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J. Biol. Chem., Vol. 281, Issue 2, 1058-1065, January 13, 2006

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The Plant Plasma Membrane Ca²⁺ Pump ACA8 Contains Overlapping as Well as Physically Separated Autoinhibitory and Calmodulin-binding Domains^*

Lone Bækgaard, Laura Luoni, Maria Ida De Michelis, and Michael G. Palmgren1

From the Department of Plant Biology, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Copenhagen, Denmark and the Dipartimento di Biologia "L. Gorini," Università di Milano, Consiglio Nazionale delle Ricerche Istituto di Biofisica-Sezione di Milano, via G. Celoria 26, 20133 Milan, Italy

In plant Ca²⁺ pumps belonging to the P_2B subfamily of P-type ATPases, the N-terminal cytoplasmic domain is responsible for pump autoinhibition. Binding of calmodulin (CaM) to this region results in pump activation but the structural basis for CaM activation is still not clear. All residues in a putative CaM-binding domain (Arg⁴³ to Lys⁶⁸) were mutagenized and the resulting recombinant proteins were studied with respect to CaM binding and the activation state. The results demonstrate that (i) the binding site for CaM is overlapping with the autoinhibitory region and (ii) the autoinhibitory region comprises significantly fewer residues than the CaM-binding region. In a helical wheel projection of the CaM-binding domain, residues involved in autoinhibition cluster on one side of the helix, which is proposed to interact with an intramolecular receptor site in the pump. Residues influencing CaM negatively are situated on the other face of the helix, likely to face the cytosol, whereas residues controlling CaM binding positively are scattered throughout. We propose that early CaM recognition is mediated by the cytosolic face and that CaM subsequently competes with the intramolecular autoinhibitor in binding to the other face of the helix.

Received for publication, July 28, 2005 , and in revised form, October 6, 2005.

^* This work was supported by the European Union's Framework 6 Program (to M. G. P.) and a grant of the Italian Ministry for Instruction, University and Research in the PRIN 2003 frame (to M. I. D. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 45-3528-2592; Fax: 45-3528-3365; E-mail: palmgren{at}kvl.dk.

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