Trypanosoma cruzi , the etiological agent of
Chagas disease , consumes
glucose and
amino acids depending on the environmental availability of each
nutrient during its complex
life cycle . For example,
amino acids are the major energy and
carbon sources in the intracellular stages of the T. cruzi
parasite , but their
consumption produces an accumulation of NH4+ in the
environment , which is toxic. These
parasites do not have a functional
urea cycle to secrete excess
nitrogen as low-
toxicity waste.
Glutamine synthetase (GS)
plays a central
role in regulating the
carbon /
nitrogen balance in the
metabolism of most living organisms. We show here that the
gene TcGS from T. cruzi encodes a functional
glutamine synthetase ; it can
complement a defect in the GLN1
gene from
Saccharomyces cerevisiae and utilizes
ATP ,
glutamate and
ammonium to yield
glutamine in vitro . Overall, its kinetic characteristics are
similar to other eukaryotic
enzymes , and it is dependent on
divalent cations . Its cytosolic/ mitochondrial
localization was confirmed by
immunofluorescence . Inhibition by
Methionine sulfoximine revealed that GS activity is indispensable under excess
ammonium conditions. Coincidently, its expression levels are maximal in the amastigote stage of the
life cycle , when
amino acids are preferably consumed, and NH4+
production is predictable. During host-
cell invasion, TcGS is required for the
parasite to escape from the parasitophorous
vacuole , a process
sine qua non for the
parasite to replicate and establish
infection in host
cells . These results are the first to establish a link between the activity of a metabolic
enzyme and the
ability of a
parasite to reach its intracellular niche to replicate and establish host-
cell infection .