Department of Neuroscience and Pharmacology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
Mol Pharmacol. 2013 Sep;84(3):335-45. doi: 10.1124/mol.113.086702. Epub 2013 Jun 13.
In addition to the 7 transmembrane receptor (7TM)-conserved disulfide bridge between transmembrane (TM) helix 3 and extracellular loop (ECL)-2, chemokine receptors (CCR) contain a disulfide bridge between the N terminus and what previously was believed to be ECL-3. Recent crystal and NMR structures of the CXC chemokine receptors (CXCR) CXCR4 and CXCR1, combined with structural analysis of all endogenous chemokine receptors indicate that this chemokine receptor-conserved bridge in fact connects the N terminus to the top of TM-7. By employing chemokine ligands that mainly target extracellular receptor regions and small-molecule ligands that predominantly interact with residues in the main binding crevice, we show that the 7TM-conserved bridge is essential for all types of ligand-mediated activation, whereas the chemokine-conserved bridge is dispensable for small-molecule activation in CCR1. However, in striking contrast to previous studies in other chemokine receptors, high-affinity CCL3 chemokine binding was maintained in the absence of either bridge. In the highly related CCR5, a completely different dependency was observed as neither activation nor binding of the same chemokines was retained in the absence of either bridge. In contrast, both bridges were dispensable for activation by the same small molecules. This indicates that CCR5 activity is independent of extracellular regions, whereas in CCR1 the preserved folding of ECL-2 is necessary for activation. These results indicate that conserved structural features in a receptor subgroup do not necessarily provide specific traits for the whole subgroup but rather provide unique traits to the single receptors.
除了 7 跨膜受体（7TM）保守的跨膜（TM）螺旋 3 和细胞外环（ECL）-2 之间的二硫键外，趋化因子受体（CCR）在 N 端和先前认为是 ECL-3 之间还存在一个二硫键。最近 CXC 趋化因子受体（CXCR）CXCR4 和 CXCR1 的晶体和 NMR 结构以及所有内源性趋化因子受体的结构分析表明，这个趋化因子受体保守的桥实际上将 N 端连接到 TM-7 的顶部。通过使用主要靶向细胞外受体区域的趋化因子配体和主要与主要结合裂缝中残基相互作用的小分子配体，我们表明 7TM 保守桥对于所有类型的配体介导的激活都是必不可少的，而趋化因子保守桥对于 CCR1 中小分子的激活是可有可无的。然而，与其他趋化因子受体的先前研究形成鲜明对比的是，在没有任何桥的情况下，高亲和力 CCL3 趋化因子结合得以维持。在高度相关的 CCR5 中，观察到完全不同的依赖性，因为在没有任何桥的情况下，这些趋化因子的激活或结合都没有保留。相比之下，在没有任何桥的情况下，两种桥对于相同的小分子的激活都是可有可无的。这表明 CCR5 的活性独立于细胞外区域，而在 CCR1 中，ECL-2 的保留折叠对于激活是必要的。这些结果表明，受体亚群中的保守结构特征不一定为整个亚群提供特定的特征，而是为单个受体提供独特的特征。
