Helman D, Herman A, Paly J, Livnah O, Elkins P A, de Vos A M, Djiane J, Gertler A
Institute of Biochemistry, Food Science and Nutrition, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, PO Box 12, Rehovot 76100, Israel.
J Endocrinol. 2001 Apr;169(1):43-54. doi: 10.1677/joe.0.1690043.
The biological activities of ovine (o) and bovine (b) placental lactogens (PLs) and their mutated analogues were compared using several binding and in vitro bioassays. In almost all cases, the biological activities of these analogues mediated through rat (r) prolactin receptor (PRLR) showed little or no change, despite a remarkable decrease in their capacity to bind to the extracellular domain of rPRLR and despite compromised stability of the 2:1 complexes. These results indicate that mutations impairing the ability of oPL or bPL to form stable complexes with lactogenic receptors do not necessarily lead to a decrease in the biological activity, because the transient existence of the homodimeric complex is still sufficient to initiate the signal transduction. In contrast, oPL and bPL analogues completely, or almost completely, lost their ability to activate homologous PRLRs, and some of them even acted as site-2 antagonists. To explain the difference between the activity transduced through homologous and that transduced through heterologous PRLRs, we propose the novel term 'minimal time of homodimer persistence'. This concept assumes that in order to initiate the signal transduction, the associated kinase JAK2 has to be transphosphorylated and this requires a 'minimal time' of homodimer existence. In the case of homologous interaction between ruminant PLs and homologous PRLRs, this 'minimal time' is met, though the interaction with homologous PRLRs has a shorter half-life than that with heterologous PRLRs. Therefore oPL or bPL are active in cells possessing both homologous and heterologous PRLRs. Mutations of oPL or bPL lead to reduced affinity and, consequently, the 'time of homodimer persistence' is shortened. Although in the case of heterologous interaction the 'minimal time' is still sufficient to initiate the biological activity, in homologous interactions, which are already weaker than heterologous interactions, further destabilization of the complex shortens its persistence to below the 'minimal time', leading to full or partial loss of biological activity.
利用多种结合试验和体外生物测定法，比较了绵羊（o）和牛（b）胎盘催乳素（PL）及其突变类似物的生物活性。在几乎所有情况下，尽管这些类似物与大鼠（r）催乳素受体（PRLR）胞外域的结合能力显著下降，且其二聚体复合物的稳定性受损，但通过rPRLR介导的这些类似物的生物活性几乎没有变化。这些结果表明，损害oPL或bPL与催乳素受体形成稳定复合物能力的突变不一定会导致生物活性降低，因为同二聚体复合物的短暂存在仍足以启动信号转导。相比之下，oPL和bPL类似物完全或几乎完全丧失了激活同源PRLR的能力，其中一些甚至起到了2型位点拮抗剂的作用。为了解释通过同源PRLR和异源PRLR转导的活性之间的差异，我们提出了“同二聚体持续存在的最短时间”这一新术语。这一概念假定，为了启动信号转导，相关激酶JAK2必须被转磷酸化，这需要同二聚体存在一段“最短时间”。在反刍动物PL与同源PRLR的同源相互作用中，尽管与同源PRLR的相互作用半衰期比与异源PRLR的相互作用短，但仍满足这一“最短时间”。因此，oPL或bPL在同时拥有同源和异源PRLR的细胞中具有活性。oPL或bPL的突变导致亲和力降低，因此“同二聚体持续存在的时间”缩短。尽管在异源相互作用中，“最短时间”仍足以启动生物活性，但在已经比异源相互作用弱的同源相互作用中，复合物的进一步不稳定会将其持续存在时间缩短至“最短时间”以下，导致生物活性完全或部分丧失。
