The glycosyltransferase involved in thurandacin biosynthesis catalyzes both O- and S-glycosylation.

The S-glycosyltransferase SunS is a recently discovered enzyme that selectively catalyzes the conjugation of carbohydrates to the cysteine thiol of proteins. This study reports the discovery of a second S-glycosyltransferase, ThuS, and shows that ThuS catalyzes both S-glycosylation of the thiol of cysteine and O-glycosylation of the hydroxyl group of serine in peptide substrates. ThuS-catalyzed S-glycosylation is more efficient than O-glycosylation, and the enzyme demonstrates high tolerance with respect to both nucleotide sugars and peptide substrates. The biosynthesis of the putative products of the thuS gene cluster was reconstituted in vitro, and the resulting S-glycosylated peptides thurandacin A and B exhibit highly selective antimicrobial activity toward Bacillus thuringiensis.

Structural and mechanistic investigations of protein S-glycosyltransferases.

Attachment of sugars to nitrogen and oxygen in peptides is ubiquitous in biology, but glycosylation of sulfur atoms has only been recently described. Here, we characterize two S-glycosyltransferases SunS and ThuS that selectively glycosylate one of five Cys residues in their substrate peptides; substitution of this Cys with Ser results in a strong decrease in glycosylation activity. Crystal structures of SunS and ThuS in complex with UDP-glucose or a derivative reveal an unusual architecture in which a glycosyltransferase type A (GTA) fold is decorated with additional domains to support homodimerization. Dimer formation creates an extended cavity for the substrate peptide, drawing functional analogy with O-glycosyltransferases involved in cell wall biosynthesis. This extended cavity contains a sharp bend that may explain the site selectivity of the glycosylation because the target Cys is in a Gly-rich stretch that can accommodate the bend. These studies establish a molecular framework for understanding the unusual S-glycosyltransferases.

Investigations into the Mechanism of Action of Sublancin.

Antimicrobial resistance is a global threat that poses a rising concern. One underlying challenge is the limited number of targets in bacteria affected by the current pool of antibiotics. To potentially help find new targets, we studied a member of the class of antimicrobial natural products named glycocins. We examined the mode of action of sublancin, which contains an unusual and essential glucosylated Cys residue, by monitoring macromolecular synthesis. Sublancin negatively affected DNA replication, transcription, and translation without affecting cell wall biosynthesis. In addition, we confirmed that the presence of the PTS sugar glucose in the medium negatively impacted antimicrobial activity of sublancin. Additionally, sublancin analogues carrying different sugars retained their antimicrobial activity regardless of which sugar was attached to the peptide or the carbon source used. These data suggest a novel mechanism upstream of transcription and translation and are consistent with previous studies suggesting that the glucose uptake system is involved.

Structure-Activity Relationships of the S-Linked Glycocin Sublancin.

Sublancin is a 37-amino acid antimicrobial peptide belonging to the glycocin family of natural products. It contains two helices that are held together by two disulfide bonds as well as an unusual S-glucosidic linkage to a Cys in a loop connecting the helices. We report the reconstitution of the biosynthetic pathway to this natural product in Escherichia coli. This technology enabled the evaluation of the structure-activity relationships of the solvent-exposed residues in the helices. The biosynthetic machinery proved tolerant of changes in both helices, and the bioactivity studies of the resulting mutants show that two residues in helix B are important for bioactivity, Asn31 and Arg33.

NMR structure of the S-linked glycopeptide sublancin 168.

Sublancin 168 is a member of a small group of glycosylated antimicrobial peptides known as glycocins. The solution structure of sublancin 168, a 37-amino-acid peptide produced by Bacillus subtilis 168, has been solved by nuclear magnetic resonance (NMR) spectroscopy. Sublancin comprises two α-helices and a well-defined interhelical loop. The two helices span residues 6-16 and 26-35, and the loop region encompasses residues 17-25. The 9-amino-acid loop region contains a ß-S-linked glucose moiety attached to Cys22. Hydrophobic interactions as well as hydrogen bonding are responsible for the well-structured loop region. The three-dimensional structure provides an explanation for the previously reported extraordinary high stability of sublancin 168.

Non-proteinogenic amino acids in lacticin 481 analogues result in more potent inhibition of peptidoglycan transglycosylation.

Lantibiotics are ribosomally synthesized and post-translationally modified peptide natural products that contain the thioether structures lanthionine and methyllanthionine and exert potent antimicrobial activity against Gram-positive bacteria. At present, detailed modes-of-action are only known for a small subset of family members. Lacticin 481, a tricyclic lantibiotic, contains a lipid II binding motif present in related compounds such as mersacidin and nukacin ISK-1. Here, we show that lacticin 481 inhibits PBP1b-catalyzed peptidoglycan formation. Furthermore, we show that changes in potency of analogues of lacticin 481 containing non-proteinogenic amino acids correlate positively with the potency of inhibition of the transglycosylase activity of PBP1b. Thus, lipid II is the likely target of lacticin 481, and use of non-proteinogenic amino acids resulted in stronger inhibition of the target. Additionally, we demonstrate that lacticin 481 does not form pores in the membranes of susceptible bacteria, a common mode-of-action of other lantibiotics.