Structure-activity relationship data and ligand-receptor interactions identify novel agonists consistent with sulfakinin tissue-specific signaling in Drosophila melanogaster heart.

BACKGROUND: The structures and activities of invertebrate sulfakinins that influence gut motility and heart rate are like the vertebrate cholecystokinin (CCK) peptides. Typical of sulfakinin precursors Drosophila melanogaster encodes non-sulfated drosulfakinin I (nsDSK I; FDDYGHMRF-NH2) and nsDSK II (GGDDQFDDYGHMRF-NH2) that bind DSK-R1 and DSK-R2. To explore the role of the nsDSK II N-terminal extension (GGDDQ) in gut we delineated its structure-activity relationship (SAR) and identified novel agonists. We then predicted the nsDSK II extension SAR is tissue specific consistent with cardiac CCK structure activity and signaling being different from gut. METHODS: To evaluate our hypothesis, we tested single-substituted alanine and asparagine analogs in heart. RESULTS: We found alanyl-substituted analogs were less active in heart than nsDSK II; in gut they include a super agonist and a protean agonist. Additionally, we discovered ns[N4]DSK II was more active than nsDSK II in pupal heart, while ns[N3]DSK II was inactive. In contrast, ns[N3]DSK II and ns[N4]DSK II were super agonists in adult heart, yet inactive in larva. Although we reported nsDSK II acts through DSK-R2 in gut, its identity in heart was unknown. CONCLUSIONS: Here we reviewed ligand-receptor interactions in conjunction with SAR data to suggest nsDSK II acts through DSK-R1 in heart consistent with sulfakinin tissue-specific signaling.

Structure-activity relationships of FMRF-NH2 peptides demonstrate A role for the conserved C terminus and unique N-terminal extension in modulating cardiac contractility.

FMRF-NH2 peptides which contain a conserved, identical C-terminal tetrapeptide but unique N terminus modulate cardiac contractility; yet, little is known about the mechanisms involved in signaling. Here, the structure-activity relationships (SARs) of the Drosophila melanogaster FMRF-NH2 peptides, PDNFMRF-NH2, SDNFMRF-NH2, DPKQDFMRF-NH2, SPKQDFMRF-NH2, and TPAEDFMRF-NH2, which bind FMRFa-R, were investigated. The hypothesis tested was the C-terminal tetrapeptide FMRF-NH2, particularly F1, makes extensive, strong ligand-receptor contacts, yet the unique N terminus influences docking and activity. To test this hypothesis, docking, binding, and bioactivity of the C-terminal tetrapeptide and analogs, and the FMRF-NH2 peptides were compared. Results for FMRF-NH2 and analogs were consistent with the hypothesis; F1 made extensive, strong ligand-receptor contacts with FMRFa-R; Y → F (YMRF-NH2) retained binding, yet A → F (AMRF-NH2) did not. These findings reflected amino acid physicochemical properties; the bulky, aromatic residues F and Y formed strong pi-stacking and hydrophobic contacts to anchor the ligand, interactions which could not be maintained in diversity or number by the small, aliphatic A. The FMRF-NH2 peptides modulated heart rate in larva, pupa, and adult distinctly, representative of the contact sites influenced by their unique N-terminal structures. Based on physicochemical properties, the peptides each docked to FMRFa-R with one best pose, except FMRF-NH2 which docked with two equally favorable poses, consistent with the N terminus influencing docking to define specific ligand-receptor contacts. Furthermore, SDNAMRF-NH2 was designed and, despite lacking the aromatic properties of one F, it binds FMRFa-R and demonstrated a unique SAR, consistent with the N terminus influencing docking and conferring binding and activity; thus, supporting our hypothesis.