides may be broadly classified into at least five different classes, and the biosynthesis of classes I-IV lanthipeptides requires catalysis by LanC cyclases that control both the site-specificity and the stereochemistry of the conjugate addition. In contrast, there are no current examples of LanCs that occur in class V biosynthetic clusters, despite the presence of lanthionine rings in these compounds. In this work, bioinformatics-guided co-occurrence analysis identifies more than 240 putative class V lanthipeptide clusters that contain a LanC cyclase. Reconstitution studies demonstrate that the cyclase-catalyzed product is notably distinct from the product formed spontaneously. Stereochemical analysis shows that the cyclase diverts the final product to a configuration that is distinct from one that is energetically favored. Structural characterization of the final product by multi-dimensional NMR spectroscopy reveals that it forms a helical stapled peptide. Mutational analysis identified a plausible order for cyclization and suggests that enzymatic rerouting to the final structure is largely directed by the construction of the first lanthionine ring. These studies show that lanthipeptide cyclases are needed for the biosynthesis of some constrained peptides, the formations of which would otherwise be energetically unfavored.

Complete synthesis of the bicyclic ring of a mutacin analog with orthogonally protected lanthionine via solid-phase intracyclization

Mutacin 1140 (MU1140) is a naturally occurring lantibiotic derived from posttranslational modifications of a ribosomally synthesized peptide during the fermentation of a bacterium called Streptococcus mutans, the etiological agent of dental cavities. A practical approach for chemically synthesizing lantibiotics would be a valuable tool to expand the MU1140 library with additional semisynthetic analogs. In turn, an expanded library may prove useful to explore additional therapeutic indications for this pipeline of novel compounds. In this work, orthogonally protected lanthionine analogs were synthesized via an aziridine ring opening strategy. This lanthionine was utilized to synthesize a cysteamine (Cya) instead of the (S)-aminovinyl-D-cysteine (AviCys) that is naturally found in MU1140. The Cya containing bicyclic C/D ring of MU1140 was synthesized by Fmoc solid-phase peptide synthesis (SPPS). The linear peptides were synthesized using OPfp ester derivatives and using various common coupling reagents such as COMU and TCTU. The linear peptide was intracyclized with DEPBT to construct the so-called bicyclic ring C/D. This is the first report on the complete chemical synthesis of the bicyclic C/D ring of a MU1140 analog using orthogonally protected lanthionines using SPPS.

A Chemical Biology Approach to Understanding Molecular Recognition of Lipid II by Nisin(1-12): Synthesis and NMR Ensemble Analysis of Nisin(1-12) and Analogues

Natural products that target lipid II, such as the lantibiotic nisin, are strategically important in the development of new antibacterial agents to combat the rise of antimicrobial resistance. Understanding the structural factors that govern the highly selective molecular recognition of lipid II by the N-terminal region of nisin, nisin(1-12), is a crucial step in exploiting the potential of such compounds. In order to elucidate the relationships between amino acid sequence and conformation of this bicyclic peptide fragment, we have used solid-phase peptide synthesis to prepare two novel analogues of nisin(1-12) in which the dehydro residues have been replaced. We have carried out an NMR ensemble analysis of one of these analogues and of the wild-type nisin(1-12) peptide in order to compare the conformations of these two bicyclic peptides. Our analysis has shown the effects of residue mutation on ring conformation. We have also demonstrated that the individual rings of nisin(1-12) are pre-organised to an extent for binding to the pyrophosphate group of lipid II, with a high degree of flexibility exhibited in the central amide bond joining the two rings.

Molecular Recognition of Lipid II by Lantibiotics: Synthesis and Conformational Studies of Analogues of Nisin and Mutacin Rings A and B

In response to the growing threat posed by antibiotic-resistant bacterial strains, extensive research is currently focused on developing antimicrobial agents that target lipid II, a vital precursor in the biosynthesis of bacterial cell walls. The lantibiotic nisin and related peptides display unique and highly selective binding to lipid II. A key feature of the nisin-lipid II interaction is the formation of a cage-like complex between the pyrophosphate moiety of lipid II and the two thioether-bridged rings, rings A and B, at the N-terminus of nisin. To understand the important structural factors underlying this highly selective molecular recognition, we have used solid-phase peptide synthesis to prepare individual ring A and B structures from nisin, the related lantibiotic mutacin, and synthetic analogues. Through NMR studies of these rings, we have demonstrated that ring A is preorganized to adopt the correct conformation for binding lipid II in solution and that individual amino acid substitutions in ring A have little effect on the conformation. We have also analyzed the turn structures adopted by these thioether-bridged peptides and show that they do not adopt the tight α-turn or β-turn structures typically found in proteins.

