Biochemistry and molecular physiology of the protein ABA3

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It is the aim of this project to study the detailed function of the Arabidopsis regulatory protein ABA3. In our previous work, we have shown that ABA3 catalyzes a novel type of posttranslational protein modification: activation by sulfuration. ABA3 is a molybdenum cofactorsulfurase that activates the two Mo-containing enzymes aldehyde oxidase (AO) and xanthine dehydrogenase (XDH). AO catalyzes that last step of abscisic acid (ABA) biosynthesis, and both AO and XDH produce ROS. The concentration of ABA and ROS can be rapidly increased by changing the ratio between inactive and active XDH and AO molecules, as catalyzed by the ABA3 protein. - The aim of this project is twofold: In the first part we want to delineate the detailed reaction mechanism of regulatory sulfur transfer catalyzed by ABA3. This sequence of reaction steps of intra- and intermolecular sulfur-transfer is novel and has no precedent. Thus it will constitute the prototype for other groups to decipher this kind of post-translational activation reactions in mammals, lower eukaryotes and bacteria where nothing is known about it. In the second part, we intend to clarify the physiological role of ABA3 as meanwhile a number of tools and assays has been worked out in our group for studying expression and activity of the low-abundant protein ABA3. We want to show which functions of ABA3 are related to the activation of AO and XDH and which functions are not connected to the activation of AO/XDH. In this context, we will analyze the importance of alternative splicing and of nuclear localization of ABA3 for which we have first experimental indications.