Aplysia allatotropin-related peptide and its newly identified D-amino acid-containing epimer both activate a receptor and a neuronal target

L-to D-residue isomerization is a post-translational modification (PTM) present in neuropeptides, peptide hormones, and peptide toxins from several animals. In most cases, the D-residue is critical for the biological function of the resulting D-amino acid-containing peptide (DAACP). Here, we provide an example in native neuropeptides in which the DAACP and its all-Lamino acid epimer are both active at their newly identified receptor in vitro and at a neuronal target associated with feeding behavior. On the basis of sequence similarity to a known DAACP from cone snail venom, we hypothesized that allatotropin-related peptide (ATRP), a neuropeptide from the neuroscience model organism Aplysia californica, may form multiple diastereomers in the Aplysia central nervous system. We determined that ATRP exists as a D-amino acid-containing peptide (D2-ATRP) and identified a specificGprotein-coupled receptor as an ATRP receptor. Interestingly, unlike many previously reported DAACPs and their all-L-residue analogs, both L-ATRP and D2-ATRP were potent agonists of this receptor and active in electrophysiological experiments. Finally, D2-ATRP was much more stable than its all-L-residue counterpart in Aplysia plasma, suggesting that in the case of ATRP, the primary role of the L-to D-residue isomerization may be to protect this peptide from aminopeptidase activity in the extracellular space. Our results indicate that L-toD-residue isomerization can occur even in an all-L-residue peptide with aknownbiological activity and that insomecases, this PTMmayhelp modulate peptide signal lifetime in the extracellular space rather than activity at the cognate receptor.