KHEYLRF-NH(2) (AF2) is the most abundant FMRFamide-related peptide (FaRP) in Ascaris suum and also in many other parasitic and free-living nematodes. The AF2 abundance in the highly diverse nematodes and its potent and profound effects on the neuromuscular systems make AF2 and its receptor(s) very attractive targets for the discovery of novel broad-spectrum anthelmintics. Although FaRP receptors are believed to belong to the large family of G-protein coupled receptors (GPCRs), to date no AF2 receptor(s) have been cloned so there is no final proof to show that they are indeed G-protein coupled. In this study, using A. suum body wall muscle membranes, we showed that: (1) AF2 effectively (EC(50) 57 nM) induced a dose-dependent stimulation of [35S]GTP gamma S binding to the membranes, which is a hallmark of G-protein activation; (2) the high affinity binding of [125I-Tyr(4)]AF2 was inhibited in a dose-dependent manner by GTP with a K(i) of 10.5 nM (so-called guanine nucleotide effect, characteristic for GPCRs). Collectively, our results provide direct evidence for G-protein involvement in AF2-triggered receptor activation and thus confirm that the receptor for AF2 in A. suum is a GPCR.