De Novo Design To Synthesize Lanthipeptides Involving Cascade Cysteine Reactions: SapB Synthesis as an Example

Lanthipeptides are a family of ribosomally synthesized peptides that have crucial biological functions. However, due to their complicated structures, the total synthesis of lanthipeptides is challenging. Here, a novel strategy to construct lanthipeptides is described, which involves cascade reactions of cysteine, including Cys disalkylation elimination, Michael reaction, and native chemical ligation. We utilized this strategy to synthesize lanthipeptide SapB as an example. This methodology has the potential to obtain lanthipeptides and their analogues for biological research and drug discovery.

Mechanistic Understanding of Lanthipeptide Biosynthetic Enzymes

Lanthipeptides are ribosomally synthesized and post-translationally modified peptides (RiPPs) that display a wide variety of biological activities, from antimicrobial to antiallodynic. Lanthipeptides that display antimicrobial activity are called lantibiotics. The post-translational modification reactions of lanthipeptides include dehydration of Ser and Thr residues to dehydroalanine and dehydrobutyrine, a transformation that is carried out in three unique ways in different classes of lanthipeptides. In a cyclization process, Cys residues then attack the dehydrated residues to generate the lanthionine and methyllanthionine thioether cross-linked amino acids from which lanthipeptides derive their name. The resulting polycyclic peptides have constrained conformations that confer their biological activities. After installation of the characteristic thioether cross-links, tailoring enzymes introduce additional post-translational modifications that are unique to each lanthipeptide and that fine-tune their activities and/or stability. This review focuses on studies published over the past decade that have provided much insight into the mechanisms of the enzymes that carry out the post-translational modifications.

Linker-free incorporation of carbohydrates into in vitro displayed macrocyclic peptides

We report a strategy for efficient post-translational modification of a library of ribosomally-translated peptides by activation and elimination of cysteine to dehydroalanine then conjugate addition of a range of exogenous thiols, with an emphasis on carbohydrates.

We report a strategy for efficient post-translational modification of a library of ribosomally-translated peptides by activation and elimination of cysteine to dehydroalanine then conjugate addition of a range of exogenous thiols, with an emphasis on carbohydrates. These reactions are selective for cysteine, and do not interfere with amplification of the nucleic acid component of an mRNA-displayed peptide. Furthermore, these reactions are shown to be compatible with two different macrocyclisation chemistries, and when applied to a peptide containing an N-terminal cysteine give a ketone that can be functionalised in an orthogonal manner. This new strategy can overcome a limitation of ribosomal translation, providing a means to incorporate untranslatable groups such as carbohydrates in amino acid side chains, and will allow for the ribosomal generation of glycopeptides, requiring only the introduction of a free thiol in the molecule to be incorporated. In combination with in vitro selection techniques, this strategy is envisaged to allow the discovery of biologically-active glycopeptides with a near-natural, but hydrolytically stable, thioglycosidic bond.

Insights into the evolution of lanthipeptide biosynthesis

Lanthipeptides are a group of posttranslationally modified peptide natural products that contain multiple thioether crosslinks. These crosslinks are formed by dehydration of Ser/Thr residues followed by addition of the thiols of Cys residues to the resulting dehydroamino acids. At least four different pathways to these polycyclic natural products have evolved, reflecting the high efficiency and evolvability of a posttranslational modification route to generate conformationally constrained peptides. The wealth of genomic information that has been made available in recent years has started to provide insights into how these remarkable pathways and their posttranslational modification machineries may have evolved. In this review, we discuss a model for the evolution of the lanthipeptide biosynthetic enzymes that has recently been developed based on the currently available data.

