Differential subcellular localization, enzymatic properties and expression patterns of γ-aminobutyric acid transaminases (GABA-Ts) in rice (Oryza sativa).

γ-Aminobutyric acid transaminase (GABA-T) catalyzes the conversion of GABA to succinic semialdehyde. The rice (Oryza sativa) genome possesses four putative GABA-T genes, which exhibit high amino acid identity (73-82%) but differ in length of the N-terminal region. Transient expression of GABA-T-green fluorescent fusion proteins in onion epidermal cells demonstrated that two of the four enzymes were targeted to mitochondria, a third to chloroplasts, and the fourth to cytosol. Enzymatic analysis of three organelle-targeted GABA-Ts revealed that they used pyruvate and glyoxylate as amino acceptors and that two of the enzymes functioned in mitochondria and chloroplasts at similar levels of activity, whereas the second mitochondrial enzyme displayed very low activity. Transcriptional analysis demonstrated that two of the four genes were more highly expressed in the vegetative organs tested, but exhibited a different pattern during seed maturation. Together, these results suggest that members of the rice GABA-T gene family vary in many respects, such as intracellular targeting, enzymatic activity and regulation of gene expression.