Auto-inhibition of Arabidopsis thaliana plasma membrane Ca2+-ATPase involves an interaction of the N-terminus with the small cytoplasmic loop

Type IIB Ca2+-ATPases have a terminal autoinhibitory, domain the action of which is suppressed by calmodulin (CaM) binding. Here, we show that a peptide (6His-M-1-I-116) corresponding to the first 116 aminoacids (aa) of At-ACA8, the first cloned isoform of Arabidopsis thaliana plasma membrane Ca2+-ATPase, inhibits the activity of the enzyme deprived of the N-terminus by controlled trypsin treatment 10-fold more efficiently than a peptide (I-41-T-63) corresponding only to the CaM-binding site. A peptide (E-268-W-348) corresponding to 81 aa of the small cytoplasmic loop of At-ACA8 binds peptide (268)His-M-1-I-116 immobilized on Ni-NTA agarose. Peptide E-268-W-348 stimulates Ca2+-ATPase activity. Its effect is not additive with that of CaM and is suppressed by tryptic cleavage of the N-terminus. These results provide the first functional identification of a site of intramolecular interaction with the terminal autoinhibitory domain of type IIB Ca2+-ATPases.