Threonine sulfation: a rare post translational modification in insect adipokinetic hormones

Peptides of the adipokinetic hormone family are responsible for metabolic roles in insects, regulating release of energy metabolites from the fat body. We report on adipokinetic hormone octapeptide sequences bearing a rarely identified post-translational modification (sulfation of a threonine residue) in two beetle subfamilies (Cetoniinae and Dynastinae) of the large superfamily of Scarabaeoidea (dung beetles, rhinoceros beetles and flower beetles), and in a bug species (family Coreidae). In the cetonids Pachnoda sinuata, Dicronorhina derbyana derbyana, Tropinota hirta, Protaetia cuprea, Cetonia aurata and Oxytherea funesta sulfated Pacsi-AKH is found (pQINLTsTGW amide), while sulfated Penid-AKH (pQVNISsTGW amide) occurs in the dynastid beetles Pentodon idiota, Xylotrupes gideon and Syrichthodontus spurius. Sulfated Schgr-AKH-II (pQLNFSsTGW amide) is found in the twig wilter Holopterna alata. Sequence elucidation was achieved by mass spectrometry, however, due to the labile nature of the sulfate group under mass spectrometric conditions, the modified amino acid could not be easily identified. Edman degradation and comparative mass spectrometry evaluations with synthetic sulfopeptide standards were therefore employed for sequence validation. This type of sulfation was previously only reported present on the protein backbone of very few proteins from vertebrates (including humans), a mollusc and a protozoan parasite.