Protein Disulfide Isomerase 2 of Chlamydomonas reinhardtii Is Involved in Circadian Rhythm Regulation

In this article, we show that the protein disulfide isomerase 2 of Chlamydomonas reinhardtii is involved in circadian rhythm regulation and interacts with a peroxiredoxin specifically during night phase. Furthermore, the recombinant protein could be characterized as a redox-active enzyme.Protein disulfide isomerases (PDIs) are known to play important roles in the folding of nascent proteins and in the formation of disulfide bonds. Recently, we identified a PDI from Chlamydomonas reinhardtii (CrPDI2) by a mass spectrometry approach that is specifically enriched by heparin affinity chromatography in samples taken during the night phase. Here, we show that the recombinant CrPDI2 is a redox-active protein. It is reduced by thioredoxin reductase and catalyzes itself the reduction of insulin chains and the oxidative refolding of scrambled RNase A. By immunoblots, we confirm a high-amplitude change in abundance of the heparin-bound CrPDI2 during subjective night. Interestingly, we find that CrPDI2 is present in protein complexes of different sizes at both day and night. Among three identified interaction partners, one (a 2-cys peroxiredoxin) is present only during the night phase. To study a potential function of CrPDI2 within the circadian system, we have overexpressed its gene. Two transgenic lines were used to measure the rhythm of phototaxis. In the transgenic strains, a change in the acrophase was observed. This indicates that CrPDI2 is involved in the circadian signaling pathway and, together with the night phase-specific interaction of CrPDI2 and a peroxiredoxin, these findings suggest a close coupling of redox processes and the circadian clock in C. reinhardtii.