Plasma membrane calcium pumps

The transport of Ca2+ out of the cytosol of eucaryotic cells, which is essential to the maintenance of cellular Ca2+ homeostasis, is accomplished by two systems: a low-affinity, high-capacity Na+/Ca2+ exchanger, which is particularly active in excitable tissues, and a high-affinity, low-capacity Ca2+-ATPase (the plasma membrane Ca2+ pump, PMCA) which is active in all eucaryotic cells. The high affinity of the ATPase enables it to interact with Ca2+ with adequate efficiency even when its concentration is at the very low level prevailing in the cytosol of cells at rest (100-200nM). Thus, the PMCA pump is the fine tuner of cell Ca2+: It counteracts the action of the plasma membrane channels, across which a limited and carefully controlled amount of Ca2+ penetrates into the cytosol [1]. The ATPase was discovered in erythrocytes in 1966 [2] and was later characterized as a P-type pump [3]. In the nearly 40 years that have elapsed since its discovery, the work has developed as in the case of other transport ATPases, gradually evolving from an initial phase focused on the properties of the transport process and on the reaction mechanism to a later phase in which the enzyme was dissected molecularly and characterized genetically. Knowledge on the PMCA pump has progressed rapidly, particularly in recent years, establishing the enzyme as a central actor in the precise control of Ca2+ homeostasis in the cells, and thus in their proper functioning. A number of reviews, comprehensively covering earlier research on the pump or focusing on particular aspects, have appeared over the years [4-13]. They should be consulted to complement the information contained in the present contribution, which will deal in a comparatively rapid way with the earlier work on the pump, to concentrate on more recent research achievements.