The redox switch of gamma-glutamylcysteine ligase via a reversible monomer-dimer transition is a mechanism unique to plants.
In plants, the first committed enzyme for glutathione biosynthesis, gamma-glutamylcysteine ligase (GCL), is under multiple controls. The recent elucidation of GCL structure from Brassica juncea (BjGCL) has revealed the presence of two intramolecular disulfide bridges (CC1, CC2), which both strongly impact on GCL activity in vitro. Here we demonstrate that cysteines of CC1 are confined to plant species from the Rosids clade, and are absent in other plant families. Conversely, cysteines of CC2 involved in the monomer-dimer transition in BjGCL are not only conserved in the plant kingdom, but are also conserved in the evolutionarily related alpha- (and some gamma-) proteobacterial GCLs. Focusing on the role of CC2 for GCL redox regulation, we have extended our analysis to all available plant (31; including moss and algal) and related proteobacterial GCL (46) protein sequences. Amino acids contributing to the homodimer interface in BjGCL are highly conserved among plant GCLs, but are not conserved in related proteobacterial GCLs. To probe the significance of this distinction, recombinant GCLs from Nicotiana tabacum (NtGCL), Agrobacterium tumefaciens (AtuGCL, alpha-proteobacteria) and Xanthomonas campestris (XcaGCL, gamma-proteobacteria) were analyzed for their redox response. As expected, NtGCL forms a homodimer under oxidizing conditions, and is activated more than threefold. Conversely, proteobacterial GCLs remain monomeric under oxidizing and reducing conditions, and their activities are not inhibited by DTT, despite the presence of CC2. We conclude that although plant GCLs are evolutionarily related to proteobacterial GCLs, redox regulation of their GCLs via CC2-dependent dimerization has been acquired later in evolution, possibly as a consequence of compartmentation in the redox-modulated plastid environment.
Gromes
Roland
R
Heidelberg Institute for Plant Sciences, Molecular Ecophysiology, Im Neuenheimer Feld 360, 69120 Heidelberg, Germany.
Hothorn
Michael
M
Lenherr
Esther D
ED
Rybin
Vladimir
V
Scheffzek
Klaus
K
Rausch
Thomas
T
D016960:Agrobacterium tumefaciens / Q000201:enzymology; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000378:metabolism; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D019143:Evolution, Molecular; D005721:Glutamate-Cysteine Ligase / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D010940:Plant Proteins / Q000378:metabolism; D055503:Protein Multimerization; D020836:Protein Structure, Quaternary; D011994:Recombinant Proteins / Q000378:metabolism; D016415:Sequence Alignment; D014026:Nicotiana / Q000201:enzymology*; D016959:Xanthomonas campestris / Q000201:enzymology
D001426:Bacterial Proteins; D004269:DNA, Bacterial; D018744:DNA, Plant; D010940:Plant Proteins; D011994:Recombinant Proteins; D005721:Glutamate-Cysteine Ligase