From the Discovery of the Ca2+ Pump in Plasma Membranes to the Demonstration of its Ubiquitousness

The vesicles from corn cells have a Ca²⁺-ATPase that can be purified by calmodulin-affinity chromatography. An unambiguous way to identify a Ca²⁺-pumping ATPase in a plasma membrane would be to demonstrate the transport of Ca²⁺ with high affinity, stoichiometrically coupled to the hydrolysis of ATP within an ample range of ATP concentration in a pure preparation of plasma membrane. By contrast, obtaining pure preparations of plasma membrane from cell containing subcellular organelles is a difficult task, because membranes from the organelles could contaminate the preparation. The discovery of the effects of calmodulin and its antagonists on the Ca2+ pump from plasma membranes has been of much help in this respect. Cells from tissues and organisms as diverse as human blood, rat kidney, hen shell gland, and plants exhibit these activities, giving support to the conclusion that the Ca²⁺ pump system is present in most plasma membrane from eukaryotic cells.