Unique topology of the internal repeats in the cardiac Na+/Ca2+ exchanger

Hydropathy analysis predicts 11 transmembrane helices in the cardiac Na+/Ca2+ exchanger. Using cysteine susceptibility analysis and epitope tagging, we here studied the membrane topology of the exchanger, in particular of the highly conserved internal alpha-1 and alpha-2 repeats. Unexpectedly, we found that the connecting loop in the alpha-1 repeat forms a re-entrant membrane loop with both ends facing the extracellular side and one residue (Asn-125) being accessible from the inside and that the region containing the alpha-2 repeat is mostly accessible from the cytoplasm. Together with other data, we propose that the exchanger may consist of nine transmembrane helices.