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

Proteases Involved in Leader Peptide Removal during RiPP Biosynthesis

Ribosomally synthesized and post-translationally modified peptides (RiPPs) have received much attention in recent years because of their promising bioactivities and the portability of their biosynthetic pathways. Heterologous expression studies of RiPP biosynthetic enzymes identified by genome mining often leave a leader peptide on the final product to prevent toxicity to the host and to allow the attachment of a genetically encoded affinity purification tag. Removal of the leader peptide to produce the mature natural product is then carried out in vitro with either a commercial protease or a protease that fulfills this task in the producing organism. This review covers the advances in characterizing these latter cognate proteases from bacterial RiPPs and their utility as sequence-dependent proteases. The strategies employed for leader peptide removal have been shown to be remarkably diverse. They include one-step removal by a single protease, two-step removal by two dedicated proteases, and endoproteinase activity followed by aminopeptidase activity by the same protease. Similarly, the localization of the proteolytic step varies from cytoplasmic cleavage to leader peptide removal during secretion to extracellular leader peptide removal. Finally, substrate recognition ranges from highly sequence specific with respect to the leader and/or modified core peptide to nonsequence specific mechanisms.

Structural and dynamic studies of the peptidase domain from Clostridium thermocellum PCAT1

The export of antimicrobial peptides is mediated by diverse mechanisms in bacterial quorum sensing pathways. One such binary system employed by gram-positive bacteria is the PCAT1 ABC transporter coupled to a cysteine protease. The focus of this study is the N-terminal C39 peptidase (PEP) domain from Clostridium thermocellum PCAT1 that processes its natural substrate CtA by cleaving a conserved -GG- motif to separate the cargo from the leader peptide prior to secretion. In this study, we are primarily interested in elucidating the dynamic and structural determinants of CtA binding and how it is coupled to cleavage efficiency in the PCAT1 PEP domain. To this end, we have characterized CtA interactions with PEP domain and PCAT1 transporter in detergent micelles using solution nuclear magnetic resonance spectroscopy. The bound CtA structure revealed the disordered C-terminal cargo peptide is linked by a sterically hindered cleavage site to a helix docked within a hydrophobic cavity in the PEP domain. The wide range of internal motions detected by amide nitrogen (N¹⁵ ) relaxation measurements in the free enzyme and substrate-bound complex suggests the binding site is relatively floppy. This flexibility plays a key role in the structural rearrangement necessary to relax steric inhibition in the bound substrate. In conjunction with previously reported PCAT1 structures, we offer fresh insight into the ATP-mediated association between PEP and transmembrane domains as a putative mechanism to optimize peptide cleavage by regulating the width and flexibility of the enzyme active site.

Secondary Metabolites Governing Microbiome Interaction of Staphylococcal Pathogens and Commensals

Various Staphylococcus species colonize skin and upper airways of warm-blooded animals. They compete successfully with many other microorganisms under the hostile and nutrient-poor conditions of these habitats using mechanisms that we are only beginning to appreciate. Small-molecule mediators, whose biosynthesis requires complex enzymatic cascades, so-called secondary metabolites, have emerged as crucial components of staphylococcal microbiome interactions. Such mediators belong to a large variety of compound classes and several of them have attractive properties for future drug development. They include, for instance, bacteriocins such as lanthipeptides, thiopeptides, and fibupeptides that inhibit bacterial competitor species; signaling molecules such as thiolactone peptides that induce or inhibit sensory cascades in other bacteria; or metallophores such as staphyloferrins and staphylopine that scavenge scant transition metal ions. For some secondary metabolites such as the aureusimines, the exact function remains to be elucidated. How secondary metabolites shape the fitness of Staphylococcus species in the complex context of other microbial and host defense factors remains a challenging field of future research. A detailed understanding will help to harness staphylococcal secondary metabolites for excluding the pathogenic species Staphylococcus aureus from the nasal microbiomes of at-risk patients, and it will be instrumental for the development of advanced anti-infective interventions.

Self-immunity to antibacterial peptides by ABC transporters

Bacteria produce under certain stress conditions bacteriocins and microcins that display antibacterial activity against closely related species for survival. Bacteriocins and microcins exert their antibacterial activity by either disrupting the membrane or inhibiting essential intracellular processes of the bacterial target. To this end, they can lyse bacterial membranes and cause subsequent loss of their integrity or nutrients, or hijack membrane receptors for internalisation. Both bacteriocins and microcins are ribosomally synthesised and several are posttranslationally modified, whereas others are not. Such peptides are also toxic to the producer bacteria, which utilise immunity proteins or/and dedicated ATP-binding cassette (ABC) transporters to achieve self-immunity and peptide export. In this review, we discuss the structure and mechanism of self-protection that is conferred by these ABC transporters.

