Arabidopsis thaliana AtNUDT7, a homodimeric Nudix hydrolase active on ADP-ribose and NADH, exerts negative control on the major signaling complex involved in plant defense activation and programmed cell death. The structural and functional consequences of altering several amino-acid residues of the AtNUDT7 protein have been examined by site-directed mutagenesis, far-UV circular dichroism (CD), attenuated total reflection-Fourier transform infrared (ATR-FTIR) and photon correlation (PCS) spectroscopy, biochemical analysis and protein-protein interaction studies. Alanine substitutions of F73 and V168 disallowed dimer formation. Both the F73A- and V168A-mutated proteins displayed no observable enzymatic activity. Alanine substitution of the V69 residue did not significantly alter the enzyme activity and had no influence on dimer arrangement. The non-conserved V26 residue, used as a negative control, did not contribute to the enzyme quaternary structure or activity. Detailed biophysical characterization of the wild-type and mutant proteins indicates that the mutations do not considerably alter the secondary structure of the enzyme but they affect dimer assembly. In addition, mutating residues V69, F73 and V168 disrupted the binding of AtNUDT7 to the regulatory 14.3.3 protein. These are the first studies of the structure-function relationship of AtNUDT7, a Nudix hydrolase of important regulatory function.
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