Kisspeptin-10

The ten-residue C-terminal fragment of kisspeptin, the product of the KISS1 gene, with an amidated C-terminus. Drawn in the PepXtide structural library, where its computed mass matches the published molecular weight.

The gene it comes from was identified in 1996 as a metastasis-suppressor gene in melanoma, and was named in Hershey, Pennsylvania, which is where the unusual spelling comes from. The gene product is processed to several peptides of different lengths sharing a common C-terminus, of which this decapeptide is the shortest that still activates the receptor, corresponding to residues 112 to 121 of the precursor. The receptor was deorphanised in 2001 by three independent groups in the same year, which is why it carries three names in the older literature.

The C-terminal amide is not decorative. This peptide belongs to the RFamide family, defined by an amidated arginine-phenylalanine C-terminus, and alanine-scanning work shows that phenylalanine 6, arginine 9 and phenylalanine 10 are critical and that alteration of the amidated C-terminus is not tolerated. A species note matters for cross-referencing: the human peptide ends in arginine-phenylalanine amide, while the rat, mouse, zebrafish and Xenopus orthologues end in arginine-tyrosine amide, so a rodent sequence is not interchangeable with this one.

The molecular target is KISS1R, also called GPR54, a 398-residue G-protein-coupled receptor that couples predominantly through Gq/11 to phospholipase C, inositol trisphosphate and intracellular calcium, with reported downstream protein kinase C, ERK1/2 and PI3K-Akt signalling. Reported affinities in CHO-K1 cells expressing the human receptor are in the low nanomolar range, with calcium-mobilisation half-maximal concentrations around one to four nanomolar, although at least one study using rat receptor in the same cell background reports values an order of magnitude weaker; a quoted number should always carry its assay system.

Selectivity is not absolute, and this is a useful design caution. The peptide has been reported to bind and activate the neuropeptide FF receptors NPFFR1 and NPFFR2 with appreciable potency, and the cross-reactivity is one-directional, so an experiment in a preparation expressing those receptors cannot attribute a response to KISS1R without a selective antagonist or knockdown control.

Its stability profile is specific and well documented. Matrix metalloproteinases 2 and 9 and several membrane-type metalloproteinases cleave the glycine-leucine bond at positions 7 and 8 of the decapeptide, abolishing activity, and the intact peptide has been reported completely digested within an hour in murine serum; non-hydrolysable isosteres of that bond were designed specifically to confer serum stability. Published preparation practice is dissolution in ultrapure water to a stock and dilution into buffer immediately before use, with identity confirmed by reversed-phase HPLC and electrospray mass spectrometry after Fmoc solid-phase synthesis.