Division of Biophysics and Neurobiology, Department of Molecular Physiology, National Institute for Physiological Sciences, Okazaki, Aichi 444-8585, Japan.
J Biol Chem. 2010 Apr 2;285(14):10291-9. doi: 10.1074/jbc.M109.077990. Epub 2010 Feb 3.
The gamma-aminobutyric acid type B receptor (GABA(B)R), one of the family C G-protein-coupled receptor members, exists as a heterodimer comprised of subunits GB1 and GB2. To clarify the ligand-induced activation mechanism of the GABA(B)R, each subunit was fused with either Cerulean or enhanced yellow fluorescent protein at its intracellular loop, and fluorescence resonance energy transfer (FRET) changes upon agonist application were monitored. As a result, FRET decreases were observed between GB1a loop 2 and GB2 loop 2 and between GB1a loop 2 and GB2 loop 1, suggesting the dissociation of intracellular domains during the receptor activation. Both intersubunit FRET pairs were expected to faithfully capture the activation of the original receptor as their pharmacological properties were highly similar to that of the wild-type receptor. However, the intrasubunit data suggest that the receptor activation does not involve major structural changes within the transmembrane domain of each subunit. By combining the results obtained from two different levels, it was concluded that the GABA(B)R activation by agonist is associated with an asymmetrical intersubunit rearrangement of GB1a and GB2 on the membrane. This type of activation mode, an intersubunit rearrangement without apparent intrahelical structural changes, appears commonly shared by the GABA(B)R and the metabotropic glutamate receptor 1alpha, another family C G-protein-coupled receptor previously studied by our group. Nevertheless, the directions of intracellular domain movements and its asymmetry observed here highlight the qualitative difference between the two receptors.
γ-氨基丁酸 B 型受体（GABA(B)R）是 C 型 G 蛋白偶联受体家族的一员，存在于由亚基 GB1 和 GB2 组成的异二聚体中。为了阐明 GABA(B)R 的配体诱导激活机制，将每个亚基的细胞内环融合 Cerulean 或增强型黄色荧光蛋白，并监测激动剂应用时的荧光共振能量转移（FRET）变化。结果，观察到 GB1a 环 2 和 GB2 环 2 之间以及 GB1a 环 2 和 GB2 环 1 之间的 FRET 减少，表明在受体激活过程中细胞内结构域的解离。由于它们的药理学特性与野生型受体非常相似，因此这两个亚基间 FRET 对都有望忠实地捕获受体的激活。然而，亚基内数据表明，受体激活不涉及每个亚基跨膜域内的主要结构变化。通过结合来自两个不同水平的结果，可以得出结论，激动剂诱导的 GABA(B)R 激活与膜上 GB1a 和 GB2 的不对称亚基重排有关。这种激活模式，即没有明显的螺旋内结构变化的亚基间重排，似乎与我们小组之前研究的另一种 C 型 G 蛋白偶联受体代谢型谷氨酸受体 1alpha 共同存在。然而，这里观察到的细胞内结构域运动方向及其不对称性突出了这两种受体之间的定性差异。