Ribosomally synthesized and post-translationally modified peptide natural products: new insights into the role of leader and core peptides during biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are a major class of natural products with a high degree of structural diversity and a wide variety of bioactivities. Understanding the biosynthetic machinery of these RiPPs will benefit the discovery and development of new molecules with potential pharmaceutical applications. In this Concept article, we discuss the features of the biosynthetic pathways to different RiPP classes, and propose mechanisms regarding recognition of the precursor peptide by the post-translational modification enzymes. We propose that the leader peptides function as allosteric regulators that bind the active form of the biosynthetic enzymes in a conformational selection process. We also speculate how enzymes that generate polycyclic products of defined topologies may have been selected for during evolution.

Structural characterization of four prochlorosins: a novel class of lantipeptides produced by planktonic marine cyanobacteria

Prochlorosins make up a class of secondary metabolites produced by strains of Prochlorococcus, single-cell, planktonic marine cyanobacteria. These polycyclic peptides contain lanthionine and methyllanthionine residues that result in thioether cross-links. In Prochlorococcus MIT9313, a single enzyme, ProcM, catalyzes the posttranslational modification of 29 linear peptide substrates to generate a library of highly diverse cyclic peptides. To investigate the catalytic promiscuity of ProcM, we chose four prochlorosins previously demonstrated to be produced by the organism for detailed structural characterization. Nuclear magnetic resonance studies allowed unambiguous assignment of the ring topologies, demonstrating a high degree of topological diversity. The stereochemistry of the lanthionine and methyllanthionine residues was determined by gas chromatography and mass spectrometry for seven prochlorosins. All methyllanthionines had the (2S,3S,6R) configuration, and the lanthionines had the (2S,6R) configuration, irrespective of the direction of cyclization, ring size, or ring topology. These findings indicate that most, if not all, of the rings in prochlorosins are formed enzymatically by ProcM lanthionine synthetase and not by a nonenzymatic process as previously suggested.

In vitro selection of functional lantipeptides

In this report we present a method to identify functional artificial lantipeptides. In vitro translation coupled with an enzyme-free protocol for posttranslational modification allows preparation of more than 10(11) different lanthionine containing peptides. This diversity can be searched for functional molecules using mRNA-lantipeptide display. We validated this approach by isolating binders toward Sortase A, a transamidase which is required for virulence of Staphylococcus aureus. The interaction of selected lantipeptides with Sortase A is highly dependent on the presence of a (2S,6R)-lanthionine in the peptide and an active conformation of the protein.

The challenge of the lantibiotics: synthetic approaches to thioether-bridged peptides

The unique antibacterial properties and structural complexity of the lantibiotics has stimulated considerable interest in the development of methodology to synthesise these peptides. One of the most challenging issues has been the synthesis of polycyclic peptides with multiple thioether bridges between side-chains, which are a characteristic feature of the lantibiotics. In this perspective, the different approaches to this problem, including solution-phase synthesis, solid-phase synthesis, biomimetic approaches and biotransformation strategies, are reviewed, highlighting the advances resulting from each of these approaches.

Nine post-translational modifications during the biosynthesis of cinnamycin

Lantibiotics are ribosomally synthesized and post-translationally modified antimicrobial peptides that are characterized by the thioether cross-linked amino acids lanthionine (Lan) and methyllanthionine (MeLan). Cinnamycin is a 19 amino acid lantibiotic that contains one Lan and two MeLan. Cinnamycin also contains an unusual lysinoalanine (Lal) bridge formed from the ε-amino group of lysine 19 and a serine residue at position 6, and an erythro-3-hydroxy-L-aspartic acid resulting from the hydroxylation of L-aspartate at position 15. These modifications are critical in mediating the interactions of cinnamycin with its target, phosphatidylethanolamine. Recently, the cinnamycin biosynthetic gene cluster (cin) from Streptomyces cinnamoneus cinnamoneus DSM 40005 was reported. Herein, we investigated the biosynthetic machinery using both in vitro studies and heterologous expression in Escherichia coli. CinX is an α-ketoglutarate/iron(II)-dependent hydroxylase that carries out the hydroxylation of aspartate 15 of the precursor peptide CinA. In addition, CinM catalyzes dehydration of four Ser and Thr residues and subsequent cyclization of Cys residues to form the three (Me)Lan bridges. The order of the post-translational modifications catalyzed by CinM and CinX is interchangeable in vitro. CinX did not require the leader sequence at the N-terminus of CinA for activity, but the leader peptide was necessary for CinM function. Although CinM dehydrated serine 6, it did not catalyze the formation of Lal. A small protein encoded by cinorf7 is critical for the formation of the cross-link between Lys19 and dehydroalanine 6 as shown by coexpression studies of CinA, CinM, CinX, and Cinorf7 in E. coli.

