Arabidopsis ATPase family gene 1-like protein 1 is a calmodulin-binding AAA+-ATPase with a dual localization in chloroplasts and mitochondria.

Members of the AAA(+)-ATPase superfamily (ATPases associated with various cellular activities) are found in all kingdoms of life and they are involved in very diverse cellular processes, including protein degradation, membrane fusion or cell division. The Arabidopsis genome encodes approximately 140 different proteins that are putative members of this superfamily, although the exact function of most of these proteins remains unknown. Using affinity chromatography on calmodulin-agarose with chloroplast proteins, we purified a 50 kDa protein encoded by AT4G30490 with similarity to the ATPase family gene 1 protein from yeast. Structural analysis showed that the protein possesses a single AAA-domain characteristic for members of the AAA(+)-ATPase superfamily and that this contains all features specific to proteins of the ATPase family gene 1-like subfamily. In vitro pull-down as well as cross-linking assays corroborate calcium-dependent binding of the protein to calmodulin. The calmodulin binding domain could be located to a region of 20 amino acids within the AAA-domain in close proximity to the Walker A motif. Our analysis further showed that the protein is localized in both mitochondria and chloroplasts, further supporting the incorporation of both endosymbiotic organelles into the calcium-signaling network of the cell. Localization of the same calmodulin-binding protein into mitochondria and chloroplasts could be a means to provide a coordinated regulation of processes in both organelles by calcium signals.

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