Conserved N-terminal Regulation of the ACA8 Calcium Pump with Two Calmodulin Binding Sites

The autoinhibited plasma membrane calcium ATPase, ACA8 from A. thaliana has an N-terminal autoinhibitory domain. Calcium-bound calmodulin binding at two sites located at residues 42-62 and 74-96 relieves autoinhibition of ACA8 activity. We investigated N-terminally truncated ACA8 constructs (WT, Delta20, Delta30, Delta35, Delta37, Delta40, Delta74 and Delta100) to explore the role of conserved motifs in the N-terminal segment preceding the calmodulin binding sites. Furthermore, we purified WT, Delta20- and Delta100-ACA8, tested activity in vitro and performed structural studies of purified Delta20-ACA8 stabilized in its native form to explore the mechanism of autoinhibition. Through activity studies and a yeast complementation assay, we show that an N-terminal segment between residues 20 and 35, upstream of the calmodulin binding sites, is important for autoinhibition and the activation by calmodulin, and that a conserved Phe32 is essential for autoinhibition. Cryo-EM structure determination at 3.3 Angstrom resolution of a beryllium fluoride inhibited form shows no autoinhibition, but a low-resolution structure for an E1 state indicates autoinhibitory domain binding consistent with the mutational studies and AlphaFold predicted structures.