INTERACTION OF CARDIAC NA-CA EXCHANGER AND EXCHANGE INHIBITORY PEPTIDE WITH MEMBRANE PHOSPHOLIPIDS

We tested the hypothesis that the exchange inhibitory peptide (XIP) domain in the cardiac Na-Ca exchanger is a regulatory site under the control of the membrane lipid environment. We found that I-125-XIP bound to liposomes composed of phosphatidylcholine (PC) and phosphatidylserine (PS) with peak binding at 1:1 PC/PS. No binding was observed in PC liposomes. XIP and pentalysine-inhibitable bovine sarcolemmal (SL) Na-Ca exchange activity was observed in reconstituted proteoliposomes composed of 1:1 PC/PS. Proteolysis of SL membranes resulted in a twofold stimulation of Na-Ca exchange activity, but the half-maximal inhibitory concentration (IC50) for XIP (3 mu M) was not significantly changed, suggesting that the XIP binding site remained intact. In contrast, the IC50 for pentalysine was decreased from 500 to 150 mu M in proteolyzed membranes. These data are consistent with a model of Na-Ca exchange regulation in which the endogenous XIP domain interacts either with another region of the exchange protein to induce an inactive conformational state or with membrane lipid to produce an active conformation.