The Plasma Membrane Ca²⁺ ATPase: Purification by Calmodulin Affinity Chromatography, and Reconstitution of the Purified Protein

Abstract

Plasma membrane Ca²⁺ ATPases (PMCA pumps) are key regulators of cytosolic Ca²⁺ in eukaryotes. They extrude Ca²⁺ from the cytosol, using the energy of ATP hydrolysis and operate as Ca²⁺-H⁺ exchangers. They are activated by the Ca²⁺-binding protein calmodulin, by acidic phospholipids and by other mechanisms, among them kinase-mediated phosphorylation. Isolation of the PMCA in pure and active form is essential for the analysis of its structure and function. In this chapter, the purification of the pump, as first achieved from erythrocyte plasma membranes by calmodulin-affinity chromatography, is described in detail. The reversible, high-affinity, Ca²⁺-dependent interaction of the pump with calmodulin is the basis of the procedure. Either phospholipids or glycerol have to be present in the isolation buffers to keep the pump active during the isolation procedure. After the isolation of the PMCA pump from human erythrocytes the pump was purified from other cell types, e.g., heart sarcolemma, plant microsomal fractions, and cells that express it ectopically. The reconstitution of the purified pump into phospholipid vesicles using the cholate dialysis method will also be described. It allows studies of transport mechanism and of regulation of pump activity. The purified pump can be stored in the reconstituted form for several days at 4 °C with little loss of activity, but it rapidly loses activity when stored in the detergent-solubilized form.