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Structural Characterization of Four Prochlorosins: a Novel Class of Lantipeptides Produced by Planktonic Marine Cyanobacteria † † ‡ Weixin Tang and Wilfred A

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Structural Characterization of Four Prochlorosins: a Novel Class of Lantipeptides Produced by Planktonic Marine Cyanobacteria † † ‡ Weixin Tang and Wilfred A Article pubs.acs.org/biochemistry Structural Characterization of Four Prochlorosins: A Novel Class of Lantipeptides Produced by Planktonic Marine Cyanobacteria † † ‡ Weixin Tang and Wilfred A. van der Donk*, , † ‡ Department of Chemistry and Howard Hughes Medical Institute, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States *S Supporting Information ABSTRACT: 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. antipeptides are ribosomally synthesized and posttransla- environments at very dilute concentrations, making it debatable L tionally modified peptides, characterized by the thioether whether prochlorosins serve as antimicrobial defenses like most amino acids lanthionine (Lan) and methyllanthionine other known lantipeptides.8 (MeLan).1,2 Lantipeptides with antimicrobial activities are Analysis of the Global Oceanic Survey showed that the genes called lantibiotics.3 In lantipeptide-producing organisms, the for production of these lanthionine-containing peptides are precursor peptides, termed LanAs, are expressed with an N- widespread.6,9 Their transcription levels respond to nitrogen terminal leader sequence and a C-terminal core peptide (e.g., starvation, suggesting that the prochlorosins are functional.6 Figure 1A). The core peptide is modified by a dehydratase, However, it remains unclear why the organisms produce this generating dehydroalanine (Dha) and dehydrobutyrine (Dhb) class of secondary metabolites and what their functions are. To from Ser and Thr, respectively (Figure 1B). Subsequently, address these questions as well as understand the remarkable intramolecular additions of the thiols of Cys to the Dha and substrate tolerance of ProcM, it is important to know the Dhb residues generate the thioether cross-links (Figure 1B). In chemical structures of prochlorosins, including their ring class II lantipeptides, these two sequential modifications are fi 4 topologies and the stereochemical con gurations of their Lan conducted by one enzyme, generically called LanM. The and MeLan residues. leader peptide is removed in the final step of lantipeptide 5 Preliminary structural characterization of several prochlor- biosynthesis to form the mature products. osins has been achieved by tandem mass spectrometry (MS).6 Prochlorosins (Pcns) make up a group of lantipeptides The presence of a thioether cross-link prohibits fragmentation produced by Prochlorococcus.6 In Prochlorococcus MIT9313, 29 of the peptide in the ring, and therefore, the fragmentation different peptide substrates are modified by a single pattern can be used for ring topology prediction.4,6 However, promiscuous enzyme, ProcM, forming the characteristic 6 tandem MS encounters difficulties in elucidating the ring thioether rings. The 29 precursor peptides have highly conserved leader peptides but highly diverse core peptides topology for structures that contain overlapping rings. In such where the posttranslational modifications occur (Figure 1A). cases, mutagenesis is usually employed to prevent the formation This class of lanthionine-containing peptides is of interest not of one or several overlapping rings such that the topology of the remaining rings can be established using tandem MS. However, only because of the remarkable promiscuity of the enzyme but fi also because the organisms carrying these genes are because ProcM exhibits a very low substrate speci city under cyanobacteria accounting for as much as half of the chlorophyll in the tropical and subtropical oceans with a lifestyle very Received: February 24, 2012 different from those of other known lantipeptide-producing Revised: May 8, 2012 organisms.7 They are single-cell organisms living in oligotrophic Published: May 10, 2012 © 2012 American Chemical Society 4271 dx.doi.org/10.1021/bi300255s | Biochemistry 2012, 51, 4271−4279 Biochemistry Article Figure 1. Precursor peptides of the 29 prochlorosins from Prochlorococcus MIT9313 and enzymatic transformations during biosynthesis of