Revealing family differences In response to low blood glucose concentrations, both the glucagon receptor (GCGR)—a family B G protein–coupled receptor (GPCR)—and the β2 adrenergic receptor (β2AR)—a family A GPCR—are activated and act through the cyclic adenosine monophosphate signaling pathway to increase glucose production. The kinetics of the response is different for the two receptors. Based on structural and spectroscopic data, Hilger et al. show that the conformation of transmembrane helix 6 in the activated state is a key differentiator (see the Perspective by Lebon). In β2AR, the helix moves toward its active conformation when an agonist binds, but in GCGR, both agonist and G protein binding are required. This likely explains why activation of its partner G protein is slower for GCGR than for β2AR. Science, this issue p. eaba3373; see also p. 507 Structural and spectroscopic data reveal differences in the activation mechanism between G protein–coupled receptors in family A and family B. INTRODUCTION Transmembrane signaling of heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCRs) is mediated by ligand-dependent conformational changes that are transmitted from the extracellular ligand binding site to the intracellular side of the receptor to allow coupling with transducers. One hallmark of GPCR activation is the outward movement of the cytoplasmic end of transmembrane domain 6 (TM6) that opens up an intracellular cavity to accommodate the Gα subunit, leading to nucleotide exchange and activation of the G protein. Comparison of family A and family B receptor-Gs protein complex structures has revealed substantial differences in the conformational changes of TM6 upon activation. In family B GPCRs, TM6 shows a disruption of the helical fold and the formation of a sharp kink. This differs from the gradual bending in TM6 observed in family A GPCRs. RATIONALE Despite the recent surge of determined GPCR–G protein complex structures, the activation mechanism of family B receptors remains poorly understood. The missing conserved structural motifs found in family A GPCRs together with the marked differences in the conformation of TM6 in the active state suggest distinct activation mechanisms between family B and family A GPCRs. In particular, the disruption of the helical fold and the unraveling of the extracellular end of TM6 suggest that the energy required to produce the fully active-state of family B GPCRs is higher than for family A GPCRs. We investigated the functional impact of these structural differences by comparing the structure and function of a prototypical family B receptor, the glucagon receptor (GCGR), with the β2 adrenergic receptor (β2AR), a family A GPCR. RESULTS We present the cryo–electron microscopy structure of the GCGR-Gs complex bound to an engineered soluble glucagon derivative. The structure shows that full activation of GCGR leads to a disruption in the α-helix of TM6 typical for family B GPCRs. Analysis of the functional consequence of this helix break on receptor-mediated G protein dissociation and guanosine triphosphate (GTP) turnover reveals that GCGR exhibits a substantially lower guanine nucleotide exchange factor (GEF) activity in comparison with the family A receptor β2AR. Characterization of G protein association, guanosine diphosphate (GDP) release, and GTP binding kinetics shows that the receptor-mediated GDP dissociation and GTP binding of Gs are slower for GCGR than for β2AR. Measurements of ligand-dependent conformational alterations of GCGR by means of fluorescence and double electron-electron resonance spectroscopy show that agonist binding alone is insufficient to promote TM6 opening, in contrast to previously studied family A GPCRs, including β2AR. The outward movement of TM6 of GCGR is only observed upon interaction with Gs, suggesting that TM6 activation is only triggered by the engagement of the α5 helix of Gαs. Furthermore, TM6 of GCGR remains in the active state for a prolonged time after disengagement of Gs, which might contribute to the persistent and sustained cyclic adenosine monophosphate (cAMP) signaling previously observed for this receptor. A comprehensive comparison of the G protein activation kinetics for a number of other family A and family B GPCRs shows that family B receptors are in general less efficient than family A GPCRs in triggering G protein signaling. CONCLUSION Our findings provide evidence for distinct activation mechanisms between family A and family B GPCRs. We propose that formation of the helix break and the sharp kink in TM6 of GCGR requires overcoming a higher energy barrier than the bending and outward movement of TM6 in family A receptors. Because of this kinetic barrier, ligand binding alone is not sufficient to stabilize the outward movement of TM6 but promotes the initial formation of the receptor–G protein complex and subsequent full engagement of the G protein at a slower time scale. Once activated by the insertion of the α5 helix of Gαs into the receptor core, as seen in the nucleotide-free complex structure, TM6 stays in the active conformation long after full disengagement of the G protein. This may be responsible for the previously described sustained and prolonged signaling of GCGR. Our comprehensive comparison of the G protein activation kinetics of family A and family B receptors suggests that the activation mechanism described for GCGR is generalizable to other family B GPCRs. Difference in the activation mechanism of family A and family B GPCRs affects G protein signaling. In contrast to family A GPCRs such as β2AR, agonist binding alone is not sufficient to induce TM6 outward movement of GCGR, most likely because of the high energy barrier for the formation of the kinked and partially unwound TM6 found in the G protein–coupled state of family B GPCRs. In comparison with family A GPCRs, this leads to a reduced G protein activation rate. Family B heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCRs) play important roles in carbohydrate metabolism. Recent structures of family B GPCR-Gs protein complexes reveal a disruption in the α-helix of transmembrane segment 6 (TM6) not observed in family A GPCRs. To