Molecular aspects of higher plant P-type Ca2+-ATPases

AbstractRecent genomic data in the model plant Arabidopsis thaliana reveal the existence of at least 11 Ca2+-ATPase genes, and an analysis of expressed sequence tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca2+-ATPases clearly form distinct groups, type IIA (or ECA for ER-type Ca2+-ATPase) and type IIB (ACA for autoinhibited Ca2+-ATPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclusively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on the high molecular masses of the encoded proteins, are good candidates for plasma membrane bound Ca2+-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore it appears that the activity of type IIB Ca2+-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respectively. Possible functions of plant calcium pumps are described and – beside second messenger functions directly linked to calcium homeostasis – new data on a putative involvement in secretory and salt stress functions are discussed