Divalent cation coordination and mode of membrane interaction in cyclotides: NMR spatial structure of ternary complex Kalata B7/Mn2+/DPC micelle

The cyclotides are the family of hydrophobic bioactive plant peptides, characterized by a circular protein backbone and three knot forming disulfide bonds. It is believed that membrane activity of the cyclotides underlines their antimicrobial, cytotoxic and hemolytic properties, but the specific interactions with divalent cations can be also involved. To assess the mode of membrane interaction and divalent cation coordination in cyclotides, the spatial structure of the Möbius cyclotide Kalata B7 from the African perennial plant Oldenlandia affinis was determined in the presence of anisotropic membrane mimetic (dodecylphosphocholine micelles). The model of peptide/cation/micelle complex was built using 5-doxylstearate and Mn²⁺ relaxation probes. Results show that the peptide binds to the micelle surface with relatively high affinity by two hydrophobic loops (loop 2 – Thr⁶-Leu⁷ and loop 5 – Trp¹⁹-Ile²¹). The partially hydrated divalent cation is coordinated by charged side-chain of Glu³, aromatic side chain of Tyr¹¹ and free carbonyls of Thr⁴ and Thr⁹, and is located in direct contact with the polar head-groups of detergent. The comparison with data about other cyclotides indicates that divalent cation coordination is the invariant property of all cyclotides, but the mode of peptide/membrane interactions is varied. Probably, the specific cation/peptide interactions play a major, but yet not known, role in the biological activity of the cyclotides.