Contulakin-G, an O-glycosylated invertebrate neurotensin

We have purified contulakin-G, a 16-amino acid O-linked glycopeptide (pGlu-Ser-Glu-Glu-Gly-Gly-Ser-Asn-Ala-Thr-Lys-Lys-Pro-Tyr-Ile-Leu-OH, pGlu is pyroglutamate) from Conus geographus venom. The major glycosylated form of contulakin-G was found to incorporate the disaccharide β-D-Galp-(1→3)-α- D-GalpNAc-(1→) attached to Thr¹⁰. The C-terminal sequence of contulakin-G shows a high degree of similarity to the neurotensin family of peptides. Synthetic peptide replicates of Gal(β→3) GalNAc(α→)Thr¹⁰ contulakin-G and its nonglycosylated analog were prepared using an Fmoc (9- fluorenylmethoxycarbonyl) protected solid phase synthesis strategy. The synthetic glycosylated contulakin-G, when administered intracerebroventricular into mice, was found to result in motor control- associated dysfunction observed for the native peptide. Contulakin-G was found to be active at 10-fold lower doses than the nonglycosylated Thr¹⁰ contulakin-G analog. The binding affinities of contulakin-G and the nonglycosylated Thr¹⁰ contulakin-G for a number of neurotensin receptor types including the human neurotensin type 1 receptor (hNTR1), the rat neurotensin type 1 and type 2 receptors, and the mouse neurotensin type 3 receptor were determined. The binding affinity of the nonglycosylated Thr¹⁰ contulakin-G was approximately an order of magnitude lower than that of neurotensin_1-13 for all the receptor types tested. In contrast, the glycosylated form of contulakin-G exhibited significantly weaker binding affinity for all of the receptors tested. However, both contulakin-G and nonglycosylated Thr¹⁰ contulakin-G were found to be potent agonists of rat neurotensin receptor type 1. Based on these results, we conclude that O- linked glycosylation appears to be a highly unusual strategy for increasing the efficacy of toxins directed against neurotransmitter receptors.