Alkaline phosphatase-somatostatin hybrid proteins as probes for somatostatin-14 receptors.

By inserting appropriate peptide ligands into surface loops on globular proteins, we expect to develop probes for the location, accessibility, and steric and electrostatic environment of these ligand-binding sites on their membrane-bound receptors. Three residues in a loop on the surface of E. coli alkaline phosphatase were substituted by an 18-residue peptide containing the receptor-binding segment of somatostatin-14 without significantly affecting the catalytic properties of the enzyme. This hybrid protein was then used to investigate the ligand-binding site of somatostatin receptors. Tryptic cleavage of the hybrid protein within the inserted sequence, and binding of the hybrid protein to antisomatostatin antibodies demonstrated the surface accessibility of the guest peptide. Both the wild-type enzyme and the hormone-enzyme hybrid displaced 125I-labeled somatostatin from rat brain membrane receptors only at high concentrations. However, chemical cationization of the hybrid protein, which again did not disturb the phosphatase activity, enhanced its receptor-binding potency to a level only 23 times lower than that of somatostatin itself and 280 times higher than that of the cationized wild-type protein. This alkaline phosphatase/somatostatin hybrid protein appears, therefore, to be a suitable starting point for the development of probes for the steric and electrostatic environment of the ligand-binding site of somatostatin receptors.