Abstract
Nudix hydrolases are a family of proteins that catalyze the hydrolysis of a variety of nucleoside diphosphate derivatives. Twenty-four genes of the Nudix hydrolase homologues (AtNUDTs) with predicted localizations in the cytosol, chloroplasts, and mitochondria exist in Arabidopsis thaliana. Here, we demonstrated the comprehensive analysis of nine types of cytosolic AtNUDT proteins (AtNUDT1, -2, -4, -5, -6, -7, -9, -10, and -11). The recombinant proteins of AtNUDT2, -6, -7, and -10 showed both ADP-ribose and NADH pyrophosphatase activities with significantly high affinities compared with those of animal and yeast enzymes. The expression of each AtNUDT is individually regulated in different tissues. These findings suggest that most cytosolic AtNUDTs may substantially function in the sanitization of potentially hazardous ADP-ribose and the regulation of the cellular NADH/NAD(+) ratio in plant cells. On the other hand, the AtNUDT1 protein had the ability to hydrolyze 8-oxo-dGTP with a K(m) value of 6.8 mum and completely suppress the increased frequency of spontaneous mutations in the Escherichia coli mutT(-) strain, indicating that AtNUDT1 is a functional homologue of E. coli MutT in A. thaliana and is involved in the prevention of spontaneous mutation. The results obtained here suggest that the plant Nudix family has evolved in a specific manner that differs from that of yeast and humans.
Highlights
Nudix1 hydrolases, which are characterized by a conserved Nudix motif, GX5EX7REVXEEXGU, where U is usually Ile, Leu, or Val (1), are widely distributed in over 120 species, ranging from viruses to humans (2)
Nudix Hydrolase Genes in A. thaliana—A search for the Nudix hydrolase genes in the National Center for Biotechnology Information Data Base showed that 24 genes (AtNUDT1–24) encoding the Nudix domain existed in A. thaliana
Only AtNUDT1 has been characterized as an NADH, dihydroneopterin triphosphate, andnucleoside triphosphate pyrophosphatases (20, 21)
Summary
Nucleoside diphosphates linked to some moiety X; hMTH1, human MutT. toxic to the cell, their intracellular levels need to be precisely regulated by these enzymes (1). In contrast to human and E. coli, little is known about the functions of Nudix hydrolases and defense systems toward oxidative damage of nucleotides by MutT proteins in higher plants (12). It is yet to be proven, it is assumed that these enzymes play an important role in maintaining the concentration of intracellular ADP-ribose at a sub-toxic level, because the accumulation of ADP-ribose can be. Enzymatic characterizations of a large number of ADP-ribose pyrophosphatases in A. thaliana clearly suggest that these isoenzymes play roles in regulating the intracellular concentration of ADP-ribose in plant cells
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