SIGNAL-TRANSDUCTION VERSUS BUFFERING ACTIVITY IN CA2+-BINDING PROTEINS

The three-dimensional structure of calbindin D-9k in the absence of Ca2+ has been determined using NMR spectroscopy in solution, allowing the first direct analysis of the consequences of Ca2+ binding for a member of the calmodulin superfamily of proteins. The overall response in calbindin D-9k is much attenuated relative to the current model for calmodulin and troponin C. These results demonstrate a novel mechanism for modulating the conformational response to Ca2+-binding in calmodulin superfamily proteins and provide insights into how their Ca2+-binding domains can be fine-tuned to remain essentially intact or respond strongly to ion binding, in relation to their functional requirements.