Folate Synthesis in Plants: The First Step of the Pterin Branch Is Mediated by a Unique Bimodular GTP Cyclohydrolase I

• Published in: Proceedings of the National Academy of Sciences - PNAS 2002-09, Vol.99 (19), p.12489-12494
• Main Authors: Basset, Gilles, Quinlivan, Eoin P., Ziemak, Michael J., de la Garza, Rocío Díaz, Fischer, Markus, Schiffmann, Susi, Bacher, Adelbert, Gregory, Jesse F., Hanson, Andrew D.
• Format: Article
• Language: English
• Subjects: Active sites; Amino Acid Sequence; Arabidopsis - genetics; Arabidopsis - growth & development; Arabidopsis - metabolism; Base Sequence; Biological Sciences; Complementary DNA; Dimerization; Dimers; DNA, Complementary - genetics; DNA, Plant - genetics; Enzyme Inhibitors - pharmacology; Enzymes; Flowers & plants; Folic Acid - biosynthesis; Gene Expression Regulation, Plant; Genes, Plant; GTP Cyclohydrolase - antagonists & inhibitors; GTP Cyclohydrolase - chemistry; GTP Cyclohydrolase - genetics; GTP Cyclohydrolase - metabolism; Guanosine Triphosphate - pharmacology; Lycopersicon esculentum - genetics; Lycopersicon esculentum - growth & development; Lycopersicon esculentum - metabolism; Mammals; Messenger RNA; Metabolism; Molecular Sequence Data; Plants; Protein Structure, Tertiary; Proteins; Pterins; Pterins - chemistry; Pterins - metabolism; RNA, Messenger - genetics; RNA, Messenger - metabolism; RNA, Plant - genetics; RNA, Plant - metabolism; Sequence Homology, Amino Acid; Species Specificity; Yeasts
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.192278499

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