KdpD/KdpE two‐component signaling system regulates expression of a high affinity potassium transporter responsible for potassium homeostasis. The C‐terminal module of KdpD consists of a GAF domain linked to a histidine kinase domain. Whereas certain GAF domains act as regulators by binding cyclic nucleotides, the role of the juxtamembrane GAF domain in KdpD is unknown. We report the high‐resolution crystal structure of KdpD GAF domain (KdpD^G) consisting of five α‐helices, four β‐sheets and two large loops. KdpD^G forms a symmetry‐related dimer, wherein parallelly arranged monomers contribute to a four‐helix bundle at the dimer‐interface, SAXS analysis of KdpD C‐terminal module reveals an elongated structure that is a dimer in solution. Substitution of conserved residues with various residues that disrupt the dimer interface produce a range of effects on gene expression demonstrating the importance of the interface in inactive to active transitions during signaling. Comparison of ligand binding site of the classic cyclic nucleotide‐binding GAF domains to KdpD^G reveals structural differences arising from naturally occurring substitutions in primary sequence of KdpD^G that modifies the canonical NKFDE sequence motif required for cyclic nucleotide binding. Together these results suggest a structural role for KdpD^G in dimerization and transmission of signal to the kinase domain.

中文翻译：

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钾生物传感器 KdpD 的近膜 GAF 结构域的结构和功能。

KdpD/KdpE 双组分信号系统调节负责钾稳态的高亲和力钾转运蛋白的表达。KdpD 的 C 端模块由连接到组氨酸激酶结构域的 GAF 结构域组成。虽然某些 GAF 域通过结合环核苷酸作为调节剂，但近膜 GAF 域在 KdpD 中的作用尚不清楚。我们报告了由五个α-螺旋、四个β-折叠和两个大环组成的KdpD GAF 结构域（KdpD ^G）的高分辨率晶体结构。KdpD ^G形成与对称相关的二聚体，其中平行排列的单体在二聚体界面形成四螺旋束，KdpD C 端模块的 SAXS 分析揭示了在溶液中是二聚体的细长结构。用破坏二聚体界面的各种残基取代保守残基对基因表达产生一系列影响，证明界面在信号传导过程中从非活性到活性转变的重要性。经典环核苷酸结合 GAF 结构域与 KdpD ^G的配体结合位点的比较揭示了由 KdpD ^G一级序列中自然发生的取代引起的结构差异修饰环核苷酸结合所需的经典 NKFDE 序列基序。这些结果一起表明 KdpD ^G在二聚化和信号传递到激酶结构域中的结构作用。