Institute for Quantitative Biosciences, The University of Tokyo, 113-0032 Tokyo, Japan.
Institute for Quantitative Biosciences, The University of Tokyo, 113-0032 Tokyo, Japan
Proc Natl Acad Sci U S A. 2020 Aug 4;117(31):18448-18458. doi: 10.1073/pnas.2006027117. Epub 2020 Jul 16.
Under physiological conditions, most Ca-ATPase (SERCA) molecules bind ATP before binding the Ca transported. SERCA has a high affinity for ATP even in the absence of Ca, and ATP accelerates Ca binding at pH values lower than 7, where SERCA is in the E2 state with low-affinity Ca-binding sites. Here we describe the crystal structure of SERCA2a, the isoform predominant in cardiac muscle, in the E2·ATP state at 3.0-Å resolution. In the crystal structure, the arrangement of the cytoplasmic domains is distinctly different from that in canonical E2. The A-domain now takes an E1 position, and the N-domain occupies exactly the same position as that in the E1·ATP·2Ca state relative to the P-domain. As a result, ATP is properly delivered to the phosphorylation site. Yet phosphoryl transfer never takes place without the filling of the two transmembrane Ca-binding sites. The present crystal structure explains what ATP binding itself does to SERCA and how nonproductive phosphorylation is prevented in E2.
在生理条件下，大多数钙 - 三磷酸腺苷酶（SERCA）分子在结合 Ca 之前结合 ATP。SERCA 对 ATP 的亲和力很高，即使在没有 Ca 的情况下也是如此，并且在 pH 值低于 7 的情况下，ATP 会加速 Ca 的结合，此时 SERCA 处于低亲和力 Ca 结合位点的 E2 状态。在这里，我们描述了在 3.0-Å 分辨率下以 E2·ATP 状态存在的心脏肌肉中主要存在的 SERCA2a 同工型的晶体结构。在晶体结构中，细胞质结构域的排列明显不同于典型的 E2。A 结构域现在占据 E1 位置，而 N 结构域相对于 P 结构域占据与 E1·ATP·2Ca 状态完全相同的位置。结果，ATP 被正确地递送到磷酸化位点。然而，在两个跨膜 Ca 结合位点未填充的情况下，磷酸化转移永远不会发生。目前的晶体结构解释了 ATP 结合本身对 SERCA 的作用，以及如何防止 E2 中的非生产性磷酸化。
