Characterization, cloning, and in vitro expression of the extremely thermostable glutamate dehydrogenase from the hyperthermophilic Archaeon, ES4

Glutamate dehydrogenase (GDH) from the hyperthermophilic Archaeon ES4 (optimal growth temperature 98°C and maximum growth temperature 110°C) was purified to homogeneity. The purified native enzyme had an M_r of 270,000 ± 5,000 and was shown by gel filtration and SDS-polyacrylamide gel electrophoresis to be a hexamer with identical subunits of M_r. =46,000 ± 3,000. The hexameric subunit composition was also evident from electron micrographs, which show a triangular antiprism structure very similar to that of bovine GDH. The enzyme is exceptionally thermostable, with a half-time of inactivation of 3.5 h at 105°C. Differential scanning calorimetry revealed a t_m for denaturation of 113°C, and a t_m for activation at 60°C. Antigenic cross-reaction with ES4 GDH was observed with the purified GDH from the thermophilic Archaea, Pyrococcus furiosus and Thermococcus litoralis as well as with bovine and yeast GDHs. The genome of ES4 was shown to contain a single copy of the gdhA gene, and this was cloned and sequenced. The deduced amino acid sequence of the GDH from ES4 corresponded to the NH₂-terminal amino acid sequence obtained from the pure protein. From the nucleotide sequence the ES4 protein is composed of 420 residues. It has a relatively high hydrophobicity and a low number of sulfur-containing residues compared with mesophilic GDHs. Relatively high homology (52%) exists between the deduced amino acid sequence of ES4 GDH and Clostridium difficile GDH. Of the two distinct families of GDH sequences known, ES4 GDH belongs to the same family as vertebrates, C. difficile, and other Archaea. The gdhA gene of ES4 was expressed in vitro in a rabbit reticulocyte cell-free lysate, thus providing a system for structural studies of the mechanisms of thermostability in hyper-thermophilic proteins.