Fig. 4.

Ct-SSTL1 clusters with human SSRP and related receptors and is activated by Ct-SSRPL1. (A) Maximum likelihood tree analysis of human and zebrafish SSRP receptors (SST and UT) and closely related receptors (opioid receptors and melanin-concentrating hormone receptor [MCHR]), ASTC-R, echinoderm SS2 receptor, and orphan receptors from Conus betulinus, Aplysia californica, and Capitella teleta. The tree was rooted with human galanin and kisspeptin receptors. The receptors tested in B and C are denoted with a star. (B) Representative concentration–response curve of Ct-SSRPL1 at the Ct-SSTL1 receptor in the PRESTO-Tango assay (error bars represent SD). The experiment was performed in four independent repeats, and the mean EC₅₀ value was 13.1 nM (pEC50 ± CI95 = 7.88 ± 0.65). (C) Representative concentration–response curve of human SS-14 and the C. betulinus nerve ring SSRP-like peptide (Cb-SSRPL1) at the human SS receptor 2 (SST₂) in the PRESTO-Tango assay (error bars represent SD), showing an activation by Cb-SSRPL1 at micromolar concentrations. The experiment was performed in three independent repeats, and the mean EC₅₀ value for Hs-SS was 4.0 nM (pEC50 ± CI95 = 8.40 ± 0.62). Species codes: Hsa: Homo sapiens, Dre: Danio rerio, Cbe: C. betulinus, Aca: A. californica, Cte: C. teleta, Aru: Asterias rubens, Dme: Drosophila melanogaster, Pdu: Platynereis dumerilli.