The self-association of the cyclotide kalata B2 in solution is guided by hydrophobic interactions

Authors

  • K. Johan Rosengren,

    1. School of Biomedical Sciences, The University of Queensland, Brisbane, QLD, Australia
    2. Institute for Molecular Bioscience, The University of Queensland, Brisbane, QLD, Australia
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  • Norelle L. Daly,

    1. Institute for Molecular Bioscience, The University of Queensland, Brisbane, QLD, Australia
    Current affiliation:
    1. Queensland Tropical Health Alliance, School of Pharmacy and Molecular Sciences and Center for Biodiscovery and Molecular Development of Therapeutics, James Cook University, Cairns, QLD, Australia
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  • Peta J. Harvey,

    1. Institute for Molecular Bioscience, The University of Queensland, Brisbane, QLD, Australia
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  • David J. Craik

    Corresponding author
    1. Institute for Molecular Bioscience, The University of Queensland, Brisbane, QLD, Australia
    • Correspondence to: David J. Craik, Institute for Molecular Bioscience, Brisbane, QLD 4072, Australia; e-mail: d.craik@imb.uq.edu.au or K. Johan Rosengren, School of Biomedical Sciences, Brisbane, QLD 4072, Australia; e-mail: j.rosengren@uq.edu.au

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  • This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com

ABSTRACT

The cyclotides are a family of small head-to-tail cyclic plant defense proteins. In addition to their cyclic backbone, cyclotides comprise three disulfide bonds in a knotted arrangement, resulting in a highly cross-braced structure that provides exceptional chemical and proteolytic stability. A number of bioactivities have been associated with cyclotides, including insecticidal, antimicrobial, anti-viral and cytotoxic, and these activities are related to an ability to target and disrupt biological membranes. Kalata B2 and to a lesser extent kalata B1, isolated from Oldenlandia affinis, self-associate to tetramers and octamers in aqueous buffers, and this oligomerization has been suggested to be relevant for their ability to form pores in membranes. Here we demonstrate by solution NMR spectroscopy analysis that the oligomerization of kalata B2 is concentration dependent and that it involves the packing of hydrophobic residues normally exposed on the surface of kalata B2 into a multimeric hydrophobic core. Interestingly, the hydrophobic surface that is “quenched” has previously been shown to be responsible for the ability of kalata B2 to insert into membranes. Thus, it seems unlikely that the oligomers observed in aqueous solution are related to any multimeric state present in a membrane environment, and responsible for the formation of pores. The ability to self-associate might alternatively provide a mechanism for preventing self-toxicity when stored at high concentrations in intracellular compartments. © 2013 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 453–460, 2013.

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