Na+/Ca2+ exchanger in Drosophila: cloning, expression, and transport differences

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Abstract
CDNAs for the Na+/Ca2+ exchanger from Drosophila melanogaster (Dmel/Nck) have been cloned by homology screening using the human heart Na+/Ca2+ exchanger cDNA. The overall deduced protein structure for Dmel/Nck is similar to that of mammalian Na+/Ca2+ exchanger genes NCX1 and NCX2, having six hydrophobic regions in the amino terminus separated from six at the carboxy-terminal end by a large intracellular loop. Sequence comparison of the Drosophila exchanger cDNAs with NCX1 and NCX2 Na+/Ca2+ exchangers are approximately 46% identical at the deduced amino acid level. Consensus phosphorylation sites for both protein kinase C and protein kinase A are present on the intracellular loop region of the Dmel/Nck. Alternative splicing for the Dmel/Nck gene is suggested in the same intracellular loop region as demonstrated for NCX1. Functionally, the Drosophila Na+/ Ca2+ exchanger expressed in oocytes differs from expressed mammalian NCX1 with regard to Ca2+ transport in Ca2+/ Ca2+ exchange and the effect of monovalent-dependent Ca2+/ Ca2+ exchange. The Dmel/Nck gene maps to chromosome 3 (93A-B) using in situ hybridization to polytene chromosomes, the same position as the Na(+)-K(+)-ATPase, a related transporter. We conclude that, although extracellular Na+ concentration-dependent Ca2+ transport is subserved by both human and Drosophila Na+/Ca2+ exchangers, there are clear and important differences in the transporters, which should be useful in deducing how the Na+/Ca2+ exchanger protein function depends on its structure.