Frog nerve-signaling peptide (Ranatachykinin C)
A short peptide from the American bullfrog that switches on a nerve-signal receiver (called NK1) involved in pain and inflammation signaling; used only as a lab research tool, not a medicine.
A researcher, an agent, or an algorithm wrote down the sequence and picked a target to hit.
An AI model like OpenFold3 or AlphaFold built a 3D structure and scored how well it fits the binding site.
A second contributor repeated the computation on their own hardware and the scores matched.
A chemistry service or a researcher ordered the sequence, it was manufactured, and mass spectrometry confirmed the right molecule was produced.
A binding or activity measurement confirmed that it actually does what the computer predicted — or didn't.
What this is
Ranatachykinin C (RTKC) is a 10-amino-acid peptide originally isolated from the brain and intestine of the American bullfrog Rana catesbeiana. Its stored sequence HNPASFIGLM is the C-terminally amidated form (HNPASFIGLM-NH₂); the amide is essential for activity and is not shown in the raw one-letter sequence. RTKC belongs to the tachykinin family, a group of short signaling peptides that share a conserved C-terminal Phe-X-Gly-Leu-Met-amide motif and act on G-protein-coupled neurokinin receptors. On the Peptidopedia card metadata, this 10-mer is registered against the human neurokinin-1 receptor (TACR1/NK1R) with a reported potency of EC50 = 13.5 nM (ChEMBL CHEMBL263185). It is studied as a comparator to substance P, the mammalian prototype tachykinin, because the two peptides activate the same receptor but produce noticeably different downstream signaling patterns.
History
The ranatachykinins were discovered in 1991 by Kozawa, Hino, Minamino, Kangawa, and Matsuo, who isolated four novel peptides — ranatachykinins A, B, C, and D — from the brain and intestine of Rana catesbeiana during a survey for unknown bioactive peptides (Kozawa et al., Biochem. Biophys. Res. Commun., 1991). Each of the four was identified by detecting stimulant activity on guinea-pig ileum smooth muscle, the classic tachykinin bioassay, followed by microsequencing and confirmatory synthesis. Ranatachykinins A, B, and C all carry the canonical tachykinin C-terminal motif Phe-X-Gly-Leu-Met-NH₂ (RTKC contains Ile in the X position), while ranatachykinin D has an unusual Phe-Tyr-Ala-Pro-Met-NH₂ tail not previously seen in any tachykinin.
Amphibian skin and gut are an unusually rich source of tachykinin diversity — more than twenty distinct tachykinins have been identified from frog skin secretions alone, all sharing the FXGLMamide signature (Nässel et al., Frontiers in Neuroscience, 2019). Ranatachykinin C in particular has become a recurring tool compound in studies comparing tachykinin pharmacology across species.
What it does
Ranatachykinin C activates neurokinin-1 receptors, the same receptors targeted by mammalian substance P. Engaging NK1 receptors triggers smooth-muscle contraction, sensory-neuron signaling, and a range of inflammatory and neurotransmission effects. In the original 1991 isolation work, RTKC produced potent contraction of guinea-pig ileum, which is how it was identified in the first place (Kozawa et al., 1991).
Although RTKC and substance P bind the same receptor, they do not produce identical responses. Studies on the bullfrog substance P receptor in a heterologous expression system reported that RTKC and related ranatachykinins elicit calcium-mobilization and receptor-desensitization profiles that differ measurably from substance P (Perrine et al., J. Med. Chem., 2000). This pattern — same receptor, divergent functional output — has made RTKC a useful probe for dissecting biased agonism at tachykinin receptors.
Evidence
- In vitro: RTKC is registered in ChEMBL (CHEMBL263185) with EC50 = 13.5 nM at the neurokinin-1 receptor (card metadata). Solution-state NMR in SDS micelles, paired with functional measurements at the bullfrog substance P receptor, characterized both the conformation and the activity of ranatachykinin peptides including RTKC (Perrine et al., J. Med. Chem., 2000). A follow-up NMR and molecular-modeling study compared SP, RTKC, and four hybrid analogs designed to isolate which residue differences drive the functional divergence between them; the analogs adopting a continuous C-terminal helix (residues 4–11) tracked with RTKC-like behavior, while those with a partial helix tracked with substance P (Beard et al., J. Med. Chem., 2007).
