Shi Y, Zou M, Ahring P, Al-Sedairy S T, Farid N R
Molecular Endocrinology Laboratory, Department of Biological and Medical Research, King Faisal Specialist Hospital and Research Center, 11211, Riyadh, Saudi Arabia.
Endocrine. 1995 Jun;3(6):409-14. doi: 10.1007/BF02935645.
The thyrotropin (TSH) receptor is a member of G protein-coupled seven-transmembrane-segment receptors. It is characterized by a large extracellular domain linked to the seven transmembrane segments and ending with a cytoplasmic tail. Sequence alignment shows that a highly conserved motif, NPXXY where X is any amino acid, exists at the boundary between the seventh transmembrane domain and proximal part of the cytoplasmic tail of virtually all G protein-coupled receptors. This motif has been implicated as an internalization signal for several cell surface receptors, such as the low density lipoprotein (LDL), insulin and insulin-like growth factor-1 (IGF-1) receptors. The potential effects of this motif on the TSH receptor signal transduction and receptor-mediated TSH internalization was analysed by replacement of the tyrosine(678) residue with an alanine residue. This mutation does not impair high affinity TSH binding, but completely abolishes the ability of cAMP response upon TSH stimulation. It also significantly reduces TSH internalization. The role of the cytoplasmic tail of the TSH receptor in receptor-mediated internalization was also assessed. Deletion of up to 56 amino acids from the C-terminus of the cytoplasmic tail enhances TSH internalization as compared to the wild-type receptor. We conclude that tyrosine(678) in the NPXXY motif is required for efficient receptor-mediated TSH internalization and G protein coupling. The cytoplasmic tail of the TSH receptor may contain sequence domains which could modulate the effects of the NPXXY internalization signal.
促甲状腺激素（TSH）受体是G蛋白偶联的七跨膜片段受体家族的成员。其特征是具有一个大的细胞外结构域，与七个跨膜片段相连，并以一个胞质尾结束。序列比对显示，在几乎所有G蛋白偶联受体的第七跨膜结构域与胞质尾近端部分的交界处，存在一个高度保守的基序NPXXY（其中X为任意氨基酸）。该基序已被认为是几种细胞表面受体（如低密度脂蛋白（LDL）、胰岛素和胰岛素样生长因子-1（IGF-1）受体）的内化信号。通过将酪氨酸（678）残基替换为丙氨酸残基，分析了该基序对TSH受体信号转导和受体介导的TSH内化的潜在影响。这种突变不损害TSH的高亲和力结合，但完全消除了TSH刺激后cAMP反应的能力。它还显著降低了TSH的内化。还评估了TSH受体胞质尾在受体介导的内化中的作用。与野生型受体相比，从胞质尾的C末端缺失多达56个氨基酸可增强TSH的内化。我们得出结论，NPXXY基序中的酪氨酸（678）是有效的受体介导的TSH内化和G蛋白偶联所必需的。TSH受体的胞质尾可能包含一些序列结构域，这些结构域可以调节NPXXY内化信号的作用。
