Identification and Characterization of CAPA Receptor in Western Flower Thrips: Specific Insights Into CAPA Signaling by Naturally Occurring Peptides and CAPA‐Derived Peptide Analogs

Insect CAPA peptides belong to the family of PRXamide(a) peptides (X, variable amino acids), which are involved in essential physiological functions, including anti-diuresis, desiccation, and myotropic activity. In this study, we identified and characterized a CAPA receptor (CAPA-R) in the western flower thrips (Frankliniella occidentalis) for two endogenous CAPA peptides: CAPA1 (EVQGLFPFPRV) and CAPA2 (QGLIPFPRV). CAPA2 exhibited approximately twice the potency of CAPA1 in activating CAPA-R. Additionally, the modified, shortened peptides, GLFPFPRV and LFPFPRV-derived from CAPA1-activated the receptor more strongly than the native CAPA1. The findings from our previous study, which showed minimal expression of the mRNA responsible for CAPA1, together with the results of the present study demonstrating that the addition of two N-terminal residues, glutamate (E) and valine (V), in CAPA1 interferes with its ability to bind to the receptor, suggest that CAPA2 serves as a primary ligand for the CAPA-R. Through the binding assays using CAPA peptides derived from species across five insect orders-Thysanoptera, Hemiptera, Diptera, Lepidoptera, and Hymenoptera, as well as a non-insect species, a mollusk (slug)-we confirmed that CAPA receptors are exclusively activated by CAPA peptides from insects, regardless of species, and show no activation by other PK peptides. Additionally, we found that F. occidentalis CAPA-R utilizes both intracellular and extracellular Ca²⁺ ions as second messengers in signal transduction triggered by CAPA receptor activation. The short peptide analogs (6-8 residues) derived from the CAPA peptides will be interesting for the development of CAPA agonists targeting the CAPA system to control F. occidentalis populations in the field.