A synthetic mirror image of kalata B1 reveals that cyclotide activity is independent of a protein receptor

Featuring a circular, knotted structure and diverse bioactivities, cyclotides are a fascinating family of peptides that have inspired applications in drug design. Most likely evolved to protect plants against pests and herbivores, cyclotides also exhibit anti-cancer, anti- HIV and hemolytic activities. In all of these activities, cell membranes appear to play an important role. However, the question of whether the activity of cyclotides depends on recognition of chiral receptors or is primarily modulated by the lipid bilayer environment has remained unknown. To determine the importance of lipid membranes on the activity of the prototypic cyclotide, kalata B1, we synthesized its all-Denantiomer and assessed its bioactivities. After the all-D-enantiomer was confirmed by 1H NMR to be the structural mirror image of the native kalata B1, it was tested for anti-HIV activity, cytotoxicity and hemolytic properties. The all-D-peptide is active in these assays, albeit with less efficiency, revealing that kalata B1 does not require chiral recognition to be active. A lower activity than the native peptide correlates with a lower affinity for phospholipid bilayers in model membranes. These results exclude a chiral receptor mechanism and supports that interaction with phospholipid membranes plays a role in the activity of kalata B1. In addition, studies with mixtures of L- and Denantiomers of kalata B1 suggested that biological activity depends on peptide oligomerization at the membrane surface, which determines affinity for membranes by modulating the associationdissociation equilibrium.