IDENTIFICATION OF A PEPTIDE INHIBITOR OF THE CARDIAC SARCOLEMMAL NA+-CA2+ EXCHANGER

The deduced amino acid sequence of the cardiac sarcolemmal Na+-Ca2+ exchanger has a region which could represent a calmodulin binding site. As calmodulin binding regions of proteins often have an autoinhibitory role, a synthetic peptide with this sequence was tested for functional effects on Na+-Ca2+ exchange activity. The peptide inhibits the Na+-dependent Ca2+ uptake (K(I) approximately 1.5-mu-M) and the Na(o)+-dependent Ca2+ efflux of sarcolemmal vesicles in a noncompetitive manner with respect to both Na+ and Ca2+. The peptide is also a potent inhibitor (K(I) approximately 0.1-mu-M) of the Na+-Ca2+ exchange current of excised sarcolemmal patches. The binding site for the peptide on the exchanger is on the cytoplasmic surface of the membrane. The exchanger inhibitory peptide binds calmodulin with a moderately high affinity. From the characteristics of the inhibition of the exchange of sarcolemmal vesicles, we deduce that only inside-out sarcolemmal vesicles participate in the usual Na+-Ca2+ exchange assay. This contrasts with the common assumption that both inside-out and right-side-out vesicles exhibit exchange activity.