Naturally occurring, pharmacologically active peptides constrained with covalent crosslinks generally have shapes that have evolved to fit precisely into binding pockets on their targets. Such peptides can have excellent pharmaceutical properties, combining the stability and tissue penetration of small-molecule drugs with the specificity of much larger protein therapeutics. The ability to design constrained peptides with precisely specified tertiary structures would enable the design of shape-complementary inhibitors of arbitrary targets. Here we describe the development of computational methods for accurate de novo design of conformationally restricted peptides, and the use of these methods to design 18–47 residue, disulfide-crosslinked peptides, a subset of which are heterochiral and/or N–C backbone-cyclized. Both genetically encodable and non-canonical peptides are exceptionally stable to thermal and chemical denaturation, and 12 experimentally determined X-ray and NMR structures are nearly identical to the computational design models. The computational design methods and stable scaffolds presented here provide the basis for development of a new generation of peptide-based drugs.

Cyclotides are plant peptides comprising a circular backbone and three conserved disulfide bonds that confer them with exceptional stability. They were originally discovered in Oldenlandia affinis based on their use in traditional African medicine to accelerate labor. Recently, cyclotides have been identified in numerous plant species of the coffee, violet, cucurbit, pea, potato, and grass families. Their unique structural topology, high stability, and tolerance to sequence variation make them promising templates for the development of peptide-based pharmaceuticals. However, the mechanisms underlying their biological activities remain largely unknown; specifically, a receptor for a native cyclotide has not been reported hitherto. Using bioactivity-guided fractionation of an herbal peptide extract known to indigenous healers as “kalata-kalata,” the cyclotide kalata B7 was found to induce strong contractility on human uterine smooth muscle cells. Radioligand displacement and second messenger-based reporter assays confirmed the oxytocin and vasopressin V_1a receptors, members of the G protein-coupled receptor family, as molecular targets for this cyclotide. Furthermore, we show that cyclotides can serve as templates for the design of selective G protein-coupled receptor ligands by generating an oxytocin-like peptide with nanomolar affinity. This nonapeptide elicited dose-dependent contractions on human myometrium. These observations provide a proof of concept for the development of cyclotide-based peptide ligands. Significance G protein-coupled receptors (GPCRs) are promising drug targets: >30% of the currently marketed drugs elicit their actions by binding to these transmembrane receptors. However, only ∼10% of all GPCRs are targeted by approved drugs. Resorting to plant-derived compounds catalogued by ethnopharmacological analyses may increase this repertoire. We provide a proof of concept by analyzing the uterotonic action of an herbal remedy used in traditional African medicine. We identified cyclic peptides, investigated the molecular mechanisms underlying their uterotonic activity, and report an oxytocic plant peptide that modulates the human oxytocin/vasopressin receptors. This naturally occurring peptide served as a template for the design of an oxytocin-like nonapeptide with enhanced receptor selectivity, highlighting the potential of cyclotides for the discovery of peptide-based GPCR ligands.

Significance: The α9α10 nicotinic AChR (nAChR) subtype is a recently identified target for the development of breast cancer chemotherapeutics and analgesics, particularly to treat neuropathic pain. Structure/function analyses of antagonists of this subtype are therefore essential for the development of specific therapeutic compounds. TheConusgenus is a rich source of pharmacologically active peptides, and we report here that the αO-conotoxin GeXIVA is a potent and selective antagonist of the α9α10 nAChR subtype. GeXIVA displays unique structural properties among otherConuspeptides and represents a previously unidentified template for molecules active against neuropathic pain.

Fungi are a newly emerging source of peptide antibiotics with therapeutic potential. Here, we report 17 new fungal defensin-like peptide (fDLP) genes and the detailed characterization of a corresponding synthetic fDLP (micasin) from a dermatophyte in terms of its structure, activity and therapeutic potential. NMR analysis showed that synthetic micasin adopts a “hallmark” cysteine-stablized α-helical and β-sheet fold. It was active on both Gram-positive and Gram-negtive bacteria, and importantly it killed two clinical isolates of methicillin-resistant Staphylococcus aureus and the opportunistic pathogen Pseudomonas aeruginosa at low micromolar concentrations. Micasin killed approximately 100% of treated bacteria within 3 h through a membrane nondisruptive mechanism of action, and showed extremely low hemolysis and high serum stability. Consistent with these functional properties, micasin increases survival in mice infected by the pathogenic bacteria in a peritonitis model. Our work represents a valuable approach to explore novel peptide antibiotics from a large resource of fungal genomes.

Tachyplesin I, II and III are host defense peptides from horseshoe crab species with antimicrobial and anticancer activities. They have an amphipathic β-hairpin structure, are highly positively-charged and differ by only one or two amino acid residues. In this study, we compared the structure and activity of the three tachyplesin peptides alongside their backbone cyclized analogues. We assessed the peptide structures using nuclear magnetic resonance (NMR) spectroscopy, then compared the activity against bacteria (both in the planktonic and biofilm forms) and a panel of cancerous cells. The importance of peptide-lipid interactions was examined using surface plasmon resonance and fluorescence spectroscopy methodologies. Our studies showed that tachyplesin peptides and their cyclic analogues were most potent against Gram-negative bacteria and melanoma cell lines, and showed a preference for binding to negatively-charged lipid membranes. Backbone cyclization did not improve potency, but improved peptide stability in human serum and reduced toxicity toward human red blood cells. Peptide-lipid binding affinity, orientation within the membrane, and ability to disrupt lipid bilayers differed between the cyclized peptide and the parent counterpart. We show that tachyplesin peptides and cyclized analogues have similarly potent antimicrobial and anticancer properties, but that backbone cyclization improves their stability and therapeutic potential.