A plant glutamate decarboxylase containing a calmodulin binding domain: cloning, sequence, and functional analysis

• Published in: The Journal of biological chemistry Vol. 268; no. 26; pp. 19610 - 19617
• Main Authors: Baum, G, Chen, Y, Arazi, T, Takatsuji, H, Fromm, H
• Format: Journal Article
• Language: English
• Published: Bethesda, MD American Society for Biochemistry and Molecular Biology 15.09.1993
• Subjects: ACTIVIDAD ENZIMATICA; ACTIVITE ENZYMATIQUE; Amino Acid Sequence; Analytical, structural and metabolic biochemistry; Base Sequence; Binding Sites; Biological and medical sciences; CALCIO; CALCIUM; Calmodulin - metabolism; Chromatography, Affinity; Cloning, Molecular; DNA; Enzymes and enzyme inhibitors; Escherichia coli - enzymology; Escherichia coli - genetics; Fundamental and applied biological sciences. Psychology; GLUTAMATE DECARBOXYLASE; Glutamate Decarboxylase - genetics; Glutamate Decarboxylase - isolation & purification; Glutamate Decarboxylase - metabolism; GLUTAMATO DECARBOXILASA; Glutathione Transferase - isolation & purification; Glutathione Transferase - metabolism; Kinetics; Lyases; Molecular Sequence Data; PETUNIA; Petunia hybrida; Plants - enzymology; Plants - genetics; Protein Conformation; PROTEINAS AGLUTINANTES; PROTEINE DE LIAISON; Recombinant Fusion Proteins - isolation & purification; Recombinant Proteins - isolation & purification; Recombinant Proteins - metabolism; SECUENCIA NUCLEICA; Sequence Homology, Amino Acid; SEQUENCE NUCLEIQUE; Carbon-carbon lyases; Vegetals; Purification; Enzyme; Carboxy-lyases; GABA; Primary structure; Lyases; Glutamate decarboxylase; Binding site; Recombinant protein; Calmodulin
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1016/s0021-9258(19)36560-3

Molecular procedures have been applied to isolate plant calmodulin-binding proteins. A petunia cDNA expression library was screened with 35S-labeled recombinant calmodulin as a probe, and a cDNA coding for a Ca(2+)-dependent calmodulin-binding protein was isolated. The deduced amino acid sequence of the petunia protein (500 amino acid residues, 58 kDa) has 67% overall amino acid sequence similarity to glutamate decarboxylase (GAD) from Escherichia coli (466 amino acid residues, 53 kDa). The recombinant protein expressed in E. coli cells displays GAD activity, i.e. catalyzes the conversion of glutamic acid to gamma-aminobutyric acid and binds calmodulin, whereas E. coli GAD does not bind calmodulin. The calmodulin binding domain in the petunia GAD was mapped by binding truncated forms of GAD immobilized on nitrocellulose membranes to recombinant petunia 35S-calmodulin as well as to biotinylated bovine calmodulin and by binding truncated forms of GAD to calmodulin-Sepharose columns. The calmodulin binding domain in petunia GAD is part of a carboxyl end extension that is not present in E. coli GAD. Polyclonal antibodies raised against the recombinant petunia GAD detect a single protein band from plant extracts of gel mobility identical to that of the recombinant GAD. Moreover, the plant protein binds calmodulin in vitro. This is the first report of the isolation of a GAD gene from plants and of a calmodulin-binding GAD from any organism. Our results raise the possibility that intracellular Ca(2+) signals via calmodulin are involved in the regulation of gamma-aminobutyric acid synthesis in plants