lantipeptides. (A) Sequence alignment of the 29 precursor peptides illustrating the high degree of conservation in their leader peptides and the high degree of diversity in their core peptides. The four peptides investigated here are denoted with pink boxes at the left. (B) Dehydration and cyclization reactions during the biosynthesis of lantibiotics and lantipeptides. − laboratory conditions and can generate a variety of ring synthetic standards.11 13 We were able to conduct these structures, it is possible that the ring topology of the mutants is experiments by producing milligram quantities of the selected not that of the wild-type prochlorosins. Indeed, a single point prochlorosins by heterologous coexpression of the precursor mutation in ProcA4.3 has been shown to change the original peptides with ProcM in Escherichia coli.14 ring pattern.6 Therefore, tandem MS has its limitations for the determination of the structures of prochlorosins, and other ■ MATERIALS AND METHODS complementary techniques are required. In this study, we used General Methods. All polymerase chain reactions (PCRs) nuclear magnetic resonance (NMR) spectroscopy to determine were conducted on a C1000 thermal cycler (Bio-Rad). DNA the ring topologies of four prochlorosins (Pcn1.1, Pcn1.7, sequencing was performed by ACGT, Inc. Preparative high- Pcn3.3, and Pcn4.3) for which the precursor genes have been performance liquid chromatography (HPLC) was performed shown to be transcribed under nitrogen starvation. using a Waters Delta 600 instrument equipped with appropriate In addition to questions about the ring patterns of columns. Matrix-assisted laser desorption ionization time-of- fi prochlorosins, the stereochemical con gurations of their Lan flight mass spectrometry (MALDI-TOF MS) was conducted on and MeLan residues have not been determined. Lan and a Voyager-DE-STR (Applied Biosystems). Overexpression and MeLan residues in lantipeptides and lantibiotics for which the purification of the modified precursor peptides were performed stereochemistry has been determined all exhibit (2S,6R) and using previously reported procedures.14 10 (2S,3S,6R)configurations, respectively. Because ProcM Construction of pRSFDuet-1 Derivatives for Coex- generates such a diversity of ring topologies, we previously pression of ProcM and ProcAs. The genes encoding speculated that perhaps only some rings are formed enzymati- ProcA1.1G−1E and ProcA4.3 were cloned from previously cally, thus preorganizing the partially cyclized peptides for reported vectors6 and inserted into multiple cloning site 1 of 6 nonenzymatic cyclization to generate the final ring structures. the pRSFDuet-1/procM-2 vector14 using the EcoRI and NotI Determination of the configuration of Lan and MeLan residues restriction sites. Primer sequences are listed in Table S1 of the in prochlorosins could potentially serve to assess whether a ring Supporting Information. Negative numbers are used for amino is formed enzymatically or nonenzymatically. acids in the leader peptide counting backward from the putative To address this hypothesis, we investigated the stereo- leader peptide cleavage site. The sequences of the constructed chemical configuration of Lan and MeLan residues of seven vectors were confirmed by DNA sequencing. The pRSFDuet- prochlorosins (Pcn1.1, Pcn1.7, Pcn2.8, Pcn2.11, Pcn3.2, 1/procM-2, pRSFDuet-1/procM-2-procA3.3G−1K, Pcn3.3, and Pcn4.3) by gas chromatography and MS (GC− pRSFDuet-1/procM-2-procA1.7G−1R, pRSFDuet-1/procM-2- MS) with a chiral stationary phase and compared them with procA2.8, pRSFDuet-1/procM-2-procA2.11G−1K, pRSFDuet- 4272 dx.doi.org/10.1021/bi300255s | Biochemistry 2012, 51, 4271−4279 Biochemistry Article Figure 2. Water-suppressed TOCSY spectrum identifying all residues of Pcn1.7. Each vertical line indicates a spin system corresponding to an amino acid. A water-suppressed NOESY spectrum (Figure S2 of the Supporting Information) along with the gene sequence was used to assign the residue numbering. 1/procM-2-procA3.2, pRSFDuet-1/procM-2-procA4.3G−1R, Illinois at Urbana-Champaign) on Varian INOVA 600 or 500 and pRSFDuet-1/procM-2-procA4.3G−1E vectors were ob- MHz spectrometers equipped with a 5 mm triple-resonance tained as reported previously.14 (1H, 13C, 15N) triaxial gradient probe. Lyophilized peptides fi fi Protease Cleavage and Puri cation
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