- Animal: Original characterization on guinea-pig ileum smooth muscle (Kozawa et al., Biochem. Biophys. Res. Commun., 1991). No mammalian in-vivo studies of RTKC as a therapeutic candidate have been published in the sources reviewed.
- Human: No human studies. RTKC is a research-tool peptide, not a drug candidate.
Known effects
- Smooth-muscle contraction (guinea-pig ileum) — Original bioassay readout used for isolation (Kozawa et al., 1991).
- NK1 receptor activation with biased signaling — Activates Ca²⁺ mobilization with a desensitization profile distinct from substance P (Perrine et al., 2000; Beard et al., 2007).
Mechanism
Tachykinin receptors (NK1, NK2, NK3) are class A G-protein-coupled receptors. The shared C-terminal Phe-X-Gly-Leu-Met-amide of vertebrate tachykinins constitutes the bioactive core: it provides the primary contacts with the receptor's orthosteric pocket, while the N-terminal residues modulate selectivity and potency (Nässel et al., Frontiers in Neuroscience, 2019). In RTKC, the conserved motif is Phe-Ile-Gly-Leu-Met-NH₂, with isoleucine occupying the variable X position. The C-terminal amide is non-negotiable for activity — replacing it with a free carboxylate abolishes binding to tachykinin receptors across the family.
NMR studies of RTKC and substance P in SDS micelles, a membrane-mimetic environment, found that the two peptides populate different conformational ensembles at their N-termini despite sharing the C-terminal pharmacophore. RTKC favors a continuous α-helical conformation spanning residues 4–11, while substance P adopts a more flexible N-terminus paired with a partial helix in the same region (Beard et al., J. Med. Chem., 2007). This structural difference is thought to underlie the functional-selectivity differences observed at the receptor.
Related peptides
- Substance P — the prototypical mammalian tachykinin (RPKPQQFFGLM-NH₂), the direct comparator in every published study of RTKC, and the endogenous NK1 ligand in humans.
- Neurokinin A and neurokinin B — the other two endogenous mammalian tachykinins, preferential ligands for NK2 and NK3 receptors respectively.
- Ranatachykinins A, B, D — the three sister peptides isolated alongside RTKC from Rana catesbeiana in the same 1991 study (Kozawa et al.).
Research directions for this peptide, selected from the current sources — hypotheses you can explore and model. None of it is proven yet; tap any one to see the full thinking.
Could the frog peptide selectively trigger only the beneficial responses at the pain receptor, while avoiding the side-effect-linked ones?
If RTKC naturally separates helpful from harmful signals at the NK1R receptor, it could serve as a template for pain or nausea drugs with fewer side effects, which would matter greatly to cancer patients on chemotherapy and to people with chronic pain.
Does the frog peptide hit only the NK1 receptor while leaving the closely related NK2 and NK3 receptors alone?
A more selective peptide would cause fewer off-target effects in the body. For patients needing NK1-targeted therapy, such as those receiving chemotherapy-induced nausea treatment, a selective compound could be safer and easier to dose.
Does the single amino acid difference between Ranatachykinin C and Substance P make the frog peptide fit the pain receptor more snugly?
If true, it could reveal a design principle for new pain-targeting drugs that are simpler and more stable than current candidates. Patients with chronic pain or nausea conditions treated via NK1R drugs could benefit from better-designed medicines.
Is the rigid proline kink in Ranatachykinin C a critical structural feature that makes the peptide fit the receptor tightly?
If the rigid bend matters, chemists could use that insight to build small, stable molecules that mimic the frog peptide, potentially leading to oral drugs that target the NK1R receptor for pain or nausea.
▸full evidence table1 metrics
| metric | value | tool |
|---|---|---|
| EC50 | 13.5 nM | GPCRDB/ChEMBL |
▸3-letter notation
▸recipeboltz-2 2.2.1
| parameter | value |
|---|---|
| model | boltz-2 2.2.1 |
| weights | — |
| hardware | vast_v100_32gb |
| mlx version | — |
| python | — |
| random seed | 1 |
| msa strategy | colabfold_local |
| runtime | — |
| predicted by | — |
| predicted at | 2026-05-22 |
▸citationbibtex
@peptide{pep10453,
sequence = {HNPASFIGLM},
target = {tacr1},
author = {peptidemodel},
year = {2026},
status = {bioassayed}
}