Glutathione
synthetase is functional both as a homodimer and as a heterotetramer in the
fission yeast Schizosaccharomyces pombe .
Nadine Phlippen (1),
Kurt Hoffmann (2), Klaus Wolf (1), Martin Zimmermann (1)
(1) Institute of Biology IV, Aachen University, Worringer Weg, Aachen, 52056,
Germany (Nadine.Phlippen@epost.de); (2) Institute of Biology VII, Aachen
University, Worringer Weg, Aachen, 52056, Germany
Glutathione is a biologically important thiol found in almost all prokaryotic and eukaryotic cells. It is synthesized enzymatically from its constituent amino acids in two consecutive steps, catalysed by gamma-glutamyl-cysteine synthetase and glutathione synthetase. In most eukaryotic organisms glutathione synthetase is a homodimer composed of two identical subunits. The fission yeast enzyme, however, has been purified and described as a heterotetramer consisting of two identical small and large subunits each. The large subunit has been assigned to the GSH2 gene. We show that the enzyme is active in vivo and in vitro in two different forms: a homodimeric protein consisting of two identical subunits and a heterotetrameric protein composed of two large and two small subunits. Both the large and the small subunit are encoded by the GSH2 open reading frame. They arise from proteolytic cleavage of a precursor protein. A stable functional homodimer can be generated by site-directed mutagenesis. Expression of the separately encoded small and large subunit in the fission yeast results in an enzymatically active protein.