Ultra-fast Ca 2+ -transport by PMCA-Neuroplastin Complexes

Plasma membrane Ca2+-ATPases (PMCA), or Ca2+-pumps, terminate Ca2+-signals in any type of cell by extruding Ca2+ from the cytosol to the extracellular space. In neurons and epithelial cells, Ca2+-extrusion occurs within tens of milliseconds, a speed that is not compatible with current knowledge on PMCA activity. Here we investigated the transport velocity of Ca2+ -pumps assembled from PMCA2 and the recently identified auxiliary subunit Neuroplastin in intact cells. Using Ca2+-activated K+ channels as fast reporters, we show that PMCA2-Neuroplastin complexes, the most abundant Ca2+-transporters in the mammalian brain, provide Ca2+-clearing in the low millisecond-range. Freeze-fracture derived immuno-EM data on densities of Ca2+-source(s) and Ca2+-transporters translated these kinetics into transport rates for PMCA2-Neuroplastin complexes of more than 6000 cycles/s. Direct comparison with the Na+/Ca2+-exchanger NCX2, an alternate-access transporter with fast kinetics, indicated similar efficiencies in Ca2+-transport. Our results establish PMCA-Neuroplastin complexes as Ca2+-transporters with unanticipated high transport rates and demonstrate that under cellular conditions ATPases may operate in the kHz range.