Abiko Layara Akemi, Vitale Phelipe M, Favaro Denize C, Hauk Pricila, Li Da-Wei, Yuan Jiaqi, Bruschweiler-Li Lei, Salinas Roberto K, Brüschweiler Rafael
Institute of Chemistry, University of São Paulo, São Paulo, SP, 05508-000, Brazil.
Campus Chemical Instrument Center, The Ohio State University, Columbus, Ohio, 43210.
Proteins. 2016 May;84(5):580-90. doi: 10.1002/prot.25003. Epub 2016 Mar 11.
The Na(+) /Ca(2+) exchanger provides a major Ca(2+) extrusion pathway in excitable cells and plays a key role in the control of intracellular Ca(2+) concentrations. In Canis familiaris, Na(+) /Ca(2+) exchanger (NCX) activity is regulated by the binding of Ca(2+) to two cytosolic Ca(2+) -binding domains, CBD1 and CBD2, such that Ca(2+) -binding activates the exchanger. Despite its physiological importance, little is known about the exchanger's global structure, and the mechanism of allosteric Ca(2+) -regulation remains unclear. It was found previously that for NCX in the absence of Ca(2+) the two domains CBD1 and CBD2 of the cytosolic loop are flexibly linked, while after Ca(2+) -binding they adopt a rigid arrangement that is slightly tilted. A realistic model for the mechanism of the exchanger's allosteric regulation should not only address this property, but also it should explain the distinctive behavior of Drosophila melanogaster's sodium/calcium exchanger, CALX, for which Ca(2+) -binding to CBD1 inhibits Ca(2+) exchange. Here, NMR spin relaxation and residual dipolar couplings were used to show that Ca(2+) modulates CBD1 and CBD2 interdomain flexibility of CALX in an analogous way as for NCX. A mechanistic model for the allosteric Ca(2+) regulation of the Na(+) /Ca(2+) exchanger is proposed. In this model, the intracellular loop acts as an entropic spring whose strength is modulated by Ca(2+) -binding to CBD1 controlling ion transport across the plasma membrane.
钠/钙交换蛋白在可兴奋细胞中提供了一条主要的钙排出途径，在控制细胞内钙浓度方面发挥着关键作用。在犬类中，钠/钙交换蛋白（NCX）的活性受钙离子与两个胞质钙结合结构域CBD1和CBD2的结合调节，使得钙离子结合激活该交换蛋白。尽管其具有生理重要性，但对该交换蛋白的整体结构知之甚少，变构钙调节机制仍不清楚。先前发现，对于无钙离子状态下的NCX，胞质环的两个结构域CBD1和CBD2是灵活连接的，而在钙离子结合后，它们会采取一种略有倾斜的刚性排列。一个关于该交换蛋白变构调节机制的现实模型不仅应解决这一特性，还应解释果蝇钠/钙交换蛋白CALX的独特行为，对于CALX，钙离子与CBD1结合会抑制钙交换。在此，利用核磁共振自旋弛豫和剩余偶极耦合来表明，钙离子以与NCX类似的方式调节CALX的CBD1和CBD2结构域间的灵活性。提出了一个关于钠/钙交换蛋白变构钙调节的机制模型。在这个模型中，细胞内环充当一个熵弹簧，其强度由钙离子与CBD1的结合调节，从而控制离子跨质膜的运输。
