Viola " inconspicua" No More: An Analysis of Antibacterial Cyclotides

J Nat Prod. 2019 Sep 27;82(9):2537-2543. doi: 10.1021/acs.jnatprod.9b00359. Epub 2019 Aug 29.

Abstract

The emergence of rapidly evolving multidrug-resistant pathogens and a deficit of new compounds entering the clinical pipeline necessitate the exploration of alternative sources of antimicrobial therapeutics. Cyclotides revealed in Viola spp. are a class of highly stable, cyclic, and disulfide-bound peptides with diverse intrinsic bioactivities. Herein we have identified a novel complement of 42 putative cyclotide masses in the plant species Viola inconspicua. Cyclotide-containing fractions of a V. inconspicua peptide library revealed potent bioactivities against the Gram-negative bacteria Escherichia coli ATCC 25922 and the highly virulent and multidrug-resistant Klebsiella pneumoniae VK148. As such, six previously uncharacterized cyclotides, cycloviolacins I1-6 (cyI1-cyI6), were prioritized for molecular characterization. Cyclotides cyI3-cyI6 contain a novel "TLNGNPGA" motif in the highly variable loop six region, expanding the already substantial sequence diversity of this peptide class. Library fractions comprised of cyclotides cyI3-cyI6 exhibited MIC values of 18 and 35 μM against E. coli and K. pneumoniae, respectively, whereas isolated cyI3 killed ∼50% of E. coli at 60 μM and isolated cyI4 demonstrated no killing at concentrations >60 μM against both pathogens. This work expands the repertoire of bioactive cyclotides found in Viola spp. and highlights the potential of these antibacterial cyclic peptides.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Anti-Bacterial Agents / chemistry
  • Anti-Bacterial Agents / isolation & purification
  • Anti-Bacterial Agents / pharmacology*
  • Cyclotides / chemistry
  • Cyclotides / isolation & purification
  • Cyclotides / pharmacology*
  • Escherichia coli / drug effects
  • Klebsiella pneumoniae / drug effects
  • Microbial Sensitivity Tests
  • Molecular Structure
  • Viola / chemistry*

Substances

  • Anti-Bacterial Agents
  • Cyclotides