Mutations in several steps of de novo
purine synthesis lead to human
inborn errors of metabolism often characterized by
mental retardation,
hypotonia,
sensorineural hearing loss,
optic atrophy, and other features. In animals, the phosphoribosylglycinamide
transformylase (GART) gene encodes a trifunctional
protein carrying out 3 steps of de novo
purine synthesis, phosphoribosylglycinamide synthase (GARS), phosphoribosylglycinamide
transformylase (also abbreviated as GART), and phosphoribosylaminoimidazole
synthetase (AIRS) and a smaller
protein that contains only the GARS domain of GART as a functional
protein. The GART gene is located on human chromosome 21 and is aberrantly regulated and overexpressed in individuals with
Down syndrome (DS), and may be involved in the phenotype of DS. The GART activity of GART requires
10-formyltetrahydrofolate and has been a target for anti-
cancer drugs. Thus, a considerable amount of information is available about GART, while less is known about the GARS and AIRS domains. Here we demonstrate that the
amino acid residue glu75 is essential for the activity of the GARS
enzyme and that the gly684 residue is essential for the activity of the AIRS
enzyme by analysis of mutations in the Chinese hamster ovary (CHO-K1) cell that require
purines for growth. We report the effects of these mutations on
mRNA and
protein content for GART and GARS. Further, we discuss the likely mechanisms by which mutations inactivating the GART
protein might arise in CHO-K1 cells.