Muyan M, Furuhashi M, Sugahara T, Boime I
Department of Molecular Biology & Pharmacology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Mol Endocrinol. 1996 Dec;10(12):1678-87. doi: 10.1210/mend.10.12.8961276.
One of the major structural differences between the LH beta and CG beta subunits is the carboxy-terminal region: beyond amino acid 114, LH beta has a hydrophobic heptapeptide stretch, while CG beta contains a 31-amino acid hydrophilic carboxy-terminal peptide (CTP) that is O-glycosylated. The CG beta subunit is secreted quantitatively as a monomer and assembles efficiently whereas secretion and assembly of LH beta is inefficient. We previously implicated the carboxy-terminal heptapeptide as a determinant for the different intracellular behavior manifested by the LH beta subunit compared with the CG beta subunit. Here we tested the function of the heptapeptide and CTP domains by fusing them to their counterparts at amino acid 114 of CG beta or LH beta subunits. The secretion and assembly of these chimeras were examined in transfected Chinese hamster ovary cells. Removal of the heptapeptide enhanced the amount of LH beta subunit secreted 4-fold compared with intact LH beta. Fusion of this heptapeptide to CG beta 114, i.e. CG beta lacking the CTP, decreased the amount of secreted subunit 2-fold compared with wild type human CG beta. Similar experiments reveal that although deleting the CTP from the CG beta subunit did not significantly alter secretion, the combination efficiency of the truncated subunit was reduced to 60%. Perturbing the native carboxy-terminal sequence of either subunit increased the heterogeneity of the secreted forms. This result suggests that these regions are also involved in the posttranslational processing of the asparagine-linked oligosaccharides of the beta-subunits. Fusion of the LH beta heptapeptide to the truncated CG beta subunit decreased combination with the alpha-subunit. These data further support the hypothesis that the carboxy-terminal regions of LH beta and CG beta subunits play a role in the intracellular behavior of the corresponding heterodimers.
促黄体生成素β亚基（LHβ）和绒毛膜促性腺激素β亚基（CGβ）之间的主要结构差异之一在于羧基末端区域：在第114位氨基酸之后，LHβ有一段疏水性七肽序列，而CGβ含有一个31个氨基酸的亲水性羧基末端肽（CTP），该肽发生了O-糖基化。CGβ亚基以单体形式定量分泌并能高效组装，而LHβ的分泌和组装效率较低。我们之前认为羧基末端七肽是导致LHβ亚基与CGβ亚基表现出不同细胞内行为的决定因素。在此，我们通过将七肽和CTP结构域分别与CGβ或LHβ亚基第114位氨基酸处的对应结构域融合，来测试它们的功能。在转染的中国仓鼠卵巢细胞中检测了这些嵌合体的分泌和组装情况。与完整的LHβ相比，去除七肽后LHβ亚基的分泌量增加了4倍。将该七肽融合到CGβ114（即缺乏CTP的CGβ）上，与野生型人CGβ相比，分泌亚基的量减少了2倍。类似实验表明，虽然从CGβ亚基中删除CTP并没有显著改变分泌情况，但截短亚基的组装效率降低到了60%。干扰任一亚基的天然羧基末端序列都会增加分泌形式的异质性。这一结果表明，这些区域也参与了β亚基天冬酰胺连接寡糖的翻译后加工过程。将LHβ七肽融合到截短的CGβ亚基上会降低其与α亚基的组装。这些数据进一步支持了以下假说：LHβ和CGβ亚基的羧基末端区域在相应异二聚体的细胞内行为中发挥作用。
