Development of a novel bioluminescent activity assay for peptide ligases

In recent years, some peptide ligases have been identified, such as bacterial sortases and certain plant asparaginyl or prolyl endopeptidases. Peptide ligases have wide applications in protein labeling and cyclic peptide synthesis. To characterize known peptide ligases or identify new ones, we propose a novel bioluminescent activity assay via the genetic fusion of a recognition motif of a peptide ligase to the C-terminus of an inactive large NanoLuc fragment (LgBiT) and the chemical introduction of a nucleophilic motif preferred by the peptide ligase to the N-terminus of the low-affinity SmBiT complementation tag. When the inactive ligation version LgBiT protein was ligated with the low-affinity ligation version SmBiT tag by the expected peptide ligase, its luciferase activity would be restored and could be quantified sensitively according to the measured bioluminescence. In the present study, we first validated the novel bioluminescent activity assay using bacterial sortase A and plant butelase-1. Subsequently, we screened novel peptide ligases from crude extracts of selected plants using two LgBiT-SmBiT ligation pairs. Among 80 common higher plants, we identified that five of them likely express asparaginyl endopeptidase-type peptide ligase and four of them likely express prolyl endopeptidase-type peptide ligase, suggesting that peptide ligases are not so rare in higher plants and more of them await discovery. The novel bioluminescent activity assay is ultrasensitive, convenient for use, and resistant to protease interference, and thus would have wide applications for characterizing known peptide ligases or screening new ones from various sources in future studies.

Backbone-Cyclized Peptides: A Critical Review

Backbone-cyclized peptides and peptidomimetics integrate the biological activity and pharmacological features necessary for successful research tools and therapeutics. In general, these structures demonstrate improved maintenance of bioactive conformation, stability and cell permeability compared to their linear counterparts, while maintaining support for a variety of side chain chemistries. We explain how backbone cyclization and cycloscan techniques allow scientists to cyclize linear peptides with retained or enhanced biological activity and improved drug-like features. We discuss head-to-tail (Cterminus to N-terminus), building unit-to-tail, building unit-to-side chain, building unit-to-building unit, and building unit-to-head backbone cyclization, with examples of building blocks, such as Nα-(ω- thioalkylene), Nα-(ω-aminoalkylene) and Nα-(ω-carboxyalkylene) units. We also present several methods for recombinant expression of backbone-cyclized peptides, including backbone cyclic peptide synthesis using recombinant elements (bcPURE), phage display and induced peptidyl-tRNA drop-off. Moreover, natural backbone-cyclized peptides are also produced by cyanobacteria, plants and other organisms; several of these compounds have been developed and commercialized for therapeutic applications, which we review. Backbone-cyclized peptides and peptidomimetics comprise a growing share of the pharmaceutical industry and will be applied to additional problems in the near future.

Recyclable hypervalent-iodine-mediated solid-phase peptide synthesis and cyclic peptide synthesis

The system of the hypervalent iodine(III) reagent FPID and (4-MeOC6H4)3P was successfully applied to solid-phase peptide synthesis and cyclic peptide synthesis. Four peptides with biological activities were synthesized through SPPS and the bioactive cyclic heptapeptide pseudostellarin D was obtained via solution-phase peptide synthesis. It is worth noting that FPID can be readily regenerated after the peptide coupling reaction.