Violaxanthin de-epoxidase and its closest relative: identification and characterization

Abstract: Light is essential for plants and algae to process photosynthesis. However, excess of light will cause damage to the organism. A process called non-photochemical quenching (NPQ) is an important way for these organisms to protect themselves from photo oxidative damage. The NPQ process is depend on the xanthophyll cycle in thylakoids, which is controlled by the Violaxanthin de-epoxidase (VDE) and zeaxanthin epoxidase (ZE). VDE converts Violaxanthin to to Zeaxanthin on the lumen side of the thylakoid membrane under acid pH conditions caused by photosynthesis. The mature VDE sequence can be divided into three domains, an N-terminal domain with conserved cysteine pattern, a lipocalin-like domain which expected to carry the substrate binding site, and a C-terminal domain rich in glutamic acids. In this work, we have constructed cysteine mutants that revealed that 12 of the 13 cysteines in VDE are essential for the activity; instead of directly contribute to catalytic function, these 12 cysteines formed disultphide bonds for VDE structural folding. The catalytic center of VDE does not appear to be only located in the cysteine-rich N-terminal domain. The expressed N-terminal domain did not show activity and the N-terminal truncation of VDE also loss catalytic ability. When mixing these two separately expressed peptides together, the activity was regained. This shows that these two domains could fold independently to their active folding, and also indicates that the active site may be located in the interface of the two domains.The closest relative of VDE has been found through bioinformatics analysis, the protein has a conserved cysteine pattern as VDE, and named as VDE related protein (VDR). From bioinformatics analysis, some characterization of VDR has been developed; VDR and VDE could be found in same organisms in tree of life, and they are suggested to have the same ancestor; cysteine-rich domain between the two protein are expected to have similar spatial structure, while the corresponding domain of lipocalin-like domain in VDR could have different structure as VDE. With 5’ RACE method and western blot, mRNA and protein level evidence of VDR expression is confirmed, together with differential centrifugation, the localization of VDR in leaf would be suggested to follow chlorophyll.