Wu H, Lustbader J W, Liu Y, Canfield R E, Hendrickson W A
Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032.
Structure. 1994 Jun 15;2(6):545-58. doi: 10.1016/s0969-2126(00)00054-x.
Human chorionic gonadotropin (hCG) is a placental hormone that stimulates secretion of the pregnancy-sustaining steroid progesterone. It is a member of a family of glycoprotein hormones that are disulfide-rich heterodimers, with a common alpha-chain and distinctive beta-chains specific to their particular G-protein linked receptors.
We have produced recombinant hCG in mammalian cells as the selenomethionyl protein, and have determined its structure (after partial deglycosylation) at 2.6 A resolution from multiwavelength anomalous diffraction (MAD) measurements. Despite only limited sequence similarity (10% identity), the alpha- and beta-subunits of hCG have similar tertiary folds. Each subunit has a cystine-knot motif at its core of extended hairpin loops. There is a very extensive subunit interface featuring two inter-chain beta-sheets and a unique, disulfide-tethered 'arm' from the beta-subunit which 'embraces' the alpha-subunit. The carboxy-terminal peptide of the beta-subunit, which is rich in O-linked sugars, is disordered.
Structural and sequence comparisons indicate an evolutionary homology, albeit remote, between the glycoprotein hormone chains and other cystine-knot proteins, notably platelet-derived growth factor. Segments of the alpha- and beta-chains that have been convincingly implicated in receptor binding by hCG are juxtaposed on one side of the molecule. A glycosylation site implicated in signal transduction but not in binding is also close to the presumed binding site suggesting a possible coupling between ligand binding and signaling. This study with selenomethionyl protein produced in mammalian cells extends the realm of MAD phasing.
人绒毛膜促性腺激素（hCG）是一种胎盘激素，可刺激维持妊娠的类固醇孕酮的分泌。它是糖蛋白激素家族的成员，这些糖蛋白激素是富含二硫键的异二聚体，具有共同的α链和特定于其特定G蛋白偶联受体的独特β链。
我们在哺乳动物细胞中产生了重组hCG作为硒代蛋氨酸蛋白，并通过多波长反常衍射（MAD）测量在2.6埃分辨率下确定了其结构（部分去糖基化后）。尽管hCG的α亚基和β亚基只有有限的序列相似性（10%的同一性），但它们具有相似的三级结构折叠。每个亚基在其延伸发夹环的核心处都有一个胱氨酸结基序。有一个非常广泛的亚基界面，其特征是两条链间β折叠片层以及β亚基伸出的一个独特的、由二硫键连接的“臂”，该“臂”“环抱”着α亚基。β亚基富含O-连接糖的羧基末端肽是无序的。
结构和序列比较表明，糖蛋白激素链与其他胱氨酸结蛋白（尤其是血小板衍生生长因子）之间存在进化同源性，尽管这种同源性较为遥远。hCG中已被证实与受体结合有关的α链和β链片段并列于分子的一侧。一个与信号转导而非结合有关的糖基化位点也靠近推测的结合位点，这表明配体结合与信号转导之间可能存在偶联。这项对哺乳动物细胞中产生的硒代蛋氨酸蛋白的研究扩展了MAD相位分析的领域。
