Cyclotides, a promising molecular scaffold for peptide-based therapeutics

Abstract

Cyclotides are a new emerging family of large plant-derived backbone-cyclized polypeptides (approximately 30 amino acids long) that share a disulfide-stabilized core (three disulfide bonds) characterized by an unusual knotted structure. Their unique circular backbone topology and knotted arrangement of three disulfide bonds make them exceptionally stable to thermal, chemical, and enzymatic degradation compared to other peptides of similar size. Currently, more than 100 sequences of different cyclotides have been characterized, and the number is expected to increase dramatically in the coming years. Considering their stability and biological activities like anti-HIV, uterotonic, and insecticidal, and also their abilities to cross the cell membrane, cyclotides can be exploited to develop new stable peptide-based drugs. We have recently demonstrated the intriguing possibility of producing libraries of cyclotides inside living bacterial cells. This opens the possibility to generate large genetically encoded libraries of cyclotides that can then be screened inside the cell for selecting particular biological activities in a high-throughput fashion. The present minireview reports the efforts carried out toward the selection of cyclotide-based compounds with specific biological activities for drug design.

Figure 1

Primary and tertiary structure of cyclotides from the plants Momordica cochinchinensis (MCoTI-II) and Oldenlandia affinis (kalata B1).– Red and blue connectors indicate backbone and disulfide bonds, respectively.

Figure 2

Biosynthetic approach for in vivo production of cyclotides kalata B1 and MCoTI–II inside live E. coli cells., Backbone cyclization of the linear precursor is mediated by a modified protein splicing unit or intein. The cyclized product then folds spontaneously in the bacterial cytoplasm.

Figure 3

Relative affinities for trypsin of a series of MCoTI-I mutants covering all the loop positions except loop 6 and Cys residues. A model of cyclotide MCoTI-I bound to trypsin is shown at the bottom indicating the position of the mutations. The side-chain of residue K6 is shown in red bound to specificity pocket of trypsin. The model was produced by homology modeling at the Swiss model workspace using the structure of CPTI-II-trypsin complex (PDB code: 2btc) as template. Structure was generated using the PyMol software package. Figure adapted from reference.