Abstract
Pituitary adenylate-cyclase-activating polypeptide (PACAP), a novel brain-gut hormone, was isolated from ovine hypothalami and represents the latest mammalian member of the secretin-glucagon peptide family. PACAP exists in two C-terminally amidated molecular forms, PACAP(1-27) and PACAP(1-38), comprising 27 or 38 amino acid residues, respectively. In order to identify a specific receptor for PACAP, we studied binding of 125I-labelled PACAP(1-27) to plasma membranes from rat brain. We identified a single high-affinity binding site (Kd, 340 pM and Bmax, 3.34 pmol/mg), specific for synthetic PACAP(1-38) and PACAP(1-27). Hormone binding was reversible and time, protein and temperature dependent. In contrast, neither the analogues PACAP(1-23), PACAP(18-38) and PACAP(3-25), nor vasoactive intestinal peptide (VIP), secretin and growth-hormone-releasing factor (GRF) revealed significant binding at concentrations up to 1 microM. A specific receptor protein, with an apparent molecular mass of 60 kDa, was identified by means of affinity cross-linking with disuccinimidyl suberate (DSS) and ethylene glycol disuccinimidyl suberate (EGS). PACAP receptors are associated with a GTP-binding protein as determined by the influence of different nucleotides on PACAP binding. PACAP-binding activity was solubilized with the detergents 3-[(3-cholamidopropyl)dimethylammonio]2-hydroxy-1-propane sulfonate (Chapso) or Triton X-100 and was characterized as a high-molecular-mass receptor complex (400 kDa) by non-reducing size-exclusion chromatography on Sepharose CL-6B. These data imply the following: high-affinity PACAP receptors are expressed abundantly on rat-brain plasma membranes; PACAP receptors are specific for PACAP and show no affinity for VIP, secretin and GRF; the PACAP receptor molecule has an apparent molecular mass of 60 kDa; the PACAP receptor complex is associated with a GTP-binding protein.
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