Structure and Dynamics of Ca^{2 +}-Binding Domain 1 of the Na ⁺/Ca^{2 +} Exchanger in the Presence and in the Absence of Ca^{2 +}

The Na ⁺/Ca^{2 +} exchanger is the major exporter of Ca^{2 +} across the cell membrane of cardiomyocytes. The activity of the exchanger is regulated by a large intracellular loop that contains two Ca^{2 +}-binding domains, calcium-binding domain (CBD) 1 and CBD2. CBD1 binds Ca^{2 +} with much higher affinity than CBD2 and is considered to be the primary Ca^{2 +} sensor. The effect of Ca^{2 +} on the structure and dynamics of CBD1 has been characterized by NMR spectroscopy using chemical shifts, residual dipolar couplings, and spin relaxation. Residual dipolar couplings are used in a new way for residue selection in the determination of the anisotropic rotational diffusion tensor from spin relaxation data. The results provide a highly consistent description across these complementary data sets and show that Ca^{2 +} binding is accompanied by a selective conformational change among the binding site residues. Residues that exhibit a significant conformational change are also sites of altered dynamics. In particular, Ca^{2 +} binding restricts the mobility of the major acidic segment and affects the dynamics of several nearby binding loops. These observations indicate that Ca^{2 +} elicits a local transition to a well-ordered coordination geometry in the CBD1-binding site.