The predicted secondary structure of the G-type glutamine amidotransferase is compatible with TIM-barrel topology

Glutamine amidotransferase (GAT) subunits or domains catalyze an important partial reaction in many complex biosynthetic reactions. The structure of one member of the F-type GATs is known, but the structure of the unrelated G-type is still unknown. Because many protein sequences are available for anthranilate synthase component II (product of the trpG gene), we have predicted its average secondary structure by a joint prediction method [Niermann and Kirschner (1991a) Protein Engng, 4, 359-370]. The predicted eight beta-strands and seven alpha-helices follow an 8-fold cyclic repetition of a beta-strand-loop-alpha-helix-loop module with helix alpha 7 missing. This pattern of secondary structure suggests that the G-type GAT domain has an 8-fold beta alpha-barrel topology, as found first in triose phosphate isomerase (TIM-barrel). This model is supported by the location of known catalytically essential residues in loops between beta-strands and alpha-helices. Evidence from published sequencing and mutational studies on selected members of the GAT superfamily (carbamoyl phosphate, imidazoleglycerol phosphate, GMP and CTP synthases) support both the secondary structure prediction and the TIM-barrel topology.