Folate synthesis in plants: The first step of the pterin branch is mediated by a unique bimodular GTP cyclohydrolase I

摘要: GTP cyclohydrolase I (GCHI) mediates the first and committing step of pterin branch folate-synthesis pathway. In microorganisms mammals, GCHI is a homodecamer ≈26-kDa subunits. Genomic approaches identified tomato Arabidopsis cDNAs specifying ≈50-kDa proteins containing two GCHI-like domains in tandem indicated that such bimodular occur other plants. Neither domain these has full set residues involved substrate binding catalysis GCHIs. The nevertheless encode functional enzymes, as shown by complementation yeast fol2 mutant assaying activity extracts complemented cells. expressed separately had activity. Recombinant formed dihydroneopterin triphosphate reaction product, do GCHIs, but unlike enzymes it did not show cooperative behavior was inhibited its substrate. Denaturing gel electrophoresis verified polypeptide cleaved vivo into component domains, size-exclusion chromatography active enzyme dimer. deduced polypeptides lack overt targeting sequences thus are presumably cytosolic, contrast to plant which mitochondrial with typical signal peptides. mRNA protein strongly unripe fruits, implying fruit folate made situ rather than imported. As ripening advances, expression declines sharply, content drops, suggesting synthesis fails keep pace turnover.