The Biological Activity of the Prototypic Cyclotide Kalata B1 Is Modulated by the Formation of Multimeric Pores

The cyclotides are a large family of circular mini-proteins containing a cystine knot motif. They are expressed in plants as defense-related proteins, with insecticidal activity. Here we investigate their role in membrane interaction and disruption. Kalata B1, a prototypic cyclotide, was found to induce leakage of the self-quenching fluorophore, carboxyfluorescein, from phospholipid vesicles. Alanine-scanning mutagenesis of kalata B1 showed that residues essential for lytic activity are clustered, forming a bioactive face. Kalata B1 was sequestered at the membrane surface and showed slow dissociation from vesicles. Electrophysiological experiments showed that conductive pores were induced in liposome patches on incubation with kalata B1. The conductance calculated from the current-voltage relationship indicated that the diameter of the pores formed in the bilayer patches is 41-47 angstrom. Collectively, the findings provide a mechanistic explanation for the diversity of biological functions ascribed to this fascinating family of ultrastable macrocyclic peptides.