Ca(2+)-dependent phosphoregulation of the plasma membrane Ca(2+)-ATPase ACA8 modulates stimulus-induced calcium signatures

Ca(2+) signals are transient, hence, upon a stimulus-induced increase in cytosolic Ca(2+) concentration, cells have to re-establish resting Ca(2+) levels. Ca(2+) extrusion is operated by a wealth of transporters, such as Ca(2+) pumps and Ca(2+)/H(+) antiporters, which often require a rise in Ca(2+) concentration to be activated. Here, we report a regulatory fine-tuning mechanism of the Arabidopsis thaliana plasma membrane-localized Ca(2+)-ATPase isoform ACA8 that is mediated by calcineurin B-like protein (CBL) and CBL-interacting protein kinase (CIPK) complexes. We show that two CIPKs (CIPK9 and CIPK14) are able to interact with ACA8 in vivo and phosphorylate it in vitro. Transient co-overexpression of ACA8 with CIPK9 and the plasma membrane Ca(2+) sensor CBL1 in tobacco leaf cells influences nuclear Ca(2+) dynamics, specifically reducing the height of the second peak of the wound-induced Ca(2+) transient. Stimulus-induced Ca(2+) transients in mature leaves and seedlings of an aca8 T-DNA insertion line exhibit altered dynamics when compared with the wild type. Altogether our results identify ACA8 as a prominent in vivo regulator of cellular Ca(2+) dynamics and reveal the existence of a Ca(2+)-dependent CBL–CIPK-mediated regulatory feedback mechanism, which crucially functions in the termination of Ca(2+) signals.