Vakonakis Ioannis, Sun Jingchuan, Wu Tianfu, Holzenburg Andreas, Golden Susan S, LiWang Andy C
Departments of Biochemistry and Biophysics, Texas A&M University, College Station, TX 77843, USA.
Proc Natl Acad Sci U S A. 2004 Feb 10;101(6):1479-84. doi: 10.1073/pnas.0305516101. Epub 2004 Jan 28.
KaiA is a two-domain circadian clock protein in cyanobacteria, acting as the positive element in a feedback loop that sustains the oscillation. The structure of the N-terminal domain of KaiA is that of a pseudo-receiver, similar to those of bacterial response regulators, which likely interacts with components of the clock-resetting pathway. The C-terminal domain of KaiA is highly conserved among cyanobacteria and enhances the autokinase activity of KaiC. Here we present the NMR structure of the C-terminal domain of KaiA from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1. This domain adopts a novel all alpha-helical homodimeric structure. Several mutations known to affect the period of the circadian oscillator are shown to be located at an exposed groove near the dimer interface. This NMR structure and a 21-A-resolution electron microscopy structure of the hexameric KaiC particle allow us to postulate a mode of KaiA-KaiC interaction, in which KaiA binds a linker region connecting two globular KaiC domains.
KaiA是蓝藻中的一种双结构域生物钟蛋白，在维持振荡的反馈回路中作为正向元件。KaiA的N端结构域具有假受体结构，类似于细菌应答调节因子的结构域，可能与生物钟重置途径的组分相互作用。KaiA的C端结构域在蓝藻中高度保守，并增强了KaiC的自激酶活性。在此，我们展示了嗜热蓝藻嗜热栖热菌BP-1中KaiA C端结构域的核磁共振结构。该结构域采用了一种新颖的全α螺旋同二聚体结构。已知几个影响生物钟振荡器周期的突变位于二聚体界面附近的一个暴露沟中。这种核磁共振结构以及六聚体KaiC颗粒的21埃分辨率电子显微镜结构，使我们能够推测KaiA与KaiC的相互作用模式，即KaiA结合连接两个球状KaiC结构域的连接区。
