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

Recent genomic data in the model plant Arabidopsis thaliana reveal the exis tence of at least 11 Ca2+-ATPase genes, and an analysis of expressed sequen ce tags suggests that the number of calcium pumps in this organism might be even higher. A phylogenetic analysis shows that 11 Ca2+-ATPases clearly fo rm distinct groups, type IIA (or ECA for (E) under bar R-type Ca2+-(A) unde r bar TPase) and type IIB (ACA for (a) under bar utoinhibited (C) under bar a(2+)-(A) under bar TPase). While plant IIB calcium pumps characterized so far are localized to internal membranes, their animal homologues are exclu sively found in the plasma membrane. However, Arabidopsis type IIB calcium pump isoforms ACA8, ACA9 and ACA10 form a separate outgroup and, based on t he high molecular masses of the encoded proteins, are good candidates for p lasma membrane bound Ca2+-ATPases. All known plant type IIB calcium ATPases seem to employ an N-terminal calmodulin-binding autoinhibitor. Therefore i t appears that the activity of type IIB Ca2+-ATPases in plants and animals is controlled by N-terminal and C-terminal autoinhibitory domains, respecti vely. Possible functions of plant calcium pumps are described and - beside second messenger functions directly linked to calcium homeostasis - new dat a on a putative involvement in secretory and salt stress functions are disc ussed. (C) 2000 Elsevier Science B.V. All rights reserved.