Structural Characterization of the PawL-Derived Peptide Family, an Ancient Subfamily of Orbitides

Abstract

Plants are an excellent source of bioactive peptides, often with disulfide bonds and/or a cyclic backbone. While focus has predominantly been directed at disulfide-rich peptides, a large family of small, cyclic but non-disulfide bonded peptides known as the orbitides has been relatively ignored. What research has been conducted has focused on discovering bioactive orbitides, with less attention to structural features. A recently discovered subfamily of the orbitides known as the PawL-Derived Peptides (PLPs) are produced during the maturation of precursors for seed storage albumins. Although their evolutionary origins have been dated, in-depth exploration of the family's structural characteristics and potential bioactivities remains to be conducted. Here we present an extensive characterization of the PLP family. Nine PLPs were produced by solid phase peptide synthesis. Although orbitides can have antimicrobial and therapeutic effects, none of these nine PLPs showed antibacterial or antifungal activity. Their structural features were studied using solution NMR spectroscopy and seven were found to possess regions of backbone order. Ordered regions consist of β-turns, with some PLPs adopting two well-defined β-turns within sequences as short as seven residues, which are largely the result of side chain interactions. Our data highlight that the sequence diversity within this family results in equally diverse shapes.