NCX1: mechanism of transport.

The plasma membrane Na(+)/Ca(2+) exchanger (NCX) plays a critical role in the maintenance of Ca(2+) homeostasis in a variety of tissues. NCX accomplishes this task by either lowering or increasing the intracellular Ca(2+) concentration, a process which depends on electrochemical gradients. During each cycle, three Na(+) are transported in the opposite direction to one Ca(2+), resulting in an electrogenic transport that can be measured as an ionic current.The residues involved in ion translocation are unknown. A residue thought to be important for Na(+) and/or Ca(2+) transport, Ser(110), was replaced with a cysteine, and the properties of the resulting exchanger mutant were analyzed using the giant patch technique. Data indicate that this residue, located in transmembrane segment 2 (part of the α-1 repeat), is important for both Na(+) and Ca(2+) translocations. Using cysteine susceptibility analysis, we demonstrated that Ser(110) is exposed to the cytoplasm when the exchanger is in the inward state configuration.