Beau Isabelle, Groyer-Picard Marie-Thérèse, Desroches Agnès, Condamine Eric, Leprince Jérôme, Tomé Jean-Philippe, Dessen Philippe, Vaudry Hubert, Misrahi Micheline
Institut National de la Santé et de la Recherche Médicale E120, Bâtiment Grégory Pincus, 80 rue du Général Leclerc, 94275 Le Kremlin Bicêtre, France.
Mol Endocrinol. 2004 Mar;18(3):733-46. doi: 10.1210/me.2003-0130. Epub 2003 Dec 23.
The mechanisms of the basolateral targeting of G protein-coupled receptors remain largely unknown. Mutagenesis experiments have allowed us to identify the basolateral sorting signals of the TSH and LH receptors expressed in Madin-Darby canine kidney cells and thyroid follicular FRT cells. Unexpectedly these signals (amino acids 731-746 and 672-689, respectively) share an unusual localization in the distal part of the intracellular domain of the receptors at a marked distance from the membrane. When grafted onto the p75-neurotropin receptor, these signals redirect this normally apically expressed protein to the basolateral cell surface. They are independent of the endocytosis signal. The basolateral sorting signals of TSH, LH, and FSH receptors do not exhibit primary sequence homology with each other or with any other known signal. Furthermore, circular dichroism studies show that the three signals exhibit distinct secondary structures. The TSH receptor has a stable helical structure, the LH receptor has both helix and beta-sheet structures, and the FSH receptor sorting signal has a main random coil structure. This means that even in closely-related receptors different secondary structures can be found for basolateral signals unrelated to internalization signals. This observation contrasts with what is known about basolateral signals related to internalization signals for which a common beta-turn structure has been described. Deletion of the basolateral sorting signals results in apical targeting of the receptors, suggesting the existence of apical sorting information. However, a soluble form of the TSH receptor, which harbors all N- and putative O-linked oligosaccharides, is secreted in a nonpolarized fashion. This implies that apical sorting information must be located elsewhere, either in the transmembrane or in the intracellular domains of the receptor.
G蛋白偶联受体基底外侧靶向的机制在很大程度上仍然未知。诱变实验使我们能够确定在Madin-Darby犬肾细胞和甲状腺滤泡FRT细胞中表达的促甲状腺激素（TSH）和促黄体激素（LH）受体的基底外侧分选信号。出乎意料的是，这些信号（分别为氨基酸731 - 746和672 - 689）在受体细胞内结构域的远端有一个不寻常的定位，与膜有明显距离。当嫁接到p75 - 神经营养蛋白受体上时，这些信号将这种通常顶端表达的蛋白重新导向基底外侧细胞表面。它们独立于内吞信号。TSH、LH和促卵泡激素（FSH）受体的基底外侧分选信号彼此之间或与任何其他已知信号均无一级序列同源性。此外，圆二色性研究表明，这三个信号具有不同的二级结构。TSH受体具有稳定的螺旋结构，LH受体具有螺旋和β折叠结构，FSH受体分选信号具有主要的无规卷曲结构。这意味着即使在密切相关的受体中，与内化信号无关的基底外侧信号也能发现不同的二级结构。这一观察结果与已知的与内化信号相关的基底外侧信号形成对比，后者已被描述为具有共同的β转角结构。基底外侧分选信号的缺失导致受体顶端靶向，提示存在顶端分选信息。然而，携带所有N - 连接和假定的O - 连接寡糖的TSH受体可溶性形式以非极化方式分泌。这意味着顶端分选信息必定位于其他地方，要么在受体的跨膜结构域，要么在细胞内结构域。
