A cystine knot is a protein structural motif containing three disulfide bridges (formed from pairs of cysteine residues).

[9] The presence of the cyclic cystine knot (CCK) motif was discovered when cyclotides were isolated from various plant families.

The CCK motif has a cyclic backbone, triple stranded beta sheet, and cystine knot conformation.

The biological activities of these molecules are partially due to the unique interlocking arrangement and cyclized peptide backbone which contains a conserved sequence shared among circulins.

[13] Studies have shown that cystine knot proteins can be incubated at temperatures of 65 °C or placed in 1N HCl/1N NaOH without loss of structural and functional integrity.