Article

Diversity in the disulfide folding pathways of cystine knot peptides

University of Queensland, Brisbane, Queensland, Australia
Letters in Peptide Science 11/2003; 10(5):523-531. DOI: 10.1007/BF02442584

ABSTRACT

The plant cyclotides are a fascinating family of circular proteins that contain a cyclic cystine knot motif (CCK). This unique
family was discovered only recently but contains over 50 known sequences to date. Various biological activities are associated
with these peptides including antimicrobial and insecticidal activity. The knotted topology and cyclic nature of the cyclotides
poses interesting questions about the folding mechanisms and how the knotted arrangement of disulfide bonds is formed. Some
studies have been performed on related inhibitor cystine knot (ICK) containing peptides, but little is known about the folding
mechanisms of CCK molecules. We have examined the oxidative refolding and reductive unfolding of the prototypic member of
the cyclotide family, kalata B1. Analysis of the rates of formation of the intermediates along the reductive unfolding pathway
highlights the stability conferred by the cystine knot motif. Significant differences are observed between the folding of
kalata B1 and an acyclic cystine knot protein, EETI-II, suggesting that the circular backbone has a significant influence
in directing the folding pathway.

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    ABSTRACT: The cyclotides are a recently discovered family of miniproteins that contain a head-to-tail cyclized backbone and a knotted arrangement of disulfide bonds. They are approximately 30 amino acids in size and are present in high abundance in plants from the Violaceae, Rubiaceae, and Cucurbitaceae families, with individual plants containing a suite of up to 100 cyclotides. They have a diverse range of biological activities, including uterotonic, anti-HIV, antitumor, and antimicrobial activities, although their natural function is likely that of defending their host plants from pathogens and pests. This review focuses on the structural aspects of cyclotides, which may be thought of as a natural combinatorial peptide template in which a wide range of amino acids is displayed on a compact molecular core made up of the cyclic cystine knot structural motif. Cyclotides are exceptionally stable and are resistant to denaturation via thermal, chemical, or enzymatic treatments. The structural features that contribute to their remarkable stability are described in this review.
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    ABSTRACT: The cyclotides are a recently discovered family of miniproteins that contain a head-to-tail cyclized backbone and a knotted arrangement of disulfide bonds. They are approximately 30 amino acids in size and are present in high abundance in plants from the Violaceae, Rubiaceae, and Cucurbitaceae families, with individual plants containing a suite of up to 100 cyclotides. They have a diverse range of biological activities, including uterotonic, anti-HIV, antitumor, and antimicrobial activities, although their natural function is likely that of defending their host plants from pathogens and pests. This review focuses on the structural aspects of cyclotides, which may be thought of as a natural combinatorial peptide template in which a wide range of amino acids is displayed on a compact molecular core made up of the cyclic cystine knot structural motif. Cyclotides are exceptionally stable and are resistant to denaturation via thermal, chemical, or enzymatic treatments. The struclural features that contribute to their remarkable stability are described ill this review. (c) 2006 Wiley Periodicals, Inc.
    No preview · Article · Jan 2006 · Biopolymers
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