PDF(Pubmed)
Abstract：
Inositol 1,4,5-trisphosphate receptors (IP3Rs) are endoplasmic reticulum Ca2+ channels whose biphasic dependence on cytosolic Ca2+ gives rise to Ca2+ oscillations that regulate fertilization, cell division and cell death. Despite the critical roles of IP3R-mediated Ca2+ responses, the structural underpinnings of the biphasic Ca2+ dependence that underlies Ca2+ oscillations are incompletely understood. Here, we collect cryo-EM images of an IP3R with Ca2+ concentrations spanning five orders of magnitude. Unbiased image analysis reveals that Ca2+ binding does not explicitly induce conformational changes but rather biases a complex conformational landscape consisting of resting, preactivated, activated, and inhibited states. Using particle counts as a proxy for relative conformational free energy, we demonstrate that Ca2+ binding at a high-affinity site allows IP3Rs to activate by escaping a low-energy resting state through an ensemble of preactivated states. At high Ca2+ concentrations, IP3Rs preferentially enter an inhibited state stabilized by a second, low-affinity Ca2+ binding site. Together, these studies provide a mechanistic basis for the biphasic Ca2+-dependence of IP3R channel activity.
摘要：
肌醇1,4,5-三磷酸受体（IP3Rs）是内质网Ca2通道，其对细胞溶质Ca2的双相依赖性会引起调节受精的Ca2振荡，细胞分裂和细胞死亡。尽管IP3R介导的Ca2+反应具有关键作用，尚未完全理解作为Ca2振荡基础的双相Ca2依赖性的结构基础。这里,我们收集具有跨越五个数量级的Ca2+浓度的IP3R的低温EM图像。无偏图像分析显示，Ca2结合并没有明确诱导构象变化，而是偏向了由静息组成的复杂构象景观，预激活,激活,和抑制状态。使用粒子计数作为相对构象自由能的代表，我们证明，在高亲和力位点的Ca2结合允许IP3Rs通过一系列预激活状态逃脱低能量静息状态而激活。在高Ca2+浓度下，IP3Rs优先进入抑制状态，稳定一秒钟，低亲和力Ca2+结合位点。一起,这些研究为IP3R通道活性的双相Ca2依赖性提供了机制基础。
