Cyclotides and conotoxins ultra-stable disulfide-rich peptides

Conotoxins and cyclotides are large families of disulfide-rich peptides present in marine cone snails and terrestrial plants respectively. The former are used by cone snails in their venoms for prey capture and the latter by some plant species as a defence against insect predation. A particularly stable structural motif, the cystine knot, is present in both families of peptides. The use of a similar structural motif in vastly different organisms for attack and defence functions appears to have evolved independently to take advantage of the exceptional stability of the cystine knot. The interlocking nature of the disulfide bonds renders cystine-knot-containing peptides highly resistant to chemical, thermal or enzymatic degradation, giving them favourable properties for pharmaceutical applications. The cystine knot in cyclotides is further stabilized by being embedded in a head-to-tail cyclised peptide backbone. Their "knot within a ring" structure perhaps makes cyclotides nature's most stable proteins and they have great potential as a protein engineering template