DIVERSITY AND REGULATION OF PLANT Ca²⁺ PUMPS: Insights from Expression in Yeast

The spatial and temporal regulation of calcium concentration in plant cells depends on the coordinate activities of channels and active transporters located on different organelles and membranes. Several Ca²⁺ pumps have been identified and characterized by functional expression of plant genes in a yeast mutant (K616). This expression system has opened the way to a genetic and biochemical characterization of the regulatory and catalytic features of diverse Ca²⁺ pumps. Plant Ca²⁺-ATPases fall into two major types: AtECA1 represents one of four or more members of the type IIA (ER-type) Ca²⁺-ATPases in Arabidopsis, and AtACA2 is one of seven or more members of the type IIB (PM-type) Ca²⁺-ATPases that are regulated by a novel amino terminal domain. Type IIB pumps are widely distributed on membranes, including the PM (plasma membrane), vacuole, and ER (endoplasmic reticulum). The regulatory domain serves multiple functions, including autoinhibition, calmodulin binding, and sites for modification by phosphorylation. This domain, however, is considerably diverse among several type IIB ATPases, suggesting that the pumps are differentially regulated. Understanding of Ca²⁺ transporters at the molecular level is providing insights into their roles in signaling networks and in regulating fundamental processes of cell biology.