FIGURE 3.

Mutations of residues coordinating Ca²⁺ at sites 3 and 4 of CBD1 decrease the exchanger Ca²⁺ affinity. A, representative outward currents recorded from oocytes expressing the indicated mutant. Residues Glu³⁸⁵, Asp⁴²¹, Asp⁴⁴⁶, Asp⁴⁴⁷, Asp⁴⁹⁸, Asp⁴⁹⁹, and Asp⁵⁰⁰ of CBD1 were mutated to Ala in mutant M7. Notice that higher Ca²⁺ concentrations are required to activate the mutant exchangers. Ca²⁺ concentrations are shown below the traces. B, dose-response curves for cytoplasmic Ca²⁺ for WT and the indicated exchanger mutants. Current amplitudes were measured at peak currents. Residual current recorded in the absence of Ca²⁺ has been subtracted. Each point is the average of between two and four experiments. The M7 peak current was normalized at the highest concentration of Ca²⁺ examined because saturation was not obtained. C, fractional activity values for each exchanger. Measurements were done in the presence of the indicated Ca²⁺ concentrations.