Characterization of tocopherol cyclases from higher plants and cyanobacteria. Evolutionary implications for tocopherol synthesis and function

Tocopherols are lipophilic antioxidants synthesized exclusively by photosynthetic organisms and collectively constitute vitamin E, an essential nutrient for both humans and animals. Tocopherol cyclase (TC) catalyzes the conversion of various phytyl quinol pathway intermediates to their corresponding tocopherols through the formation of the chromanol ring. Herein, the molecular and biochemical characterization of TCs from Arabidopsis (VTE1 VITAMIN E 1]), Zea mays (SXD1 Sucrose Export Deficient 1]) and Synechocystis sp. PCC6803 (slr1737) are described. Mutations in the VTE1, SXD1, or slr1737 genes resulted in both tocopherol deficiency and the accumulation of 2,3-dimethyl-6-phytyl-1,4-benzoquinone (DMPBQ), a TC substrate. Recombinant SXD1 and VTE1 proteins are able to convert DMPBQ to gamma-tocopherol in vitro. In addition, expression of maize SXD1 in a Synechocystis sp. PCC6803 slr1737 knockout mutant restored tocopherol synthesis, indicating that TC activity is evolutionarily conserved between plants and cyanobacteria. Sequence analysis identified a highly conserved 30-amino acid C-terminal domain in plant TCs that is absent from cyanobacterial orthologs. vte1-2 causes a truncation within this C-terminal domain, and the resulting mutant phenotype suggests that this domain is necessary for TC activity in plants. The defective export of Suc in sxd1 suggests that in addition to presumed antioxidant activities, tocopherols or tocopherol breakdown products also function as signal transduction molecules, or, alternatively, the DMPBQ that accumulates in sxd1 disrupts signaling required for efficient Suc export in maize.