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Calcium-binding Proteins; Calmodulin; Cellular Regulation; Crystal Structure; Membrane Transport; pH Regulation; Sodium Proton Exchange; Structural Biology
Abstract:
The ubiquitous mammalian Na+/H+ exchanger NHE1 has critical functions in regulating intracellular pH, salt concentration, and cellular volume. The regulatory C-terminal domain of NHE1 is linked to the ion-translocating N-terminal membrane domain and acts as a scaffold for signaling complexes. A major interaction partner is calmodulin (CaM), which binds to two neighboring regions of NHE1 in a strongly Ca2+-dependent manner. Upon CaM binding, NHE1 is activated by a shift in sensitivity toward alkaline intracellular pH. Here we report the 2.23 Å crystal structure of the NHE1 CaM binding region (NHE1CaMBR) in complex with CaM and Ca2+. The C- and N-lobes of CaM bind the first and second helix of NHE1CaMBR, respectively. Both the NHE1 helices and the Ca2+-bound CaM are elongated, as confirmed by small angle x-ray scattering analysis. Our x-ray structure sheds new light on the molecular mechanisms of the phosphorylation-dependent regulation of NHE1 and enables us to propose a model of how Ca2+ regulates NHE1 activity.
