The species specificity of growth hormone requires the cooperative interaction of two motifs

Abstract

Primate growth hormones (GH) activate both primate and non-primate somatotrophic receptors (GH receptors), but non-primate GHs do not activate primate GH receptors. Previous studies argued the interaction of Asp171 of human GH and Arg43 of the receptor produced an attractive ionic interaction. In non-primate GHs, His170 replaces the homologous Asp171, producing a repulsive interaction with Arg43 of the primate receptor which was believed to reduce the attraction of non-primate GH for the human GH receptor, thus providing species specificity. In this report, H170D bovine GH had activity and affinity for human GH receptors approaching those of human GH. In contrast, replacing Asp171 of human GH with His did not significantly reduce somatotrophic activity, indicating that species specificity is not wholly explained by this residue’s interaction with Arg43 of the receptor. Deletion of either Phe44 (a residue present only in primate GHs) or residues 32–46 (20-kDa form of human GH) each only marginally reduced somatotrophic activities. But the combination of the D171H mutation with either ΔPhe44 or Δ32–46 in human GH reduced binding and activity in a greater than additive fashion, indicated a functional interaction between these distant structural features. In bovine GH addition of phenylalanine at position 44 increased the somatotrophic activity and receptor affinity in cells containing the human GH receptor. The combination of the H170D mutation and the addition of phenylalanine at position 44 created a bovine GH with activity indistinguishable from wild-type human GH. Based on evidence from both bovine and human GHs, the cooperative interaction of these two distant motifs determined the species specificity and indicated that structural plasticity was a critical feature necessary for the species specificity of somatotrophic activity.