Disulfide bond formation in chloroplasts: Formation of disulfide bonds in signaling chloroplast proteins

Abstract

Regulatory protein disulfide bonds serve as key signaling elements in chloroplasts in a manner that appears independent of the generally highly reducing intra-organellar conditions. This suggests that both the formation and the reduction reactions of the disulfides are specifically catalyzed. Regulatory disulfides are preferentially reduced by the dithiol reductant, thioredoxin, but their oxidant counterpart is yet to be identified. Regulatory disulfides are found in chloroplast proteins in the dark as well as under low illumination, implying that the source of the oxidative equivalents might not be limited to certain lighting conditions. Several plausible oxidants required for regulatory disulfide formation are discussed herein. By the same token, the recent finding of oxidative protein folding in chloroplasts implies an involvement of an enzymatic system for disulfide formation. Mechanisms for oxidative folding in prokaryotes and eukaryotes share a common design, comprising of a thiol oxidase and an oxidative-type thioredoxin such as protein disulfide isomerase. While the localization of protein disulfide isomerases to chloroplasts seems well established, the identity of a chloroplast thiol oxidase is yet to be determined. The understanding of disulfide formation in chloroplasts should prove key to our understanding of redox signaling in general.