A role for chlorophyllide <i>a</i> oxygenase in the regulated import and stabilization of light-harvesting chlorophyll <i>a</i> / <i>b</i> proteinsChristiane Reinbothe, Sandra Bartsch, Laura L. Eggink et al.|Proceedings of the National Academy of Sciences|2006 The Arabidopsis CAO gene encodes a 52-kDa protein with predicted localization in the plastid compartment. Here, we report that CAO is an intrinsic Rieske iron-sulfur protein of the plastid-envelope inner and thylakoid membranes. Activity measurements revealed that CAO catalyzes chlorophyllide a to chlorophyllide b conversion in vitro and that the enzyme was only slightly active with protochlorophyllide a, the nonreduced precursor of chlorophyllide a. Protein import and organelle fractionation studies identified CAO to be distinct from Ptc52 in the substrate-dependent transport pathway of NADPH:protochlorophyllide oxidoreductase A but instead to be part of a separate translocon complex. This complex was involved in the regulated import and stabilization of the chlorophyllide b-binding light-harvesting proteins Lhcb1 (LHCII) and Lhcb4 (CP29) in chloroplasts. Together, our results provide insights into the plastid subcompartmentalization and evolution of chlorophyll precursor biosynthesis in relation to protein import in higher plants.
Three thioredoxin targets in the inner envelope membrane of chloroplasts function in protein import and chlorophyll metabolismSandra Bartsch, Julie Monnet, Kristina Selbach et al.|Proceedings of the National Academy of Sciences|2008 Thioredoxins (Trxs) are ubiquitous small proteins with a redox-active disulfide bridge. In their reduced form, they constitute very efficient protein disulfide oxidoreductases. In chloroplasts, two types of Trxs (f and m) coexist and play central roles in the regulation of the Calvin cycle and other processes. Here, we identified a class of Trx targets in the inner plastid envelope membrane of chloroplasts that share a CxxC motif approximately 73 aa from their carboxyl-terminal end. Members of this group belong to a superfamily of Rieske iron-sulfur proteins involved in protein translocation and chlorophyll metabolism. These proteins include the protein translocon protein TIC55, the precursor NADPH:protochlorophyllide oxidoreductase translocon protein PTC52, which operates as protochlorophyllide a-oxygenase, and the lethal leaf spot protein LLS1, which is identical with pheophorbide a oxygenase. The role of these proteins in dark/light regulation and oxidative control by the Trx system is discussed.