On the Role of Helix-Disrupting Amino Acid Residues in Supporting the Activity of Helical Antimicrobial Peptides Isolated from Australian Tree Frogs

Abstract

The peptides Aurein 1.2, Citropin 1.1, Maculatin 1.1 and Caerin 1.1 are members of four structurally related families of antimicrobial peptides produced in the skin secretions of Australian tree frogs. Although largely unstructured in aqueous solution, these peptides exhibit a high propensity for folding into amphipathic alpha helices when partitioned into lipid bilayers or dissolved in membrane mimetic media. Some of the distinguishing features of these families of antimicrobial peptides are that they usually form alpha helical structures with large hydrophobic surfaces (hydrophobic angle ∼200-240°), and the amino acid sequences of many of the larger members (i.e. those with sequence lengths >∼18 aa residues) usually contain significant amounts of helix-disrupting residues such as glycine and proline, the presence of which seems to be essential for the retention of antimicrobial activity. These helix-disrupting amino acid residues seem to be preferentially located in the C-terminal regions of the peptide where they tend to disrupt the break up the helical rod into two or more helical sections separated by disordered “flexible hinge” regions. The role of these “flexible hinges” has been the subject of considerable study and speculation. Our studies show that because of their large hydrophobic surfaces, they form helices with a high propensity for self association in aqueous media, and this property markedly diminishes the aqueous monomeric solubility of such peptides. Our results suggest the helix-disrupting amino acid residues may be essential for maintaining the aqueous solubility of these antimicrobial peptides