Yi Soojin, Bernat Bryan, Pál Gábor, Kossiakoff Anthony, Li Wen-Hsiung
Department of Ecology and Evolution, University of Chicago, Chicago, Illinois 60637, USA.
Mol Biol Evol. 2002 Jul;19(7):1083-92. doi: 10.1093/oxfordjournals.molbev.a004166.
Primate growth hormone (GH) has evolved rapidly, having undergone approximately 30% amino acid substitutions from the inferred ancestral eutherian sequence. Nevertheless, human growth hormone (hGH) is physiologically effective when administered to nonprimate mammals. In contrast, its functional counterpart, the human growth hormone receptor (hGHR), has evolved species specificity so that it responds only to Old World primate GHs. It has been proposed that this species specificity of the hGHR is largely caused by the Leu --> Arg change at position 43 after a prior His --> Asp change at position 171 of the GH. Sequence analyses supported this hypothesis and revealed that the transitional phase in the GH:GHR coevolution still persists in New World monkeys. For example, although the GH of the squirrel monkey has the His --> Asp substitution at position 171, residue 43 of its GHR is a Leu, the nonprimate residue. If the squirrel monkey truly represents an intermediate stage of GH:GHR coevolution, its GHR should respond to both hGH and nonprimate GH. Also, if the emergence of species specificity was a result of the selection for a more efficient GH:GHR interaction, then changing residue 43 of the squirrel monkey growth hormone receptor (smGHR) to Arg should increase its binding affinity toward higher primate GH. To test these hypotheses, we performed protein-binding assays between the smGHR and both human and rat GHs, using the surface plasmon resonance methodology. Furthermore, the effects of reciprocal mutations at position 43 of human and squirrel monkey GHRs are measured for their binding affinities toward human and squirrel monkey GHs. The results from the binding kinetic assays clearly demonstrate that the smGHR is in the intermediate state of the evolution of species specificity. Interestingly, the altered residue Arg at position 43 of the smGHR does not lead to an increased binding affinity. The implications of these results on the evolution of the GH:GHR interaction and on functional evolution are discussed.
灵长类动物生长激素（GH）进化迅速，与推测的原始真兽类序列相比，氨基酸替换率约为30%。然而，人类生长激素（hGH）在给予非灵长类哺乳动物时具有生理活性。相比之下，其功能对应物人类生长激素受体（hGHR）具有物种特异性，因此仅对旧世界灵长类动物的GH有反应。有人提出，hGHR的这种物种特异性很大程度上是由GH第171位的His→Asp变化后第43位的Leu→Arg变化引起的。序列分析支持了这一假设，并表明GH:GHR共同进化的过渡阶段在新世界猴中仍然存在。例如，虽然松鼠猴的GH在第171位有His→Asp替换，但其GHR的第43位残基是Leu，即非灵长类动物的残基。如果松鼠猴真的代表了GH:GHR共同进化的中间阶段，其GHR应该对hGH和非灵长类动物的GH都有反应。此外，如果物种特异性的出现是选择更有效GH:GHR相互作用的结果，那么将松鼠猴生长激素受体（smGHR）的第43位残基改为Arg应该会增加其对高等灵长类动物GH的结合亲和力。为了验证这些假设，我们使用表面等离子体共振方法进行了smGHR与人类和大鼠GH之间的蛋白质结合试验。此外，还测量了人类和松鼠猴GHR第43位相互突变对其与人类和松鼠猴GH结合亲和力的影响。结合动力学试验结果清楚地表明，smGHR处于物种特异性进化的中间状态。有趣的是，smGHR第43位改变后的残基Arg并没有导致结合亲和力增加。讨论了这些结果对GH:GHR相互作用进化和功能进化的影响。
