Mouse cDNAs encoding a trifunctional protein of de novo purine synthesis and a related single-domain glycinamide ribonucleotide synthetase.

Three of the enzymatic activities of de novo purine synthesis, glycinamide ribonucleotide synthetase (GARS), aminoimidazole ribonucleotide synthetase (AIRS) and glycinamide ribonucleotide formyltransferase (GART), can be catalyzed by a single 110-kDa protein in mouse cells. Western blots using a polyclonal antibody (Ab) to this protein identified two species, the trifunctional 110-kDa protein and a 50-kDa cytosolic protein with GARS, but not GART activity. We used Ab and, subsequently, oligodeoxyribonucleotide screens to isolate cDNAs corresponding to these two proteins from mouse T-cell cDNA expression libraries. The sequence of one class of these cDNAs and the partial sequence of a corresponding genomic clone defined an open reading frame (ORF) encoding a 1010-amino-acid (aa) protein, individual domains of which showed high homology to each of the monofunctional bacterial GARS, AIRS and GART proteins, and to each domain of chicken and human trifunctional GARS-AIRS-GARTs. cDNAs corresponding to the smaller protein contained a 1.3-kb ORF with complete identity to the GARS domain of, but with a 3' untranslated region different from, the trifunctional cDNAs. Hence, both cDNAs appear to derive from the same gene due to either differential splicing or use of an intronic polyadenylation signal. The functional requirement for the expression of both trifunctional protein with GARS activity and monofunctional, catalytically active GARS is unknown.