Wall M E, Clarage J B, Phillips G N
Department of Biochemistry and Cell Biology, The WM Keck Center for Computational Biology, Rice University Houston, TX 77005-1892, USA,
Structure. 1997 Dec 15;5(12):1599-612. doi: 10.1016/s0969-2126(97)00308-0.
Calmodulin is a calcium-activated regulatory protein which can bind to many different targets. The protein resembles a highly flexible dumbbell, and bends in the middle as it binds. This and other motions must be understood to formulate a realistic model of calmodulin function.
Using the Bragg reflections from X-ray crystallography, a multiple-conformer refinement of a calmodulin-peptide complex shows anisotropic displacements, with high variations of dihedral angles in several nonhelical domains: the flexible linker; three of the four calcium-binding sites (including both of the N-terminal sites); and a turn connecting the C-terminal EF-hand calcium-binding domains. Three-dimensional maps of the large scale diffuse X-ray scattering data show isotropic liquid-like motions with an unusually small correlation length. Three-dimensional maps of the small scale diffuse streaks show highly coupled, anisotropic motions along the head-to-tail molecular packing direction in the unit cell. There is also weak coupling perpendicular to the head-to-tail packing direction, particularly across a cavity occupied by the disordered linker domain of the molecule.
Together, the Bragg and diffuse scattering present a self-consistent description of the motions in the flexible linker of calmodulin. The other mobile regions of the protein are also of great interest. In particular, the high variations in the calcium-binding sites are likely to influence how strongly they bind ions. This is especially important in the N-terminal sites, which regulate the activity of the molecule.
钙调蛋白是一种钙激活调节蛋白，可与许多不同的靶标结合。该蛋白类似于一个高度灵活的哑铃，结合时在中间弯曲。必须了解这种运动及其他运动，才能构建出一个真实的钙调蛋白功能模型。
利用X射线晶体学的布拉格反射，对钙调蛋白-肽复合物进行多构象精修，结果显示出各向异性位移，在几个非螺旋结构域中，二面角变化很大：柔性连接子；四个钙结合位点中的三个（包括两个N端位点）；以及连接C端EF手型钙结合结构域的一个转角。大尺度漫散射X射线数据的三维图谱显示出各向同性的类液体运动，其相关长度异常小。小尺度漫散射条纹的三维图谱显示出沿着晶胞中分子头对尾堆积方向的高度耦合的各向异性运动。在垂直于头对尾堆积方向上也存在弱耦合，特别是跨越分子无序连接子结构域占据的一个空腔。
布拉格散射和漫散射共同对钙调蛋白柔性连接子中的运动给出了自洽的描述。该蛋白的其他可移动区域也非常有趣。特别是，钙结合位点的高度变化可能会影响它们与离子结合的强度。这在调节分子活性的N端位点尤为重要。
