Initial localization of regulatory regions of the cardiac sarcolemmal Na⁺-Ca²⁺ exchanger

We have analyzed the regulatory properties of the wild-type cardiac Na⁺-Ca²⁺ exchanger expressed in Xenopus laevis oocytes using the giant excised patch technique. The exchanger is activated by cytoplasmic application of chymotrypsin and exhibits a number of properties that can be changed or abolished by chymotrypsin treatment, including cytoplasmic Na⁺-dependent inactivation, secondary regulation by free cytoplasmic Ca²⁺, and inhibition by exchanger inhibitory peptide. Thus, the cloned exchanger expressed in oocytes exhibits regulatory properties similar to those of the native sarcolemmal exchanger. The exchanger protein contains a large (520 amino acids) hydrophilic domain modeled to be intracellular. The role of this region in exchanger function and regulation was examined by deletion mutagenesis. Mutants with residues 240-679 and 562-685 deleted exhibited exchange activity, indicating that this extensive region is not essential for transport function. Both mutants were stimulated by chymotrypsin treatment. Neither mutant demonstrated regulation by free cytoplasmic Ca²⁺ (Ca²⁺_i) or inhibition by exchanger inhibitory peptide (XIP). However, mutant Δ562-685 but not Δ240-679 displayed Na⁺-dependent inactivation. The data suggest that the binding sites for XIP and regulatory Ca²⁺ may reside in the region encompassed by residues 562-685. A chimera made from renal and cardiac exchangers has normal regulatory characteristics and helps to further define these sites.