Bernard M P, Myers R V, Moyle W R
Department of OBGYN, Robert Wood Johnson (Rutgers) Medical School, 675 Hoes Lane, Piscataway, NJ 08854, USA.
Biochem J. 1998 Nov 1;335 ( Pt 3)(Pt 3):611-7. doi: 10.1042/bj3350611.
Human chorionic gonadotropin (hCG) and bovine lutropin (bLH), a hormone chemically more similar to most mammalian lutropins than hCG, interact with the extracellular domains of their gonadal lutropin receptors (LHRs). These portions of the rat and human LHRs are 85% identical and both receptors bind hCG with high, albeit not identical, affinity. However, at least 1000-fold more bLH is required to inhibit binding of radiolabelled hCG to the human LHR than to the rat LHR, a phenomenon that proved useful for identifying regions of the extracellular domain that contact lutropins. Previous studies using truncated receptors and lutropin/follitropin receptor chimaeras localized most, if not all, high-affinity ligand contacts to the N-terminal three-fifths of the rat LHR extracellular domain. We report here that 10-fold more bLH was needed to inhibit binding of labelled hCG to rat/human LHR chimaeras containing the N-terminal three-fifths of the human LHR extracellular domain than to the rat LHR. Unexpectedly, 100-fold more bLH was required to inhibit binding of labelled hCG to chimaeras containing the C-terminal one-fifth of the human LHR extracellular domain than to the rat LHR. The ability of the C-terminal portion of the human LHR extracellular domain to inhibit bLH binding suggests this region of the receptor also contacts the ligand even though it is not needed for ligand binding. The extracellular domains of all the glycoprotein hormone receptors are thought to be horseshoe-shaped, a consequence of their leucine-rich repeat motifs. Portions of the ligand that become located within the cavity created by the concave surface of the horseshoe would have the opportunity to contact residues in the C-terminal portion of the extracellular domain. Changes to the ligand or receptor that influence this interaction would be expected to alter binding and confound efforts to identify residues in key ligand-receptor contacts.
人绒毛膜促性腺激素(hCG)和牛促黄体素(bLH),一种在化学结构上比hCG更类似于大多数哺乳动物促黄体素的激素,与它们的性腺促黄体素受体(LHRs)的细胞外结构域相互作用。大鼠和人LHRs的这些部分有85%的同一性,并且两种受体都以高亲和力(尽管不完全相同)结合hCG。然而,抑制放射性标记的hCG与人LHR结合所需的bLH比与大鼠LHR结合所需的bLH至少多1000倍,这一现象被证明有助于确定细胞外结构域中与促黄体素接触的区域。先前使用截短受体和促黄体素/促卵泡素受体嵌合体的研究将大部分(如果不是全部)高亲和力配体接触定位到大鼠LHR细胞外结构域的N端五分之三。我们在此报告,抑制标记的hCG与包含人LHR细胞外结构域N端五分之三的大鼠/人LHR嵌合体结合所需的bLH比与大鼠LHR结合所需的bLH多10倍。出乎意料的是,抑制标记的hCG与包含人LHR细胞外结构域C端五分之一的嵌合体结合所需的bLH比与大鼠LHR结合所需的bLH多100倍。人LHR细胞外结构域C端部分抑制bLH结合的能力表明,受体的这一区域也与配体接触,尽管配体结合不需要它。所有糖蛋白激素受体的细胞外结构域被认为是马蹄形的,这是其富含亮氨酸重复基序的结果。位于马蹄形凹面形成的腔内的配体部分将有机会接触细胞外结构域C端部分的残基。预期影响这种相互作用的配体或受体的变化会改变结合,并混淆识别关键配体-受体接触中残基的努力。