On the mechanism of calcium-dependent activation of NADPH oxidase 5 (NOX5). Elisa Millana Fañanás Sofia Todesca Alessandro Sicorello Laura Masino Petr Pompach Francesca Magnani Annalisa Pastore Andrea Mattevi 10779/crick.12571178.v1 https://crick.figshare.com/articles/journal_contribution/On_the_mechanism_of_calcium-dependent_activation_of_NADPH_oxidase_5_NOX5_/12571178 It is now accepted that reactive oxygen species (ROS) are not only dangerous oxidative agents but also chemical mediators of the redox cell signaling and innate immune response. A central role in ROS-controlled production is played by the NADPH oxidases (NOXs), a group of seven membrane-bound enzymes (NOX1-5 and DUOX1-2) whose unique function is to produce ROS. Here, we describe the regulation of NOX5, a widespread family member present in cyanobacteria, protists, plants, fungi, and the animal kingdom. We show that the calmodulin-like regulatory EF-domain of NOX5 is partially unfolded and detached from the rest of the protein in the absence of calcium. In the presence of calcium, the C-terminal lobe of the EF-domain acquires an ordered and more compact structure that enables its binding to the enzyme dehydrogenase (DH) domain. Our spectroscopic and mutagenesis studies further identified a set of conserved aspartate residues in the DH domain that are essential for NOX5 activation. Altogether, our work shows that calcium induces an unfolded-to-folded transition of the EF-domain that promotes direct interaction with a conserved regulatory region, resulting in NOX5 activation. 2020-07-01 11:55:05 EF-hands NMR calcium activation enzyme structure SB NMR-ack Biochemistry & Molecular Biology 0601 Biochemistry and Cell Biology 1101 Medical Biochemistry and Metabolomics 0304 Medicinal and Biomolecular Chemistry