Structure of the Thioether Cross-Links in Roseocin's Alpha Component

Area of Science:

• Microbiology
• Biochemistry
• Structural Biology

Background:

• Roseocin is a novel lantibiotic produced by *Streptomyces roseosporus*.
• It consists of two peptides (Rosα and Rosβ) exhibiting synergistic activity against Gram-positive pathogens.
• Lantibiotics are characterized by thioether cross-links, including lanthionine (Lan) and methyllanthionine (MeLan).

Purpose of the Study:

• To systematically assign the ring pattern of the alpha component of roseocin.
• To investigate the role of specific lanthionine and methyllanthionine rings in roseocin's structure and bioactivity.
• To elucidate the stereochemistry of the thioether cross-links.

Main Methods:

• Site-directed mutagenesis was used to create six individual ring variants of the alpha peptide by substituting key Ser/Thr and Cys residues with Ala.
• Marfey's analysis was employed to determine the stereochemistry of the lanthionine and methyllanthionine residues.
• Minimum Inhibitory Concentration (MIC) assays were performed to assess the impact of ring disruption on bioactivity.

Main Results:

• The study proposes a specific ring pattern for roseocin's alpha peptide: Cys10-Ser2 (Lan ring A), Cys11-Thr7 (MeLan ring B), Ser18-Cys23 (Lan ring C), and Thr21-Cys27 (MeLan ring D).
• All identified lanthionine and methyllanthionine residues possess DL stereochemistry.
• Disruption of lanthionine rings significantly impacted roseocin's structure and bioactivity, while methyllanthionine rings, though expendable, were important for optimal activity, as evidenced by increased MIC values upon their disruption.

Conclusions:

• The precise arrangement of lanthionine and methyllanthionine rings in roseocin is crucial for its synergistic antibacterial efficacy.
• The formation of the two lanthionine rings appears particularly decisive for achieving the native ring pattern essential for activity.
• This research provides critical insights into lantibiotic structure-function relationships, aiding in the development of novel antimicrobial agents.