Localization and targeting of the VP14 epoxy-carotenoid dioxygenase to chloroplast membranes

Abscisic acid (ABA) is a key regulator of seed dormancy and plant responses to environmental challenges. ABA is synthesized via an oxidative cleavage of 9-cis epoxy-carotenoids, the first committed and key regulatory step in the ABA biosynthetic pathway. Vp14 of maize encodes an epoxy-carotenoid dio xygenase that is soluble when expressed in E coli. An important goal has be en to determine how the soluble VP14 protein is targeted to epoxy-carotenoi d substrates that are located in the thylakoid and envelope membranes of ch loroplasts and other plastids. Using an in vitro chloroplast import assay, we have shown that VP14 is imported into chloroplasts with cleavage of a sh ort stroma-targeting domain. The mature VP14 exists in two forms, one which is soluble in stroma and the other bound to thylakoid membranes. Analysis of a series of truncated VP14 mutants mapped the membrane targeting signal to the 160 amino acid N-terminal sequence. A putative amphipathic alpha -he lix within this region is essential, but not sufficient, for the membrane t argeting. Either deletion of or insertion of helix breaking residues into t his region abolished the membrane binding, whereas a chimeric protein carry ing just the amphipathic region fused with bacterial glutathione S-transfer ase failed to associate with the thylakoid membrane. The membrane-bound VP1 4 was partially resistant to chaotropic washes such as 0.1 M Na2CO3 (pH 11. 5) and 6 M urea. Unlabelled recombinant VP14 inhibited the tight binding of imported VP14, suggesting that VP14 is associated with specific components of the thylakoid membrane.