Abstract
Small, circular proteins are reported to have antimicrobial, cytotoxic and a host of other bioactivities. In bacteria and fungi they can be made by non-ribosomal peptide synthetases, but in plants they are exclusively ribosomal and the ligation reaction is performed by specialised endoproteases. Cyclic peptides from the Annona genus display cytotoxic and anti-inflammatory activities, but their biosynthesis is unknown. The medicinal soursop plant, Annona muricata, has been reported to contain annomuricatins A (cyclo-PGFVSA) and B (cyclo-PNAWLGT). Here, using de novo transcriptomics and tandem mass spectrometry, we identify a suite of short transcripts for precursor proteins for ten validated annomuricatins, nine of which are novel. In their precursors, annomuricatins are preceded by an absolutely conserved Glu and each peptide sequence has a conserved proto-terminal Pro, revealing parallels with the segetalin orbitides from the seeds of Vaccaria hispanica, which are processed through ligation by a prolyl oligopeptidase in a transpeptidation reaction.
Short Summary By assembling a transcriptome of Annona muricata de novo, we show that the known orbitide, annomuricatin A, is encoded by a short transcript that also encodes a novel second peptide, annomuricatin D. We discovered and sequenced eight additional annomuricatins encoded by five transcripts.
