Structural study for the novel regulation mechanism by calcium ion

钙信号转导机制被所有真核生物广泛用于调控基因表达和各种细胞过程。在植物中，许多细胞外信号，如光、干旱、寒冷、盐度、胁迫因子，都会引起细胞内钙浓度的变化。钙感受器蛋白经常以钙依赖的方式改变其构象，并与其他信号蛋白相互作用来传递信号。在高等植物中已鉴定出几个钙感受器蛋白家族。最近，从拟南芥中发现了一种新的具有EF-Hand基序的钙感受器蛋白，称为AtCBL(拟南芥钙调神经磷酸酶B-like Protein)。一个SNF1家族被确定为CBL蛋白(7-9)的靶标。这些是丝氨酸-苏氨酸蛋白激酶，称为AtCIPK(拟南芥CBL相互作用蛋白激酶)，形成了迄今为止仅在植物中发现的一个新的蛋白质家族。AtCBL2的晶体结构已在2.1a分辨率下确定。该蛋白质形成一个紧凑的a-螺旋结构，具有两对EF-Hand基序。该结构在整体折叠拓扑结构上与钙调神经磷酸酶B和神经元钙感受器1的结构相似，但在局部构象上有显著差异。两个钙离子在第一和第四个EF-手基序中配位，而第二和第三个EF-手基序通过内部氢键保持开放状态，而不与钙离子配位。基于三维结构，讨论了目标与AtCBL2结合的可能位置和可能的机制。

Calcium signaling mechanisms are widely employed by all eukaryotic organisms to regulate gene expression and a variety of cellular processes. In plants, many extracellular signals, such as light, drought, cold, salinity, stress factors, elicit changes in cellular calcium concentration. The calcium sensor protein often changes its conformation in a calcium-dependent manner and interacts with other signaling proteins to relay the signal. Several families of calcium sensor proteins have been identified in higher plants. Recently, novel calcium sensor proteins with EF-hand motifs from Arabidopsis thaliana have been identified, referred to as AtCBL(Arabidopsis thaliana calcineurin B-like protein). A family of SNF1 has been identified as targets for CBL proteins(7-9).These are serine-threonine protein kinases, referred to as AtCIPK(Arabidopsis thaliana CBL-interacting protein kinase), that form a novel family of proteins found so far only in plants.The crystal structure of AtCBL2 has been determined at 2.1A resolution. The protein forms a compact a-helical structure with two pairs of EF-hand motifs. The structure is similar in overall folding topology to the structures of calcineurin B and neuronal calcium sensor 1 , but differs significantly in local conformation. The two calcium ions are coordinated in the first and fourth EF-hand motifs, while the second and third EF-hand motifs are maintained in the open form by internal hydrogen bonding without coordination of calcium ions. Both a possible site and a possible mechanism for the target binding to AtCBL2 are discussed based on the three-dimensional structure.