Direct Observation of Ca2+‐Induced Calmodulin Conformational Transitions in Intact Xenopus laevis Oocytes by 19F NMR Spectroscopy

The Ca²⁺‐mediated conformational transition of the protein calmodulin (CaM) is essential to a variety of signal transduction pathways. Whether the transition in living cells is similar to that observed in buffer is not known. Here, we report the direct observation by ¹⁹F NMR spectroscopy of the transition of the Ca²⁺‐free and ‐bound forms in Xenopus laevis oocytes at different Ca²⁺ levels. We find that the Ca²⁺‐bound CaM population increased greatly upon binding the target protein myosin light‐chain kinase (MLCK) at the same Ca²⁺ level. Paramagnetic NMR spectroscopy was also exploited for the first time to obtain long‐range structural constraints in cells. Our study shows that ¹⁹F NMR spectroscopy can be used to obtain long‐range structural constraints in living eukaryotic cells and paves the way for quantification of protein binding constants.