Unravelling the molecular mechanisms of regulation of plant type 2B Ca2+-ATPases using Arabidopsis thaliana plasma membrane isoform ACA8 as a model system

Type 2B Ca2+-ATPases of plants (ACAs) have an extended cytosolic N-terminus containing an auto-inhibitory domain which by interacting with the catalytic head hampers pump activity. Using the Arabidopsis thaliana plasma membrane isoform ACA8 as a model, we have shown that fine-tuning of plant type 2B Ca2+-ATPases depends on multiple molecular mechanisms. Calmodulin (CaM) is the best known regulator of type 2B Ca2+-ATPases: CaM-binding to ACA8 at two sites in the N-terminus suppresses auto-inhibition and determines both an increase of Vmax and a decrease of the K0.5 for free Ca2+. Beside CaM, acidic phospholipids (APL) – as e.g. phosphatidylinositol-4P – stimulate ACA8 activity via a dual mechanism, involving different APL binding sites. APL binding to the N-terminus suppresses its auto-inhibitory action similarly to CaM, while binding to another, yet unidentified, site further increases the enzyme affinity for Ca2+. In addition, the N-terminus of ACA8 contains several Ser residues