Yeast glutamine-fructose-6-phosphate aminotransferase (Gfa1) requires methionine aminopeptidase activity for proper function

Methionine aminopeptidase (MetAP) catalyzes the co-translational processing of initiator methionine from nascent proteins. A cellular requirement for MetAP activity is likely due to dysfunction of MetAP substrates that require methionine removal for proper protein function. Glutamine-fructose-6-phosphate aminotransferase (Gfa1) is an essential enzyme in yeast that catalyzes the first and rate-limiting step in hexosamine biosynthesis. The alpha-amino group of Gfa1 Cys-1 has been proposed to act as a nucleophile in the catalytic mechanism. We used two mutational strategies to evaluate whether removal of initiator methionine, catalyzed by MetAP, is required for Gfa1 function. Our results demonstrate that exposure of the alpha-amino group of Cys-1 is required for normal Gfa1 function as failure to do so results in decreased enzyme activity and slow growth. Further, either isoform of MetAP in yeast is sufficient for Gfa1 processing in vivo. These results are the first demonstration of an endogenous yeast protein that requires the exposure of the alpha-amino group by MetAP action for normal function. Additionally, Gfa1 will be a relevant target in therapeutic or physiological applications in which MetAP activity is inhibited.