The Single Disulfide-Directed β-Hairpin Fold. Dynamics, Stability, and Engineering

Characterization of nanodisc-forming peptides for membrane protein studies

Lipid-bilayer nanodiscs provide a stable, native-like membrane environment for the functional and structural studies of membrane proteins and other membrane-binding molecules. Peptide-based nanodiscs having unique properties are developed for membrane protein studies and other biological applications. While the self-assembly process rendering the formation of peptide-nanodiscs is attractive, it is important to understand the stability and suitability of these nanodisc systems for membrane protein studies. In this study, we investigated the nanodiscs formation by the anti-inflammatory and tumor-suppressing peptide AEM28. AEM28 is a chimeric peptide containing a cationic-rich heparan sulfate proteoglycan- (HSPG)-binding domain from human apolipoprotein E (hapoE) (141-150) followed by the 18A peptide's amino acid sequence. AEM28-based nanodiscs made with different types of lipids were characterized using various biophysical techniques and compared with the nanodiscs formed using 2F or 4F peptides. Variable temperature dynamic light-scattering and 31P NMR experiments indicated the fusion and size heterogeneity of nanodiscs at high temperatures. The suitability of AEM28 and Ac-18A-NH2- (2F-) based nanodiscs for studying membrane proteins is demonstrated by reconstituting and characterizing a drug-metabolizing enzyme, cytochrome-P450 (CYP450), or the redox complex CYP450-CYP450 reductase. AEM28 and 2F were also tested for their efficacies in solubilizing E. coli membranes to understand the possibility of using them for detergent-free membrane protein isolation. Our experimental results suggest that AEM28 nanodiscs are suitable for studying membrane proteins with a net positive charge, whereas 2F-based nanodiscs are compatible with any membrane proteins and their complexes irrespective of their charge. Furthermore, both peptides solubilized E. coli cell membranes, indicating their use in membrane protein isolation and other applications related to membrane solubilization.

Characterisation of Elevenin-Vc1 from the Venom of Conus victoriae: A Structural Analogue of α-Conotoxins

Elevenins are peptides found in a range of organisms, including arthropods, annelids, nematodes, and molluscs. They consist of 17 to 19 amino acid residues with a single conserved disulfide bond. The subject of this study, elevenin-Vc1, was first identified in the venom of the cone snail Conus victoriae (Gen. Comp. Endocrinol. 2017, 244, 11-18). Although numerous elevenin sequences have been reported, their physiological function is unclear, and no structural information is available. Upon intracranial injection in mice, elevenin-Vc1 induced hyperactivity at doses of 5 or 10 nmol. The structure of elevenin-Vc1, determined using nuclear magnetic resonance spectroscopy, consists of a short helix and a bend region stabilised by the single disulfide bond. The elevenin-Vc1 structural fold is similar to that of α-conotoxins such as α-RgIA and α-ImI, which are also found in the venoms of cone snails and are antagonists at specific subtypes of nicotinic acetylcholine receptors (nAChRs). In an attempt to mimic the functional motif, Asp-Pro-Arg, of α-RgIA and α-ImI, we synthesised an analogue, designated elevenin-Vc1-DPR. However, neither elevenin-Vc1 nor the analogue was active at six different human nAChR subtypes (α1β1εδ, α3β2, α3β4, α4β2, α7, and α9α10) at 1 µM concentrations.

Modulation of Lymphocyte Potassium Channel KV1.3 by Membrane-Penetrating, Joint-Targeting Immunomodulatory Plant Defensin

We describe a cysteine-rich, membrane-penetrating, joint-targeting, and remarkably stable peptide, EgK5, that modulates voltage-gated K_V1.3 potassium channels in T lymphocytes by a distinctive mechanism. EgK5 enters plasma membranes and binds to K_V1.3, causing current run-down by a phosphatidylinositol 4,5-bisphosphate-dependent mechanism. EgK5 exhibits selectivity for K_V1.3 over other channels, receptors, transporters, and enzymes. EgK5 suppresses antigen-triggered proliferation of effector memory T cells, a subset enriched among pathogenic autoreactive T cells in autoimmune disease. PET-CT imaging with ¹⁸F-labeled EgK5 shows accumulation of the peptide in large and small joints of rodents. In keeping with its arthrotropism, EgK5 treats disease in a rat model of rheumatoid arthritis. It was also effective in treating disease in a rat model of atopic dermatitis. No signs of toxicity are observed at 10-100 times the in vivo dose. EgK5 shows promise for clinical development as a therapeutic for autoimmune diseases.

The Single Disulfide-Directed β-Hairpin Fold: Role of Disulfide Bond in Folding and Effect of an Additional Disulfide Bond on Stability

Disulfide bonds play a key role in the oxidative folding, conformational stability, and functional activity of many peptides. A few disulfide-rich peptides with privileged architecture such as the inhibitor cystine knot motif have garnered attention as templates in drug design. The single disulfide-directed β-hairpin (SDH), a novel fold identified more recently in contryphan-Vc1, has been shown to possess remarkable thermal, conformational, and chemical stability and can accept a short bioactive epitope without compromising the core structure of the peptide. In this study, we demonstrated that the single disulfide bond is critical in maintaining the native fold by replacing both cysteine residues with serine. We also designed an analogue with an additional, non-native disulfide bridge by replacing Gln1 and Tyr9 with Cys. Contryphan-Vc11–22[Q1C, Y9C] was synthesised utilising orthogonal cysteine protection and its solution structure determined using solution NMR spectroscopy. This analogue maintained the overall fold of native contryphan-Vc1. Previous studies had shown that the β-hairpin core of contryphan-Vc1 was resistant to proteolysis by trypsin and α-chymotrypsin but susceptible to cleavage by pepsin. Contryphan-Vc11–22[Q1C, Y9C] proved to be completely resistant to pepsin, thus confirming our design strategy. These results highlight the role of the disulfide bond in maintaining the SDH fold and provide a basis for the design of more stable analogues for peptide epitope grafting.

Structure, folding and stability of a minimal homologue from Anemonia sulcata of the sea anemone potassium channel blocker ShK

Peptide toxins elaborated by sea anemones target various ion-channel sub-types. Recent transcriptomic studies of sea anemones have identified several novel candidate peptides, some of which have cysteine frameworks identical to those of previously reported sequences. One such peptide is AsK132958, which was identified in a transcriptomic study of Anemonia sulcata and has a cysteine framework similar to that of ShK from Stichodactyla helianthus, but is six amino acid residues shorter. We have determined the solution structure of this novel peptide using NMR spectroscopy. The disulfide connectivities and structural scaffold of AsK132958 are very similar to those of ShK but the structure is more constrained. Toxicity assays were performed using grass shrimp (Palaemonetes sp) and Artemia nauplii, and patch-clamp electrophysiology assays were performed to assess the activity of AsK132958 against a range of voltage-gated potassium (K_V) channels. AsK132958 showed no activity against grass shrimp, Artemia nauplii, or any of the K_V channels tested, owing partly to the absence of a functional Lys-Tyr dyad. Three AsK132958 analogues, each containing a Tyr in the vicinity of Lys19, were therefore generated in an effort to restore binding, but none showed activity against any of K_V channels tested. However, AsK132958 and its analogues are less susceptible to proteolysis than that of ShK. Our structure suggests that Lys19, which might be expected to occupy the pore of the channel, is not sufficiently accessible for binding, and therefore that AsK132958 must have a distinct functional role that does not involve K_V channels.