Kudo M, Chen T, Nakabayashi K, Hsu S Y, Hsueh A J
Department of Gynecology and Obstetrics, Stanford University School of Medicine, California 94305-5317, USA.
Mol Endocrinol. 2000 Feb;14(2):272-84. doi: 10.1210/mend.14.2.0422.
The receptors for LH, FSH, and TSH belong to the large G protein-coupled, seven-transmembrane protein family and are unique in having a large N-terminal extracellular (ecto-) domain containing leucine-rich repeats important for interactions with the large glycoprotein hormone ligands. Recent studies indicated the evolution of an expanding family of homologous leucine-rich repeat-containing, G protein-coupled receptors (LGRs), including the three known glycoprotein hormone receptors; mammalian LGR4 and LGR5; and LGRs in sea anemone, fly, and snail. We isolated nematode LGR cDNA and characterized its gene from the Caenorhabditis elegans genome. This receptor cDNA encodes 929 amino acids consisting of a signal peptide for membrane insertion, an ectodomain with nine leucine-rich repeats, a seven-TM region, and a long C-terminal tail. The nematode LGR has five potential N-linked glycosylation sites in its ectodomain and multiple consensus phosphorylation sites for protein kinase A and C in the cytoplasmic loop and C tail. The nematode receptor gene has 13 exons; its TM region and C tail, unlike mammalian glycoprotein hormone receptors, are encoded by multiple exons. Sequence alignments showed that the TM region of the nematode receptor has 30% identity and 50% similarity to the same region in mammalian glycoprotein hormone receptors. Although human 293T cells expressing the nematode LGR protein do not respond to human glycoprotein hormones, these cells exhibited major increases in basal cAMP production in the absence of ligand stimulation, reaching levels comparable to those in cells expressing a constitutively activated mutant human LH receptor found in patients with familial male-limited precocious puberty. Analysis of cAMP production mediated by chimeric receptors further indicated that the ectodomain and TM region of the nematode LGR and human LH receptor are interchangeable and the TM region of the nematode LGR is responsible for constitutive receptor activation. Thus, the identification and characterization of the nematode receptor provides the basis for understanding the evolutionary relationship of diverse LGRs and for future analysis of mechanisms underlying the activation of glycoprotein hormone receptors and related LGRs.
促黄体生成素（LH）、促卵泡生成素（FSH）和促甲状腺激素（TSH）的受体属于大型G蛋白偶联的七跨膜蛋白家族，其独特之处在于具有一个大的N端细胞外（胞外）结构域，该结构域含有富含亮氨酸的重复序列，对与大型糖蛋白激素配体的相互作用很重要。最近的研究表明，一个不断扩大的同源富含亮氨酸重复序列的G蛋白偶联受体（LGR）家族在进化，包括三种已知的糖蛋白激素受体；哺乳动物的LGR4和LGR5；以及海葵、果蝇和蜗牛中的LGR。我们从秀丽隐杆线虫基因组中分离出线虫LGR cDNA并对其基因进行了表征。该受体cDNA编码929个氨基酸，包括一个用于膜插入的信号肽、一个具有九个富含亮氨酸重复序列的胞外结构域、一个七跨膜区域和一个长的C端尾巴。线虫LGR在其胞外结构域有五个潜在的N-糖基化位点，在细胞质环和C尾巴中有多个蛋白激酶A和C的共有磷酸化位点。线虫受体基因有13个外显子；与哺乳动物糖蛋白激素受体不同，其跨膜区域和C尾巴由多个外显子编码。序列比对显示，线虫受体的跨膜区域与哺乳动物糖蛋白激素受体的同一区域有30%的同一性和50%的相似性。虽然表达线虫LGR蛋白的人293T细胞对人糖蛋白激素无反应，但这些细胞在没有配体刺激的情况下基础cAMP产生大幅增加，达到与表达家族性男性性早熟患者中发现的组成型激活突变型人LH受体的细胞相当的水平。对嵌合受体介导的cAMP产生的分析进一步表明，线虫LGR和人LH受体的胞外结构域和跨膜区域是可互换的，线虫LGR的跨膜区域负责受体的组成型激活。因此，线虫受体的鉴定和表征为理解不同LGR的进化关系以及未来分析糖蛋白激素受体和相关LGR激活的潜在机制提供了基础。
