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.