investigate the functional impact of this structural difference, we compared the structure and function of the glucagon receptor (GCGR; family B) with the β2 adrenergic receptor (β2AR; family A). We determined the structure of the GCGR-Gs complex by means of cryo–electron microscopy at 3.1-angstrom resolution. This structure shows the distinct break in TM6. Guanosine triphosphate (GTP) turnover, guanosine diphosphate release, GTP binding, and G protein dissociation studies revealed much slower rates for G protein activation by the GCGR compared with the β2AR. Fluorescence and double electron-electron resonance studies suggest that this difference is due to the inability of agonist alone to induce a detectable outward movement of the cytoplasmic end of TM6.

中文翻译：

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对 A 族和 B 族 GPCR 对 G 蛋白激活差异的结构洞察

揭示家族差异 为了应对低血糖浓度，胰高血糖素受体 (GCGR)——一种家族 BG 蛋白偶联受体 (GPCR)——和 β2 肾上腺素能受体 (β2AR)——一种家族 A GPCR——都被激活并通过环磷酸腺苷信号通路增加葡萄糖产量。两种受体的反应动力学不同。根据结构和光谱数据，Hilger 等人。表明处于激活状态的跨膜螺旋 6 的构象是一个关键的区分因素（参见 Lebon 的观点）。在 β2AR 中，当激动剂结合时，螺旋向其活性构象移动，但在 GCGR 中，激动剂和 G 蛋白结合都需要。这可能解释了为什么 GCGR 其伙伴 G 蛋白的激活比 β2AR 慢。科学，这个问题 p。eaba3373; 另见第 507 结构和光谱数据揭示了 A 族和 B 族中 G 蛋白偶联受体激活机制的差异。 引言 异源三聚鸟嘌呤核苷酸结合蛋白（G 蛋白）偶联受体 (GPCR) 的跨膜信号传导由配体依赖性介导从细胞外配体结合位点传递到受体细胞内的构象变化，以允许与换能器偶联。GPCR 激活的一个标志是跨膜结构域 6 (TM6) 的细胞质末端向外移动，打开细胞内腔以容纳 Gα 亚基，导致核苷酸交换和 G 蛋白的激活。家族 A 和家族 B 受体-Gs 蛋白复合物结构的比较揭示了激活后 TM6 构象变化的实质性差异。在 B 族 GPCR 中，TM6 显示出螺旋折叠的破坏和尖锐扭结的形成。这与在 A 族 GPCR 中观察到的 TM6 逐渐弯曲不同。基本原理尽管最近确定的 GPCR-G 蛋白复合物结构激增，但对 B 族受体的激活机制仍知之甚少。在 A 族 GPCR 中发现的缺失保守结构基序以及 TM6 在活性状态下构象的显着差异表明 B 族和 A 族 GPCR 之间存在不同的激活机制。特别是，螺旋折叠的破坏和 TM6 细胞外末端的解开表明产生 B 族 GPCR 完全激活状态所需的能量高于 A 族 GPCR。我们通过比较原型家族 B 受体胰高血糖素受体 (GCGR) 与 β2 肾上腺素能受体 (β2AR) 家族 A GPCR 的结构和功能，研究了这些结构差异的功能影响。结果我们展示了与工程可溶性胰高血糖素衍生物结合的 GCGR-Gs 复合物的冷冻电子显微镜结构。该结构显示 GCGR 的完全激活导致 TM6 的 α-螺旋破坏，这对于 B 族 GPCR 是典型的。对这种螺旋断裂对受体介导的 G 蛋白解离和三磷酸鸟苷 (GTP) 转换的功能后果的分析表明，与家族 A 受体 β2AR 相比，GCGR 表现出显着较低的鸟嘌呤核苷酸交换因子 (GEF) 活性。G 蛋白结合、二磷酸鸟苷 (GDP) 释放和 GTP 结合动力学的表征表明，GCGR 的受体介导的 GDP 解离和 Gs 的 GTP 结合比 β2AR 慢。通过荧光和双电子-电子共振光谱法测量 GCGR 的配体依赖性构象改变表明，与先前研究的家族 A GPCR（包括 β2AR）相比，单独的激动剂结合不足以促进 TM6 开放。GCGR 的 TM6 向外运动仅在与 Gs 相互作用时观察到，表明 TM6 激活仅由 Gαs 的 α5 螺旋的接合触发。此外，GCGR 的 TM6 在 Gs 脱离后长时间保持活跃状态​​，这可能有助于先前针对该受体观察到的持久和持续的环磷酸腺苷 (cAMP) 信号传导。许多其他 A 族和 B 族 GPCR 的 G 蛋白激活动力学的综合比较表明，在触发 G 蛋白信号传导方面，B 族受体通常不如 A 族 GPCR 有效。结论 我们的发现为 A 族和 B 族 GPCR 之间不同的激活机制提供了证据。我们建议GCGR TM6 中螺旋断裂和尖锐扭结的形成需要克服比TM6 在A 族受体中弯曲和向外运动更高的能量障碍。由于这种动力学障碍，单独的配体结合不足以稳定 TM6 的向外运动，但会促进受体-G 蛋白复合物的初始形成以及随后 G 蛋白在较慢时间尺度上的完全结合。一旦通过将 Gαs 的 α5 螺旋插入受体核心而激活，如无核苷酸复合结构中所见，TM6 在 G 蛋白完全脱离后很长时间内仍保持活性构象。这可能是造成先前描述的 GCGR 持续和延长信号的原因。我们对 A 族和 B 族受体的 G 蛋白激活动力学的综合比较表明，针对 GCGR 描述的激活机制可推广到其他 B 族 GPCR。A 族和 B 族 GPCR 激活机制的差异影响 G 蛋白信号传导。与 β2AR 等 A 族 GPCR 相比，单独的激动剂结合不足以诱导 GCGR 的 TM6 向外运动，很可能是因为在 G 蛋白偶联状态下形成扭结和部分解开的 TM6 具有高能量屏障B 族 GPCR。与 A 族 GPCR 相比，这导致 G 蛋白激活率降低。B族异源三聚鸟嘌呤核苷酸结合蛋白（G蛋白）偶联受体（GPCRs）在碳水化合物代谢中起重要作用。家族 B GPCR-Gs 蛋白复合物的最新结构揭示了跨膜片段 6 (TM6) 的α-螺旋的破坏，在家族 A GPCR 中未观察到。为了研究这种结构差异的功能影响，我们比较了胰高血糖素受体（GCGR；B 族）与 β2 肾上腺素能受体（β2AR；A 族）的结构和功能。我们通过低温电子显微镜以 3.1 埃的分辨率确定了 GCGR-Gs 复合物的结构。这种结构显示了 TM6 中的明显断裂。三磷酸鸟苷 (GTP) 周转、二磷酸鸟苷释放、GTP 结合和 G 蛋白解离研究表明，与 β2AR 相比，GCGR 对 G 蛋白的激活速度要慢得多。