Abstract
Cyclotides are a family of macrocyclic peptides that combine the unique features of a head-to-tail cyclic backbone and a cystine knot motif, the combination of which imparts them with extraordinary stability. The prototypic cyclotide kalata B1 is toxic against two economically important gastrointestinal nematode parasites of sheep, Haemonchus contortus and Trichostrongylus colubriformis. A lysine scan was conducted to examine the effect of the incorporation of positive charges into the kalata B1 cyclotide framework. Each of the non-cysteine residues in this 29-amino acid peptide was successively substituted with lysine, and the nematocidal and hemolytic activities of the suite of mutants were determined. Substitution of 11 residues within kalata B1 decreased the nematocidal activity dramatically. On the other hand, six other residues that are clustered on the surface of kalata B1 were tolerant to Lys substitution, and indeed the introduction of positively charged residues into this region increased nematocidal activity. This activity was increased further in double and triple lysine mutants, with a maximal increase (relative to the native kalata B1) of 13-fold obtained with a triple lysine mutant (mutated at positions Thr-20, Asn-29, and Gly-1). Hemolytic activity correlated with the nematocidal activity of all lysine mutants. Our data clearly highlight the residues crucial for nematocidal and hemolytic activity in cyclotides, and demonstrate that the nematocidal activity of cyclotides can be increased by incorporation of basic amino acids.
Highlights
Stock production globally [1,2,3]
Each of the non-cysteine residues in this 29-amino acid peptide was successively replaced with lysine using solid-phase peptide synthesis, and the suite of mutants was assayed against H. contortus and T. colubriformis
In this study the structural and functional role of positive charge in the cyclotide framework was assessed by synthesis of a suite of lysine mutants, each with a single point mutation of the prototypic cyclotide kalata B1
Summary
Select double and triple lysine mutants were designed and assayed. Many of the substitutions resulted in a total loss of activity, helping to define residues implicated in bioactivity, but some substitutions resulted in increased anthelmintic activity, which was further increased in some double and triple mutants. Overall, this biochemical mutagenesis study has identified key residues in the prototypic cyclotide kalata B1 important for anthelmintic activity and demonstrated the ability to enhance activity through incorporation of positively charged residues into a localized region of the peptide. We refer to this localized region as the “amendable” face of cyclotides and describe a mechanistic model explaining its role in modulating cyclotide bioactivity
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