Synthesis of peptides containing overlapping lanthionine bridges on the solid phase: an analogue of rings D and E of the lantibiotic nisin

A methodology for the solid-phase synthesis of the overlapping lanthionine bridges found in many lantibiotics has been developed. A novel Teoc/TMSE-protected lanthionine derivative has been synthesized, and this lanthionine, and an Aloc/allyl-protected lanthionine derivative, have been incorporated into a linear peptide using solid-phase peptide synthesis. Selective deprotection of the silyl protecting groups, followed by sequential cyclization, deprotection of the allyl protecting groups, and further cyclization, enabled the regioselective formation of an analogue of rings D and E of nisin.

Ribosomal peptide natural products: bridging the ribosomal and nonribosomal worlds

Ribosomally synthesized bacterial natural products rival the nonribosomal peptides in their structural and functional diversity. The last decade has seen substantial progress in the identification and characterization of biosynthetic pathways leading to ribosomal peptide natural products with new and unusual structural motifs. In some of these cases, the motifs are similar to those found in nonribosomal peptides, and many are constructed by convergent or even paralogous enzymes. Here, we summarize the major structural and biosynthetic categories of ribosomally synthesized bacterial natural products and, where applicable, compare them to their homologs from nonribosomal biosynthesis.

Use of lantibiotic synthetases for the preparation of bioactive constrained peptides

Stabilization of biologically active peptides is a major goal in peptide-based drug design. Cyclization is an often-used strategy to enhance resistance of peptides toward protease degradation and simultaneously improve their affinity for targets by restricting their conformational flexibility. Among the various cyclization strategies, the use of thioether crosslinks has been successful for various peptides including enkephalin. The synthesis of these thioethers can be arduous, especially for longer peptides. Described herein is an enzymatic strategy taking advantage of the lantibiotic synthetase LctM that dehydrates Ser and Thr residues to the corresponding dehydroalanine and dehydrobutyrine residues and catalyzes the Michael-type addition of Cys residues to form thioether crosslinks. The use of LctM to prepare thioether containing analogs of enkephalin, contryphan, and inhibitors of human tripeptidyl peptidase II and spider venom epimerase is demonstrated.

Synthesis of bicyclic alkene-/alkane-bridged nisin mimics by ring-closing metathesis and their biochemical evaluation as lipid II binders: toward the design of potential novel antibiotics

This report describes the design, synthesis, and biochemical evaluation of alkene- and alkane-bridged AB(C)-ring mimics of the lantibiotic nisin. Nisin belongs to a class of natural antimicrobial peptides, and has a unique mode of action: its AB(C)-ring system binds to the pyrophosphate moiety of lipid II. This mode of action was the rationale for the design of smaller nisin-derived peptides to obtain novel potential antibiotics. As a conformational constraint the thioether bridge was mimicked by an alkene- or alkane isostere. The peptides of the linear individual ring precursors were synthesized on solid support or in solution, and cyclized by ring-closing metathesis in solution with overall yields of between 36 and 89 %. The individual alkene-bridged macrocycles were assembled in solution by using carbodiimide-based synthesis protocols for the corresponding AB(C)-ring mimics. These compounds were tested for their binding affinity toward lipid II by evaluation of their potency to inhibit nisin-induced carboxyfluorescein release from large unilamellar vesicles. It was found that these AB(C)-ring mimics were not able to induce membrane leakage; however, they acted by inhibiting nisin-induced carboxyfluorescein release; this indicates their affinity toward lipid II. These results imply that an alkene or alkane moiety is a suitable thioether bridge mimic.

Step-wise and pre-organization induced synthesis of a crossed alkene-bridged nisin Z DE-ring mimic by ring-closing metathesis

This paper describes two approaches for the synthesis of a crossed alkene-bridged mimic of the thioether ring system of the nisin Z DE-fragment. The first approach comprised the stepwise total synthesis featuring a cross metathesis and a macrolactamization on a solid support followed by a ring-closing metathesis in solution. Via this route the title compound was obtained in an overall yield of 7% (85% on average for 16 reaction steps). In the second approach, the linear precursor peptide was subjected to ring-closing metathesis and the bicyclic peptide with the correct side chain connectivity pattern was obtained in yields up to 95%. The preferred formation of the bicyclic crossed alkene-bridged mimic of the DE-ring suggests a favorable pre-organization of the linear precursor peptide.