Biosynthesis and secretion of the microbial sulfated peptide RaxX and binding to the rice XA21 immune receptor

The rice immune receptor XA21 is activated by the sulfated microbial peptide required for activation of XA21-mediated immunity X (RaxX) produced by Xanthomonas oryzae pv. oryzae (Xoo). Mutational studies and targeted proteomics revealed that the RaxX precursor peptide (proRaxX) is processed and secreted by the protease/transporter RaxB, the function of which can be partially fulfilled by a noncognate peptidase-containing transporter component B (PctB). proRaxX is cleaved at a Gly-Gly motif, yielding a mature peptide that retains the necessary elements for RaxX function as an immunogen and host peptide hormone mimic. These results indicate that RaxX is a prokaryotic member of a previously unclassified and understudied group of eukaryotic tyrosine sulfated ribosomally synthesized, posttranslationally modified peptides (RiPPs). We further demonstrate that sulfated RaxX directly binds XA21 with high affinity. This work reveals a complete, previously uncharacterized biological process: bacterial RiPP biosynthesis, secretion, binding to a eukaryotic receptor, and triggering of a robust host immune response.

CylA is a sequence-specific protease involved in toxin biosynthesis

CylA is a subtilisin-like protein belonging to a recently expanded serine protease family related to class II lanthipeptide biosynthesis. As a leader peptidase, CylA is responsible for maturation of the enterococcal cytolysin, a lantibiotic important for Enterococcus faecalis virulence. In vitro reconstitution of CylA reveals that it accepts both linear and modified cytolysin peptides with a preference for cyclized peptides. Further characterization indicates that CylA activates itself by removing its N-terminal 95 amino acids. CylA achieves sequence-specific traceless cleavage of non-cognate peptides even if they are post-translationally modified, which makes the peptidase a powerful tool for mining novel lanthipeptides by providing a general strategy for leader peptide removal. Knowledge about the substrate specificity of CylA may also facilitate the development of protease inhibitors targeting cytolysin biosynthesis as a potential therapeutic approach for enterococcal infections.

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.

One-pot synthesis of class II lanthipeptide bovicin HJ50 via an engineered lanthipeptide synthetase

Lanthipeptides are a large class of bacteria-produced, ribosomally-synthesized and post-translationally modified peptides. They are recognized as peptide antibiotics because most of them exhibit potent antimicrobial activities against Gram-positive bacteria especially those that are phylogenetically related to producers. Maturation of class II lanthipeptide like bovicin HJ50 undergoes precursor modification by LanM and a subsequent leader peptide cleavage by LanT. Herein, via co-expression of precursor gene bovA, modification gene bovM and transporter gene bovT in Escherichia coli C43 (DE3), bioactive bovicin HJ50 was successfully produced and secreted. To further achieve in vitro one-pot synthesis of bovicin HJ50, an engineered bovicin HJ50 synthetase BovT150M was obtained by fusing the peptidase domain of BovT (BovT150) to the N-terminus of BovM. BovT150M exhibited dual functions of precursor modification and leader peptide cleavage to release mature bovicin HJ50. Under the guidance of BovA leader peptide, BovT150M exhibited substrate tolerance to modify non-native substrates including suicin and lacticin 481. This work exemplifies the feasibility of enzyme chimera of peptidase domain (LanT150) and modification enzyme (LanM) as a one-pot lanthipeptide synthetase.

Applications of the class II lanthipeptide protease LicP for sequence-specific, traceless peptide bond cleavage

The class II lanthipeptide protease LicP maturates through self-cleavage and enables sequence-specific, traceless peptide bond cleavage.

The final step of lanthipeptide biosynthesis involves the removal of leader peptides by dedicated proteases. In vitro characterization of LicP, a class II LanP protease involved in the biosynthesis of the lantibiotic lichenicidin, revealed a self-cleavage step that removes 100 amino acids from the N-terminus. The 2.35 Å resolution crystal structure provides insights into the active site geometry and substrate specificity, and unveiled an unusual calcium-independent maturation mechanism of a subtilisin family member. LicP processes LicA2 peptides with or without post-translational modifications, but dehydrated and cyclized LicA2 is favored. Investigation of its substrate specificity demonstrated that LicP can serve as an efficient sequence-specific traceless protease and may have great utility in basic research and biotechnology. Encouraged by these findings for LicP, we identified 13 other class II LanPs, ten of which were previously unknown, and suggest that these proteins may serve as a pool of proteases with diverse recognition sequences for general traceless tag removal applications, expanding the current toolbox of proteases.

Secretome of obligate intracellular Rickettsia

The genus Rickettsia (Alphaproteobacteria, Rickettsiales, Rickettsiaceae) is comprised of obligate intracellular parasites, with virulent species of interest both as causes of emerging infectious diseases and for their potential deployment as bioterrorism agents. Currently, there are no effective commercially available vaccines, with treatment limited primarily to tetracycline antibiotics, although others (e.g. josamycin, ciprofloxacin, chloramphenicol, and azithromycin) are also effective. Much of the recent research geared toward understanding mechanisms underlying rickettsial pathogenicity has centered on characterization of secreted proteins that directly engage eukaryotic cells. Herein, we review all aspects of the Rickettsia secretome, including six secretion systems, 19 characterized secretory proteins, and potential moonlighting proteins identified on surfaces of multiple Rickettsia species. Employing bioinformatics and phylogenomics, we present novel structural and functional insight on each secretion system. Unexpectedly, our investigation revealed that the majority of characterized secretory proteins have not been assigned to their cognate secretion pathways. Furthermore, for most secretion pathways, the requisite signal sequences mediating translocation are poorly understood. As a blueprint for all known routes of protein translocation into host cells, this resource will assist research aimed at uniting characterized secreted proteins with their apposite secretion pathways. Furthermore, our work will help in the identification of novel secreted proteins involved in rickettsial 'life on the inside'.

