A Compact intermediate state of calmodulin in the process of target binding

Yamada, Yoshiteru*; Matsuo, Tatsuhito; Iwamoto, Hiroyuki*; Yagi, Naoto*

Calmodulin undergoes characteristic conformational changes by binding Ca. We measured the conformational changes of calmodulin upon Ca and mastoparan binding using the time-resolved small-angle X-ray scattering technique combined with flash photolysis of caged calcium. This measurement system covers the time range of 0.5-180 ms. Within 10 ms of the stepwise increase in Ca concentration, we identified a distinct compact conformational state with a drastically different molecular dimension. This process is too fast to study with a conventional stopped-flow apparatus. The compact conformational state was also observed without mastoparan, indicating that the calmodulin forms a compact globular conformation by itself upon Ca binding. This new conformational state of calmodulin seems to regulate Ca binding and conformational changes in the N-terminal domain.