The role of conserved Glu residue on cyclotide stability and activity: a structural and functional study of kalata B12, a naturally occurring Glu to Asp mutant.

Cyclotides are a family of plant defense proteins with a unique cyclic backbone and cystine knot. Their remarkable stability under harsh thermal, enzymatic, and chemical conditions, combined with their range of bioactivities, including anti-HIV activity, underpins their potential as protein drug scaffolds. The vast majority of cyclotides possess a conserved glutamate residue in loop 1 of the sequence that is involved in a structurally important network of hydrogen bonds to an adjacent loop (loop 3). A single native cyclotide sequence, kalata B12, has been discovered that has an aspartic acid in this otherwise conserved position. Previous studies have determined that methylation of the glutamate or substitution with alanine abolishes the membrane disrupting activity that is characteristic of the family. To further understand the role of this conserved structural feature, we studied the folding, structure, stability, and activity of the natural aspartic acid variant kalata B12 and compared it to the prototypical cyclotide kalata B1, along with its glutamate to alanine or aspartate mutants. We show that the overall fold of kalata B12 is similar to the structure of other cyclotides, confirming that the cyclotide framework is robust and tolerant to substitution, although the structure appears to be more flexible than other cyclotides. Modification of the glutamate in kalata B1 or replacing the aspartate in kalata B12 with a glutamate reduces the efficiency of oxidative folding relative to the native peptides. The bioactivity of all modified glutamate cyclotides is abolished, suggesting an important functional role of this conserved residue. Overall, this study shows that the presence of a glutamic acid in loop 1 of the cyclotides improves stability and is essential for the membrane disrupting activity of cyclotides.