ANOMALOUS REGULATION OF THE DROSOPHILA NA-CA2+ EXCHANGER BY CA2+()

The Na+-Ca2+ exchanger from Drosophila was expressed in Xenopus oocyte s and characterized electrophysiologically using the giant excised pat ch technique. This protein, termed Calx, shares 49% amino acid identit y to the canine cardiac Na+-Ca2+ exchanger, NCX1. Calx exhibits proper ties similar to previously characterized Na+-Ca2+ exchangers including intracellular Na+ affinities, current-voltage relationships, and sens itivity to the peptide inhibitor, XIP. However, the Drosophila Na+-Ca2 + exchanger shows a completely opposite response to cytoplasmic Ca2+. Previously cloned Na+-Ca2+ exchangers (NCX1 and NCX2) are stimulated b y cytoplasmic Ca2+ in the micromolar range (0.1-10 mu M) This stimulat ion of exchange current is mediated by occupancy of a regulatory Ca2binding site separate from the Ca2+ transport site. In contrast, Calx is inhibited by cytoplasmic Ca2+ over this same concentration range. T he inhibition of exchange current is evident for both forward and reve rse modes of transport The characteristics of the inhibition are consi stent with the binding of Ca2+ at a regulatory site distinct from the transport site. These data provide a rational basis for subsequent str ucture-function studies targeting the intracellular Ca2+ regulatory me chanism.