Rational design of receptor-specific variants of human growth hormone

Human growth hormone (hGH) binds to both the growth hormone (GH) and the prolactin (PRL) receptors. Competition experiments followed by mutational analysis show that the epitope on hGH for hPRL receptor consists of strong determinants in the middle of helix 1 (comprising residues His-18, His-21, and Phe-25), a loop region (including Ile-58, Ser-62, and Asn-63), and the central portion of helix 4 (containing residues Arg-167, Lys-168, Lys-172, Glu-174, Phe-176, and Arg-178). When these residues are mapped on a structural model of hGH, they form a patch that overlaps but is not identical to that previously determined for the hGH receptor. Three of these side chains (His-18, His-21, and Glu-174) are ligands for binding Zn2+, which is required for high-affinity hGH-hPRL receptor complex formation. By introducing pairs of mutations into hGH that exploit the zinc dependency for hPRL receptor binding and remove side chains in the nonoverlapping regions, we have shifted the binding preference toward the hGH receptor by a factor of 34,000 or toward the hPRL receptor by a factor of 150 without substantial loss in binding affinity for the preferred receptor. The energetic effects of the individual mutations are additive within the double mutants, suggesting that each functions independently and does not introduce gross perturbations in structure. Such receptor-selective variants of hGH should be useful molecular probes to link specific receptor binding events to the various biological activities of hGH.