C-terminal extension of rice glutamate decarboxylase (OsGAD2) functions as an autoinhibitory domain and overexpression of a truncated mutant results in the accumulation of extremely high levels of GABA in plant cells

• Published in: Journal of experimental botany 2007-08, Vol.58 (10), p.2699-2707
• Main Authors: Akama, Kazuhito, Takaiwa, Fumio
• Format: Article
• Language: English
• Subjects: Agrobacterium; amino acid; Amino Acid Sequence; Amino acids; Binding Sites; Biological and medical sciences; calmodulin; Calmodulin - metabolism; Cell biochemistry; Cell lines; Cell physiology; Complementary DNA; Enzymes; Fundamental and applied biological sciences. Psychology; GABA; GAD; gamma-Aminobutyric Acid - metabolism; glutamate decarboxylase; Glutamate Decarboxylase - chemistry; Glutamate Decarboxylase - genetics; Glutamate Decarboxylase - physiology; Molecular Sequence Data; Mutagenesis; Oryza - enzymology; Oryza - genetics; Oryza - metabolism; Oryza sativa; overexpression; Petunia - enzymology; Petunia - genetics; Petunia - metabolism; Plant cells; Plant physiology and development; Plant Proteins - chemistry; Plant Proteins - genetics; Plant Proteins - physiology; Plants; Plants, Genetically Modified - metabolism; Protein Structure, Tertiary; Proteins; Research Papers; Rice; Sequence Alignment; Transgenic plants; γ-aminobutyric acid
• ISSN: 0022-0957; 1460-2431
• DOI: 10.1093/jxb/erm120

Glutamate decarboxylase (GAD) converts L-glutamate to γ-aminobutyric acid (GABA), which is a non-protein amino acid present in all organisms. Plant GADs carry a C-terminal extension that binds to Ca2+/calmodulin (CaM) to modulate enzyme activity. However, rice possesses two distinct types of GAD, OsGAD1 and OsGAD2. Although they both have a C-terminal extension, the former peptide contains an authentic CaM-binding domain (CaMBD), which is common to dicotyledonous plants, while the latter does not. Therefore, the role of the C-terminal extension in functional expression of OsGAD2 was investigated. An in vitro enzyme assay using recombinant OsGAD2 proteins revealed low activity in the presence or absence of Ca2+/CaM. However, a truncated version of GAD2 (OsGAD2ΔC) had over 40-fold higher activity than wild-type GAD at physiological pH. These two DNA constructs were introduced simultaneously into rice calli via Agrobacterium to establish transgenic cell lines. Free amino acids were isolated from several lines for each construct to determine GABA content. Calli overexpressing OsGAD2 and OsGAD2ΔC had about 6-fold and 100-fold the GABA content of wild-type calli, respectively. Regenerated OsGAD2ΔC rice plants had aberrant phenotypes such as dwarfism, etiolated leaves, and sterility. These data suggest that the C-terminal extension of OsGAD2 plays a role as a strong autoinhibitory domain, and that truncation of this domain causes the enzyme to act constitutively, with higher activity both in vitro and in vivo.