Characterization of Noctilisin, a Heat-Stable Glycopeptide from Sirex Nocilio (F.) Venom That Causes Needle Wilt in Pines

During oviposition, female Sirex noctilio (F.) woodwasps inject their conifer hosts with a venom that facilitates lethal infection of the tree by a fungal pathogen, Amylostereum areolatum. A heat-stable factor in venom that can migrate from the site of oviposition to needles in the tree crown was purified by size-fractionation and RP-HPLC using assays based on defense gene induction and needle wilt response in pine shoot explants. The activity was associated with an 11-amino acid peptide (SEGPROGTKRP) encoded by a highly abundant transcript identified from S. noctilio venom gland RNA. The peptide displayed several post-translational modifications, including hydroxylation of the Pro residue as well as O-glycosylation of Ser and Thr residues. The O-linked sugars were identical α-linked N-acetylgalactosamine residues modified at the C6 position by addition of phosphoethanolamine moieties. A synthetic version of the hydroxylated peptide backbone lacking the glycosyl side chains failed to induce pine defense genes or cause needle wilt. This peptide, hereafter called noctilisin, appears to be related to a group O-glycosylated short-chain proline-rich antimicrobial peptides that includes drosocin. The unusual structure of the noctilisin glycosyl side chains suggests possibilities for how this factor evades surveillance by the pine defense response system.