A protein oxidase catalysing disulfide bond formation is localized to the chloroplast thylakoids

In chloroplasts, thiol/disulfide-redox-regulated proteins have been linked to numerous metabolic pathways. However, the biochemical system for disulfide bond formation in chloroplasts remains undetermined. In the present study, we characterized an oxidoreductase, AtVKOR-DsbA, encoded by the gene At4g35760 as a potential disulfide bond oxidant in Arabidopsis. The gene product contains two distinct domains: an integral membrane domain homologous to the catalytic subunit of mammalian vitamin K epoxide reductase (VKOR) and a soluble DsbA-like domain. Transient expression of green fluorescent protein fusion in Arabidopsis protoplasts indicated that AtVKOR-DsbA is located in the chloroplast. The first 45 amino acids from the N-terminus were found to act as a transit peptide targeting the protein to the chloroplast. An immunoblot assay of chloroplast fractions revealed that AtVKOR-DsbA was localized in the thylakoid. A motility complementation assay showed that the full-length of AtVKOR-DsbA, if lacking its transit peptide, could catalyze the formation of disulfide bonds. Among the 10 cysteine residues present in the mature protein, eight cysteines (four in the AtVKOR domain and four in the AtDsbA domain) were found to be essential for promoting disulfide bond formation. The topological arrangement of AtVKOR-DsbA was assayed using alkaline phosphatase sandwich fusions. From these results, we developed a possible topology model of AtVKOR-DsbA in chloroplasts. We propose that the integral membrane domain of AtVKOR-DsbA contains four transmembrane helices, and that both termini and the cysteines involved in catalyzing the formation of disulfide bonds face the oxidative thylakoid lumen. These studies may help to resolve some of the issues surrounding the structure and function of AtVKOR-DsbA in Arabidopsis chloroplasts.