Line up the neuropeptides called NPFF, kisspeptin, prolactin-releasing peptide, QRFP, and RFRP side by side and they look like strangers. Their lengths differ. Their sequences barely overlap. They turn up in different parts of the brain doing different jobs. Yet biochemists file them into a single group—the RFamide peptide family—for one reason: they all end the same way. Every member of this family, sold strictly for research use only, finishes with the chemical signature Arg-Phe-NH2. This overview explains what that ending is, which five groups share it, how their receptors map out, and why the family has become such a productive area of laboratory study.
The Arg-Phe-NH2 Motif — What Actually Defines the Family
Here is the one-line answer to what makes an RFamide peptide an RFamide peptide: the family is defined by how a molecule ends, not by its overall shape or length. A peptide is a chain of amino acids, and this family is grouped by the last two links in that chain plus a small chemical cap.
Read the signature left to right. "Arg" is arginine, an amino acid abbreviated with the single letter R. "Phe" is phenylalanine, abbreviated F. The "-NH2" marks that the final phenylalanine carries an amide group rather than the free acid group most peptides end with. That last detail isn't cosmetic; C-terminal amidation changes how the molecule is shaped and how tightly it fits its target. Put the three parts together and you get Arg-Phe-NH2, the family's shared surname.
Think of the C-terminal tail as a handle. The receptors that recognize these peptides read that handle first, which is how such structurally different molecules end up in the same club. Past the shared ending, the N-terminal regions diverge widely, and that is where each group gets its own identity—a pattern you also see in other motif-defined families such as the melanocortin peptides and their HFRW motif. Not every membership is settled, though. Kisspeptin carries the Arg-Phe-NH2 ending and has long been grouped here, but some analyses argue it fits better elsewhere—a debate we come back to below (Wikipedia: RFamide peptide family).
From a Clam Heart in 1977 — Where the Family Came From
Why care about a peptide ending first spotted in a shellfish? Because it turns out to be one of the oldest and most widely conserved signaling tags in the animal kingdom, and that deep history is part of what makes the family scientifically interesting.
The story starts in 1977. Researchers isolated a small cardioexcitatory peptide—one that stimulated heart tissue—from the ganglia of the clam Macrocallista nimbosa. They named it FMRFamide after its amino acid sequence, and its Arg-Phe-amide ending became the template the whole family is named for (Editorial: A Comparative Survey of the RF-Amide Peptide Superfamily).
From that starting point, the motif shows up across an enormous evolutionary range. RFamide-type signaling traces back to the common ancestor of bilaterian animals, and related peptides—SIFamides in some invertebrate lineages, SALMFamides in others—share a common origin with the vertebrate versions. For years this stayed a niche corner of comparative biology. Then researchers found that kisspeptin, working through a receptor called GPR54, is essential for the onset of puberty and reproduction. That single finding pulled the entire family into the spotlight and drove a wave of research into every group it contains.
Meet the Five Groups
Five genes, five groups, one shared ending. In mammals, researchers sort RFamide peptides into five families. Each is encoded by its own gene and built around a different N-terminal sequence layered on top of the common Arg-Phe-NH2 tail (RFamide Peptides: Structure, Function, Mechanisms and Pharmaceutical Potential).