The polypeptide PHI discriminates a GTP-insensitive form of vip receptor in livermembranes

Abstract

In early reports on125I-VIP binding experiments in liver membranes, it has been proposed that, the VIP binding sites were partially sensitive to GTP. Here we confirm that the VIP binding sites of chicken liver membranes consisted mainly in bivalent VIP/PACAP receptors and that about 50% of the125I-VIP binding capacity was not affected by the GTP analogue GppNHp. Part of these bivalent receptors also appeared to represent PHI binding sites. In GppNHp-treated membranes, the GTP-insensitive VIP binding sites displayed a 17-fold higher relative affinity than in control membranes for the VIP analogue PHI. Such data suggested that GTP-insensitive VIP receptors may correspond to a subclass of high-affinity PHI receptors. Cross-linking of125I-VIP or125I-PHI to their receptors, revealed 2 components of 48 and 60kDa. The radiolabelling of the 60kDa component was strongly affected by increasing concentrations of the GTP analogue but was modestly abolished by an excess of PHI. Conversely, the radiolabelling of the 48kDa molecular form was not affected by the GTP analogue but was efficiently abolished by increasing concentrations of PHI. Taken together, the data suggest that the 48kDa component expressed in chicken liver membranes display the properties of a GTP-insensitive VIP/PHI receptor that can be pharmacologically discriminated from the GTP-sensitive 60kDa form, through its much higher affinity for PHI.