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Differences in bactericidal, hemolytic and anti-HIV efficiencies of the cyclotide
kalata B1 are explained by distinct cell membrane compositions
Sónia Troeira Henriques, 1, 2 Yen-Hua Huang, 1 K Johan Rosengren, 1 Henri G Franquelim,2
Filomena A Carvalho,2 Adam Johnson,3 Secondo Sonza,3 Gilda Tachedjian, 3 Miguel ARB
Castanho, 2 Norelle L Daly, 1 David J Craik1
1
The University of Queensland, Institute for Molecular Bioscience, Brisbane, QLD 4072,
Australia.
2
Institute of Molecular Medicine, Medicine School, University of Lisbon, 1649-028 Lisbon,
Portugal.
3
Centre for Virology, Burnet Institute, 3004 Melbourne, Victoria, Australia
Keywords: Cyclotides, peptide-membrane interactions, anti-HIV peptide, antimicrobial peptide,
phosphatidylethanolamine
Cyclotides are an intriguing family of cyclic peptides found in plants. With a very stable structure
and a range of bioactivities, cyclotides have potential pharmaceutical and agricultural
applications. As well as plant host defence activities, cyclotides possess antimicrobial, anticancer
and anti-HIV activities. The interaction of cyclotides with membranes seems to be the key-feature
of these various biological activities. To unravel the importance of lipid membranes on the
activity of the prototypic cyclotide, kalata B1 (kB1), a comprehensive characterization of the
binding of kB1, and modified analogs, to lipid membranes was evaluated. Model membranes of
different compositions were studied and compared with natural membranes (red blood cells,
bacteria, and HIV particles). We show that the bioactivity of kB1 is dependent on the lipid
composition of the target cell membrane. In particular, kB1 targets membranes through specific
interactions with phospholipids containing phosphatidylethanolamine (PE) headgroups, whereas
raft-like domains facilitate insertion inside membranes through nonspecific hydrophobic
interactions. In addition, negativelycharged phospholipids have no effect on kB1 affinity. This
lipid selectivity explains the hemolytic properties of kB1 and its lack of antimicrobial activity: it
inserts in the membranes of red blood cells, which have some PE headgroups and raft domains in
their outer layer, but does not target the bacterial cell wall, which is negatively-charged and
lacking PE. In addition, we have found that kB1 has direct virucidal activity against HIV particles
by targeting and disrupting the HIV membrane-envelope, a raft-like membrane with a high
proportion of PE phospholipids.