Electrophysiological characterization of the archaeal transporter NCX_Mj using solid supported membrane technology

Sodium-calcium exchangers (NCXs) are membrane transporters that play an important role in Ca(2+) homeostasis and Ca(2+) signaling. The recent crystal structure of NCX_Mj, a member of the NCX family from the archaebacterium Methanococcus jannaschii, provided insight into the atomistic details of sodium-calcium exchange. Here, we extend these findings by providing detailed functional data on purified NCX_Mj using solid supported membrane (SSM)-based electrophysiology, a powerful but unexploited tool for functional studies of electrogenic transporter proteins. We show that NCX_Mj is highly selective for Na(+), whereas Ca(2+) can be replaced by Mg(2+) and Sr(2+) and that NCX_Mj can be inhibited by divalent ions, particularly Cd(2+) By directly comparing the apparent affinities of Na(+) and Ca(2+) for NCX_Mj with those for human NCX1, we show excellent agreement, indicating a strong functional similarity between NCX_Mj and its eukaryotic isoforms. We also provide detailed instructions to facilitate the adaption of this method to other electrogenic transporter proteins. Our findings demonstrate that NCX_Mj can serve as a model for the NCX family and highlight several possible applications for SSM-based electrophysiology.