Amino Acids in Highly Conserved Regions Near the C-Terminus of Rat Prolactin (PRL) Play Critical Roles Similar to Those in Binding of Human GH to the PRL Receptor

Abstract

The fourth helix (helix 4) in the human growth hormone (hGH) molecule plays a role in binding to the GH receptor as well as the PRL receptor through topologically different amino acids. This study investigated the function of amino acids in the predicted helix 4 of rat prolactin (rPRL) using site-directed mutagenesis. Twenty mutants for 7 amino acid residues were expressed in COS-1 cells and their receptor binding and Nb2 proliferation activities were assayed. It was found that R174 (R at residue number 174), K179, and D181 are indispensable for PRL function, while D176, S177, and C189 are important to some extent. When these results were compared with those reported for binding of hGH to its receptors, the binding of PRL to the PRL receptor was shown to involve amino acids topologically similar to those in the binding of hGH to the PRL receptor, rather than those in the binding of hGH to the GH receptor.