Abstract：
S100A1, a small homodimeric EF-hand Ca2+-binding protein (~21 kDa), plays an important regulatory role in Ca2+ signaling pathways involved in various biological functions including Ca2+ cycling and contractile performance in skeletal and cardiac myocytes. One key target of the S100A1 interactome is the ryanodine receptor (RyR), a huge homotetrameric Ca2+ release channel (~2.3 MDa) of the sarcoplasmic reticulum. Here, we report cryoelectron microscopy structures of S100A1 bound to RyR1, the skeletal muscle isoform, in absence and presence of Ca2+. Ca2+-free apo-S100A1 binds beneath the bridging solenoid (BSol) and forms contacts with the junctional solenoid and the shell-core linker of RyR1. Upon Ca2+-binding, S100A1 undergoes a conformational change resulting in the exposure of the hydrophobic pocket known to serve as a major interaction site of S100A1. Through interactions of the hydrophobic pocket with RyR1, Ca2+-bound S100A1 intrudes deeper into the RyR1 structure beneath BSol than the apo-form and induces sideways motions of the C-terminal BSol region toward the adjacent RyR1 protomer resulting in tighter interprotomer contacts. Interestingly, the second hydrophobic pocket of the S100A1-dimer is largely exposed at the hydrophilic surface making it prone to interactions with the local environment, suggesting that S100A1 could be involved in forming larger heterocomplexes of RyRs with other protein partners. Since S100A1 interactions stabilizing BSol are implicated in the regulation of RyR-mediated Ca2+ release, the characterization of the S100A1 binding site conserved between RyR isoforms may provide the structural basis for the development of therapeutic strategies regarding treatments of RyR-related disorders.
摘要：
S100A1，一种小同二聚体EF-手Ca2+结合蛋白（~21kDa），在涉及各种生物学功能的Ca2+信号通路中起着重要的调节作用，包括骨骼和心肌细胞的Ca2+循环和收缩性能。S100A1相互作用体的一个关键靶标是ryanodine受体（RyR），肌浆网巨大的同四聚体Ca2释放通道（〜2.3MDa）。这里,我们报道了与RyR1结合的S100A1的低温电子显微镜结构，在不存在和存在Ca2+的情况下。不含Ca2的apo-S100A1在桥接螺线管（BSol）下方结合，并与接合螺线管和RyR1的壳-核接头形成接触。在Ca2+结合时，S100A1经历构象变化,导致已知充当S100A1的主要相互作用位点的疏水性口袋的暴露。通过疏水口袋与RyR1的相互作用，与Ca2结合的S100A1比apo形式更深地侵入BSol下方的RyR1结构，并引起C端BSol区域向相邻RyR1质子发生器的侧向运动，从而导致更紧密的质子间接触。有趣的是,S100A1-二聚体的第二疏水口袋大部分暴露在亲水表面，使其易于与局部环境相互作用，这表明S100A1可能参与与其他蛋白质伴侣形成更大的RyRs杂复合物。由于稳定BSol的S100A1相互作用与RyR介导的Ca2释放的调节有关，RyR同工型之间保守的S100A1结合位点的表征可能为开发有关RyR相关疾病的治疗策略提供结构基础.