Orthogonally protected lanthionines: synthesis and use for the solid-phase synthesis of an analogue of nisin ring C

[structure: see text] Lanthionine, a thioether analogue of cystine, is a key component of the lantibiotics, a family of modified peptides bearing multiple thioether bridges resulting from posttranslational modifications between side chains. It is also used as a conformational constraint in medicinally active peptides. We have explored two synthetic routes to give lanthionine, orthogonally protected with Alloc/allyl and Fmoc groups. One route utilized a carbamate-protected iodoalanine that yielded a mixture of diastereoisomers, and one utilized a trityl-protected iodoalanine, formed via a Mitsunobu reaction, that gave the single desired lanthionine, in complete regio- and diastereoselectivity. We then used this orthogonally protected lanthionine in the solid-phase synthesis of an analogue of a fragment of nisin containing its ring C. The chemoselective deprotection of the allyl and Alloc groups of the incorporated lanthionine unit was followed by regio- and stereoselective cyclization on resin to give the desired lanthionine-bridged peptide.

Understanding base-assisted desulfurization using a variety of disulfide-bridged peptides

A recently rediscovered reaction of base-assisted lanthionine formation has been applied to several systems of disulfide-bridged peptides. In addition to previously described nonapeptides consisting of i, i+3 cystine linkages, the reaction has now been extended to systems consisting of shorter (i, i+2) and longer (i, i+4) disulfide bridges. The desulfurization reaction is also compatible with disulfide bridges formed through homocysteines and penicillamines, yielding unusual amino acids such as cystathionine and beta,beta-dimethyl lanthionine (referred to as "penthionine") in a peptide chain, respectively. Systematic study of this transformation has provided several new insights into its mechanism. We have observed formation of dehydroalanine and dehydrovaline residues resulting from i, i+2-bridged cysteines and i, i+3-bridged cysteine/penicillamine peptides, respectively, thereby supporting a beta-elimination/Michael-addition mechanism for this transformation. Amino acid analysis and NMR data from total correlation spectroscopy (TOCSY) and (1)H-(13)C heteronuclear single quantum correlation (HSQC) experiments show three diastereomeric lanthionine-bridged peptides in the product mixture. But in the case of desulfurization of a cysteine/homocysteine containing disulfide-bridged peptide, Michael addition appears to be stereoselective, yielding a single stereoisomer of cystathionine within the peptide. According to molecular modeling and CD spectroscopy, constrained peptides such as those containing penicillamine are less likely to undergo facile desulfurization. Flexibility of the torsional angles (C(alpha)H-C(alpha)-C(beta)-S) corresponding to the cysteine residues and temperature appear to be contributing factors determining the extent of desulfurization.

Biomimetic stereoselective formation of methyllanthionine

Fmoc-(2R,3S)-3-methyl-Se-phenylselenocysteine was used for the synthesis of dehydrobutyrine (Dhb)-containing peptides. Biomimetic cyclization via Michael addition of Cys to a Dhb yielded the B-ring of the lantibiotic subtilin as a single diastereomer. The methyllanthionine product was shown to have the natural configuration by preparation of the authentic stereoisomer. The formation of a single isomer suggests that the prepeptide has a strong intrinsic preference for the stereochemistry observed in lantibiotics. [reaction: see text]

Facile chemoselective synthesis of dehydroalanine-containing peptides

Useful methodology is described for the synthesis of dehydroalanine residues (II) within peptides. The unnatural amino acid (Se)-phenylselenocysteine (I) can be incorporated into growing peptide chains via standard peptide synthesis procedures. Subsequent oxidative elimination affords a dehydroalanine at the desired position. The oxidation conditions are mild and tolerate functionalities commonly found in peptides, including variously protected cysteine residues. To illustrate its utility, cyclic lanthionines have been synthesized by this method.

