Activation of the Ca2+-Mg2+-ATPase by Ca-EGTA in plasmalemma from the roots of Cucumber

Purification of the Plasma Membrane Ca2+-ATPase from Radish Seedlings by Calmodulin-Agarose Affinity Chromatography

The Ca2+-ATPase of the plasma membrane (PM) of germinating radish (Raphanus sativus L.) seeds was purified by calmodulin (CaM)-affinity chromatography using a batch procedure. PM purified by aqueous two-phase partitioning was solubilized with n-dodecyl beta-d-maltoside and applied to a CaM-agarose matrix. After various washings with decreasing Ca2+ concentrations, the Ca2+-ATPase was eluted with 5 mm ethylenediaminetetraacetate (EDTA). The EDTA-eluted fraction contained about 25% of the loaded Ca2+-ATPase activity, with a specific activity 70-fold higher than that of the starting PM fraction. The EDTA-eluted fraction was highly enriched in a 133-kD polypeptide, which was identified as the PM Ca2+-ATPase by 125I-CaM overlay and fluorescein-isothiocyanate labeling. The PM Ca2+-ATPase cross-reacted with an antiserum against a putative Ca2+-ATPase of the Arabidopsis thaliana chloroplast envelope.

PM-type calcium pumps are associated with higher plant cell intracellular membranes

The supposition that all eukaryotic cell types contain a plasma membrane (PM)-type Ca pump (i.e. a Ca pump which is directly-stimulated by calmodulin and located exclusively at the PM) has been questioned by recent data from higher plant cells. These studies suggest the presence of Ca pumps directly stimulated by calmodulin associated with an intracellular membrane (probably the endoplasmic reticulum, ER) in a variety of monocotelydonous and dicotelydonous species. Thus plants have a 'PM-type' Ca pump at an intracellular membrane. The evidence for this includes studies on isolated membranes, purification and functional reconstitution and phosphorylated intermediate formation. Plant cells also contain a homologue of the sarcoplasmic reticulum/endoplasmic reticulum (SR/ER) Ca pump, probably located at the ER. The implications of these new data for our appreciation of the structure, function and location of eukaryotic Ca pumps are discussed, together with recent data from the use of inhibitors specific to mammalian ER/SR Ca pumps.