Structural investigations of redox regulation in ATFKBP13

Abstract

Change in redox status has long been known to link light to the posttranslational regulation of chloroplast enzymes. So far, studies have been conducted primarily with thioredoxin-linked members of the stroma that function in a broad array of biosynthetic and degradation processes. Consequently, little is known about the role of redox in regulating the growing number of enzymes found to occur in the lumen, the site of oxygen evolution in thylakoid membranes. We have determined the 1.85 ?? resolution structure of AtFKBP13, an FKBP-type immunophilin earlier shown to interact with a redox-active protein of the lumen. The protein contains a pair of disulfide bonds, which in protein mutagenesis experiments have been shown to be essential for the associated peptidyl-prolyl isomerase activity. They are unique to chloroplast FKBPs and are absent in animal and yeast counterparts. Both disulfide bonds were redox-active and were reduced by thioredoxin from either chloroplast or bacterial sources in a reaction that led to loss of enzyme activity. The reduced protein structure has also been determined to investigate if there is any disulfide mediated structural changes. The reduced structure reveals a previously unrecognized paradigm for redox regulation in chloroplasts in which activation by light is achieved in concert with oxygen evolution by the oxidation of sulfhydryl groups (conversion of SH to SS). Such a mechanism, occurring in the thylakoid lumen, is in direct contrast to regulation of enzymes in the stroma, where reduction of disulfides targeted by thioredoxin (SS converted to SH) leads to an increase in activity in the light.