Calcium transporting ATPases in plant cells.

Plant cells carry Ca2+ ATPases to maintain the low resting level of cytosolic Ca2+ (0.1-0.2 µM) found in all eukaryotes. Plant Ca2+ ATPases are mainly located in the plasma membrane, the endoplasmic reticulum and in the vacuolar membrane, and consequently large Ca2+ gradients are created across these membranes. The Ca2+ gradients are used by the cell to transmit information in a variety of physiological processes, such as in tip growth, cell division and stress responses. Ca2+ signals involve the co-ordinated action of Ca2+ channels and Ca2+ pumps. Upon Ca2+ influx via Ca2+ channels, cytosolic Ca2+ increases. The subsequent extrusion of Ca2+, by Ca2+ ATPases, ends the Ca2+ signal and restores the resting cytosolic Ca2+ level. During signal transduction, Ca2+ ATPases may also function in evoking the specificity in Ca2+ signals, i.e. by affecting the oscillating pattern of the Ca2+ signal. To understand how the distribution and regulation of Ca2+ ATPases in the plant cell can relate to a role of Ca2+ ATPases in signal transduction, specific Ca2+ ATPases were studied using isolated plant membrane vesicles.

Plasma membrane Ca2+ ATPases in wheat and spinach were characterised by high affinities for Ca2+ (Km * 0.6 µM for the activated form), pH optima at 7.2-7.3 and low substrate specificities (30-50% activities with GTP and ITP in comparison to ATP). The plasma membrane Ca2+ ATPases in wheat and spinach were stimulated by calmodulin 2 to 4-fold. The stimulations were through an increase in Vmax as well as in the affinity for Ca2+, and involved a 11-kDa terminal fragment (spinach). Calmodulin binding to the 120-kDa spinach plasma membrane Ca2+ ATPase was strictly Ca2+ dependent. An endoplasmic reticulum Ca2+ ATPase in wheat was stimulated by calmodulin up to 5-fold, but differed from the plasma membrane Ca2+ ATPases in showing a lower pH optimum (pH 6.8) and a high specificity for ATP.

Differences in pH optima between Ca2+ ATPases in different subcellular membranes may have physiological significance since pH fluctuations may specify Ca2+ signals. Ca2+-calmodulin interaction with Ca2+ ATPases provides a feedback mechanism for regulation of cytosolic Ca2+ extrusion, which can agree with a role of Ca2+ ATPases in modulating Ca2+ oscillations.