Journal of Biological Chemistry
Volume 278, Issue 8, 21 February 2003, Pages 6314-6322
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PROTEIN STRUCTURE AND FOLDING
Disulfide Folding Pathways of Cystine Knot Proteins: TYING THE KNOT WITHIN THE CIRCULAR BACKBONE OF THE CYCLOTIDES*

https://doi.org/10.1074/jbc.M210492200Get rights and content
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The plant cyclotides are a fascinating family of circular proteins that contain a cyclic cystine knot motif. The knotted topology and cyclic nature of the cyclotides pose interesting questions about folding mechanisms and how the knotted arrangement of disulfide bonds is formed. In the current study we have examined the oxidative refolding and reductive unfolding of the prototypic cyclotide, kalata B1. A stable two-disulfide intermediate accumulated during oxidative refolding but not in reductive unfolding. Mass spectrometry and NMR spectroscopy were used to show that the intermediate contained a native-like structure with two native disulfide bonds topologically similar to the intermediate isolated for the related cystine knot protein EETI-II (Le-Nguyen, D., Heitz, A., Chiche, L., El Hajji, M., and Castro B. (1993) Protein Sci.2, 165–174). However, the folding intermediate observed for kalata B1 is not the immediate precursor of the three-disulfide native peptide and does not accumulate in the reductive unfolding process, in contrast to the intermediate observed for EETI-II. These alternative pathways of linear and cyclic cystine knot proteins appear to be related to the constraints imposed by the cyclic backbone of kalata B1 and the different ring size of the cystine knot. The three-dimensional structure of a synthetic version of the two-disulfide intermediate of kalata B1 in which Ala residues replace the reduced Cys residues provides a structural insight into why the two-disulfide intermediate is a kinetic trap on the folding pathway.

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*

This work was supported in part by a grant from the Australian Research Council (to D. J. C.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplementary materials.

The atomic coordinates and the structure factors (code 1N1U) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).