Diversity in the disulfide folding pathways of cystine knot peptides

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.