XANTHOPHYLL CYCLE ENZYMES ARE MEMBERS OF THE LIPOCALIN FAMILY, THE FIRST IDENTIFIED FROM PLANTS

Violaxanthin de-epoxidase and zeaxanthin epoxidase catalyze the additi on and removal of epoxide groups in carotenoids of the xanthophyll cyc le in plants. The xanthophyll cycle is implicated in protecting the ph otosynthetic apparatus from excessive light. Two new sequences for vio laxanthin de-epoxidase from tobacco and Arabidopsis are described. Alt hough the mature proteins are well conserved, the transit peptides of these proteins are divergent, in contrast to transit peptides from oth er proteins targeted to the thylakoid lumen. Sequence analyses of both violaxanthin de-epoxidase and zeaxanthin epoxidase establish the xant hophyll cycle enzymes as members of the lipocalin family of proteins. The lipocalin family is a diverse group of proteins that bind small hy drophobic (lipophilic) molecules and share a conserved tertiary struct ure of eight P-strands forming a barrel configuration. This is the fir st reported identification of lipocalin proteins in plants.