Expression, purification, and characterization of EpiC, an enzyme involved in the biosynthesis of the lantibiotic epidermin, and sequence analysis of Staphylococcus epidermidis epiC mutants

The plasmid-encoded epidermin biosynthetic gene epiC of Staphylococcus epidermidis Tü3298 was expressed in Escherichia coli by using the T7 RNA polymerase-promoter system, and the gene product EpiC was identified by Western blotting (immunoblotting) with an anti-EpiC-peptide antiserum. EpiC was a hydrophobic but soluble protein. EpiC was purified by hydrophobic-interaction chromatography. The determined amino-terminal amino acid sequence was M I N I N N I .... The electrophoretic migration behavior of EpiC depended on the oxidation state of the enzyme, indicating the formation of an intramolecular disulfide bridge between C-274 and C-321. The cysteine residues in the motifs WC-274YG and C-321HG of EpiC are conserved in all lantibiotic enzymes of the C type (so-called LanC proteins) and in the CylM protein. Mutated epiC genes from S. epidermidis epiC mutants were cloned and expressed in E. coli. Sequence analysis revealed that the mutations occurred in the two motifs -S-X-X-X-G-X-X-G- and -N-X-G-X-A-H-G-X-X-G-, which are conserved in all LanC proteins. For the investigation of EpiC-EpiA interactions, precursor peptide EpiA was coupled to N-hydroxysuccinimide-activated Sepharose High Performance Material (HiTrap). Under reducing conditions, EpiC was retarded on the EpiA-HiTrap column. In the incubation experiments, EpiC did not react with EpiA, with proepidermin, or with oxidative decarboxylated peptides.

Post-translational modifications of lantibiotics

Several newly reported post-translational modification reactions are involved in lantibiotic biosynthesis. A short overview of the present knowledge on the post-translational modifications and on the enzymes involved in lantibiotic biosynthesis is given. The oxidative decarboxylation of the epidermin precursor peptide EpiA is described in detail. The FMN-containing oxidoreductase EpiD is involved in the formation of the C-terminal S-[(Z)-2-aminovinyl]-D-cysteine residue of epidermin: under reducing conditions the side chain of the C-terminal cysteine residue of EpiA is converted to an enethiol. EpiD has no absolute substrate specificity and can be used for modification of peptides having the C-terminal consensus motif [V/I/L/(M)/F/Y/W]-[A/S/V/T/C/(I/L)]-C.

Isolation and characterization of genetically engineered gallidermin and epidermin analogs

Gallidermin (Gdm) and epidermin (Epi) are highly homologous tetracyclic polypeptide antibiotics that are ribosomally synthesized by a Staphylococcus gallinarum strain and a Staphylococcus epidermidis strain, respectively. These antibiotics are secreted into media and are distinguished by the presence of the unusual amino acids lanthionine, 3-methyllanthionine, didehydrobutyrine, and S-(2-aminovinyl)-D-cysteine, which are formed by posttranslational modification. To study the substrate specificities of the modifying enzymes and to obtain variants that exhibit altered or new biological activities, we changed certain amino acids by performing site-specific mutagenesis with the Gdm and Epi structural genes (gdmA and epiA, respectively). S. epidermidis Tü3298/EMS6, an epiA mutant of the Epi-producing strain, was used as the expression host. This mutant synthesized Epi, Gdm, or analogs of these antibiotics when the appropriate genes were introduced on a plasmid. No Epi or Gdm analogs were isolated from the supernatant when (i) hydroxyamino acids involved in thioether amino acid formation were replaced by nonhydroxyamino acids (S3N and S19A); (ii) C residues involved in thioether bridging were deleted (delta C21, C22 and delta C22); or (iii) a ring amino acid was replaced by an amino acid having a completely different character (G10E and Y20G). A strong decrease in production was observed when S residues involved in thioether amino acid formation were replaced by T residues (S16T and S19T). A number of conservative changes at positions 6, 12, and 14 on the Gdm backbone were tolerated and led to analogs that had altered biological properties, such as enhanced antimicrobial activity (L6V) or a remarkable resistance to proteolytic degradation (A12L and Dhb14P). The T14S substitution led to simultaneous production of two Gdm species formed by incomplete posttranslational modification (dehydration) of the S-14 residue. The fully modified Dhb14Dha analog exhibited antimicrobial activity similar to that of Gdm, whereas the Dhb14S analog was less active. Both peptides were more sensitive to tryptic cleavage than Gdm was.