Antibacterial peptides "bacteriocins": an overview of their diverse characteristics and applications

Bacteriocins are ribosomally synthesized antibacterial peptides produced by bacteria that inhibit the growth of similar or closely related bacterial strains. A number of bacteriocins from a wide variety of bacteria have been discovered, and their diverse structures have been reported. Growing evidence suggests that bacteriocins have diverse structures, modes of action, mechanisms of biosynthesis and self-immunity, and gene regulation. Bacteriocins are considered as an attractive compound in food and pharmaceutical industries to prevent food spoilage and pathogenic bacterial growth. Furthermore, elucidation of their biosynthesis has led to the use of bacteriocin-controlled gene-expression systems and the biosynthetic enzymes of lantibiotics, a class of bacteriocins, as tools to design novel peptides. In this review, we summarize and discuss currently known information on bacteriocins produced by Gram-positive bacteria and their applications.

Generation of an actagardine A variant library through saturation mutagenesis

The lantibiotic actagardine A is nineteen amino acids in length and comprises three intertwined C-terminal methyllanthionine-bridged rings and an N-terminal lanthionine-bridged ring. Produced by the actinomycete Actinoplanes garbadinensis ATCC 31049, actagardine A demonstrates antibacterial activity against important Gram-positive pathogens. This activity combined with its ribosomal synthesis makes it an attractive target for the generation of lantibiotic variants with improved biological activity. A variant generation system designed to allow the specific substitution of amino acids at targeted sites throughout the actagardine A peptide has been used to generate a comprehensive library by site-directed mutagenesis. With the exception of residues involved in bridge formation, each amino acid in the actagardine A peptide as well as the alanine (ala(0)) at position -1 relative to the mature peptide, has been systematically substituted with all remaining 19 amino acids. A total of 228 mutants have been engineered with 44 produced in good yield. The mutant V15F in particular demonstrates improved activity against a range of notable Gram-positive pathogens including Clostridium difficile, when evaluated alongside actagardine A. The scope of variants generated provides an insight into the flexibility of the actagardine A processing machinery and will undoubtedly assist in future mutational studies.

Discovery, biosynthesis, and engineering of lantipeptides

Aided by genome-mining strategies, knowledge of the prevalence and diversity of ribosomally synthesized natural products (RNPs) is rapidly increasing. Among these are the lantipeptides, posttranslationally modified peptides containing characteristic thioether cross-links imperative for bioactivity and stability. Though this family was once thought to be a limited class of antimicrobial compounds produced by gram-positive bacteria, new insights have revealed a much larger diversity of activity, structure, biosynthetic machinery, and producing organisms than previously appreciated. Detailed investigation of the enzymes responsible for installing the posttranslational modifications has resulted in improved in vivo and in vitro engineering systems focusing on enhancement of the therapeutic potential of these compounds. Although dozens of new lantipeptides have been isolated in recent years, bioinformatic analyses indicate that many hundreds more await discovery owing to the widespread frequency of lantipeptide biosynthetic machinery in bacterial genomes.

Lantibiotic transporter requires cooperative functioning of the peptidase domain and the ATP binding domain

Lantibiotics are ribosomally synthesized and post-translationally modified peptide antibiotics that contain unusual amino acids such as dehydro and lanthionine residues. Nukacin ISK-1 is a class II lantibiotic, whose precursor peptide (NukA) is modified by NukM to form modified NukA. ATP-binding cassette (ABC) transporter NukT is predicted to cleave off the N-terminal leader peptide of modified NukA and secrete the mature peptide. Multiple sequence alignments revealed that NukT has an N-terminal peptidase domain (PEP) and a C-terminal ATP binding domain (ABD). Previously, in vitro reconstitution of NukT has revealed that NukT peptidase activity depends on ATP hydrolysis. Here, we constructed a series of NukT mutants and investigated their transport activity in vivo and peptidase activity in vitro. Most of the mutations of the conserved residues of PEP or ABD resulted in failure of nukacin ISK-1 production and accumulation of modified NukA inside the cells. NukT(N106D) was found to be the only mutant capable of producing nukacin ISK-1. Asn(106) is conserved as Asp in other related ABC transporters. Additionally, an in vitro peptidase assay of NukT mutants demonstrated that PEP is on the cytosolic side and all of the ABD mutants as well as PEP (with the exception of NukT(N106D)) did not have peptidase activity in vitro. Taken together, these observations suggest that the leader peptide is cleaved off inside the cells before peptide secretion; both PEP and ABD are important for NukT peptidase activity, and cooperation between these two domains inside the cells is indispensable for proper functioning of NukT.