SODIUM PROTON ANTIPORTER IN THE EURYHALINE CRAB CARCINUS-MAENAS - MOLECULAR-CLONING, EXPRESSION AND TISSUE DISTRIBUTION/

Gill epithelial cells of euryhaline crustaceans demonstrate net inward transport of sodium ions, possibly via apical Na+/H+ antiporters, NaK+/2Cl(-) cotransporters or Na+ channels working in series with the ba solateral Na++K+-ATPase. We have identified and sequenced the cDNA cod ing for a crustacean Na+/H+ antiporter, starting with mRNA isolated fr om gills of the euryhaline green shore crab Carcinus maenas. The compl ete 2595-base-pair cDNA includes an open reading frame coding for a 67 3-amino-acid protein. A search of GenBank revealed more than 20 high-s coring matches, all Na+/H+ antiporter sequences from mammalian, amphib ian, teleost and nematode species. Injection of Xenopus laevis oocytes with cRNA transcribed from the cloned crab sequence substantially enh anced Na+-dependent H+ efflux from the oocytes. Analysis of crab tissu e antiporter mRNA levels by semi-quantitative reverse transcription-po lymerase chain reaction revealed that posterior and anterior gills of Carcinus maenas expressed this antiporter the most strongly, followed in decreasing order by skeletal muscle, hepatopancreas, hypodermis and heart. Hydropathy and transmembrane alpha-helix analysis suggested a 10-helix membrane-spanning topology of the antiporter protein. It is c lear from this study that Carcinus maenas gills vigorously transcribe a gene coding for a Na+/H+ antiporter. Whether these gills also expres s a gene coding for an epithelial Na+ channel or Na+K+/2Cl(-) cotransp orter remains to be demonstrated.