The calcium pumps of plant cell membranes

Active calcium transport in higher plant cell membranes involves both H(+)-linked antiport (at the tonoplast) and direct (P-type) calcium pumping ATPases. Both systems act to remove calcium from the cytoplasm either by pumping it into intracellular stores or into the apoplast. This chapter considers recent advances in our knowledge of the calcium-pumping ATPases of the plant cell, located both at the plasma membrane and in intracellular membranes. Progress in characterising the types of Ca2+ pump in plant cells is particularly important as it becomes increasingly clear that designations applicable to other eukaryotic Ca2+ pumps ('PM-type' and 'SR/ER type') are much less relevant for plant cells. Responses of plant Ca2+ pumps to mammalian Ca2+ pump inhibitors and differences in estimated relative molecular mass also underline the differences between plant and animal Ca2+ pumps. Molecular cloning has resulted in the identification of an SR/ER type Ca2+ pump in plants strongly homologous to that of mammals. These advances are put into the context of research aims in characterising the function and mechanisms of the plant Ca2+ pumps, and their role not only in regulating cytosolic free calcium concentrations, but also in providing intracellular signalling pools and in the regulation of secretion is discussed.