Formyl-methenyl-methylenetetrahydrofolate synthetase(combined) from yeast. Biochemical characterization of the protein from an ade3 mutant lacking the formyltetrahydrofolate synthetase function.

A protein from Saccharomyces cerevisiae mutant ade3-1050, a formyltetrahydrofolate synthetase-deficient mutant, has been purified to apparent homogeneity. The purified mutant enzyme shows both methylenetetrahydrofolate dehydrogenase and methenyltetrahydrofolate cyclohydrolase activities, but lacks formyltetrahydrofolate synthetase activity. The biochemical characterization of the mutant protein described in this paper is consistent with genetic data which indicate that the 1050 mutation is a point mutation at the ade3 locus of chromosome VII of S. cerevisiae. The molecular weight of the native mutant protein (Mr = 227,000 by exclusion chromatography), as well as the number and size of its subunits are exactly the same as those of the trifunctional wild type enzyme. In addition, both proteins have the same sedimentation behavior in a glycerol density gradient (s20,w = 9.4 S), and their activities and structures are equally affected by exposure to mild tryptic degradation. ATP protects both enzymes from tryptic degradation, but NADP+ does not. Some of the kinetic properties of the activities of both enzymes were also determined and were essentially similar. Although both enzymes require the presence of metals for maximal synthetase and dehydrogenase activities, metals are not necessary to maintain their structures intact.