Intra- and Interspecific Divergence of FXPRLamide Neuropeptide-Receptor Interactions in flies (Diptera)

Pyrokinin (PK) neuropeptides are characterized by a conserved C-terminal FXPRLamide motif and modulate a range of physiological functions and behaviors in species spanning the Eumetazoa. The pban gene is conserved across the insects and encodes pheromone biosynthesis activating neuropeptide (PBAN) and 1-3 additional pyrokinins. The pban gene of the basal Diptera resembles that of other insect Orders while in more derived Diptera (where it is referred to as the hugin gene) the PBAN peptide coding sequence is absent and another peptide, hugin, serves novel functions in the fruit fly Drosophila melanogaster. In this paper, structural models are generated for the pyrokinins PBAN and {gamma}-SGNP and their receptor PBANR of the basal species Aedes aegypti, and hugin and its receptor PK2_1 of the more derived Drosophila melanogaster. The binding pockets for all three peptides overlap even when amino acids are non-synonymous between the two species. Peptide C-terminal core sequences preferentially bind more conserved transmembrane regions of the receptor while the variable N-terminal peptide residues are relegated to less conserved sites in the extracellular loops. The PBAN peptide forms rigid secondary and tertiary structures with its long N-terminus, binding unique amino acids in the extracellular loops of PBANR, providing a basis for functional differentiation from the short and flexible {gamma}-SGNP peptide. A kink introduced into the full peptide backbones by proline residues directs the N-terminus along the surface of the ECLs, suggesting roles for secondary and tertiary structure in peptide recognition. D. melanogaster hugin binds residues that contact PBAN in A. aegypti, suggesting that loss of PBAN in higher Diptera facilitated divergence of peptide-receptor interactions. This data provides a potential basis for differing roles of multiple peptides in A. aegypti, and coevolution of ligand and receptor in D. melanogaster that may have facilitated functional evolution.