A chloroplast sulfate transporter modulates glutathione-mediated redox cycling to regulate cell division

Glutathione redox cycling is important for cell cycle regulation. However, the underlying mechanisms remain unclear. We previously identified a cell-size mutant, suppressor of mat3 15-1 ( smt15-1 ), that has elevated cellular glutathione, increased number of cell divisions, and small daughter cells. Here, we demonstrated that SMT15 is a chloroplast-associated membrane protein that is capable of transporting sulfate. Reducing expression of γ-GLUTAMYLCYSTEINE SYNTHETASE , which encodes the rate-limiting enzyme required for glutathione biosynthesis, corrected the size defect of smt15-1 cells. Moreover, overexpressing GLUTATHIONE SYNTHETASE recapitulated the small-size phenotype of smt15-1 mutant, confirming the role of glutathione in modulation of the cell division. Hence, SMT15 may regulate chloroplast sulfate concentration to modulate cellular glutathione levels. Interestingly, glutathione was found to accumulate in the cytosol at the G1 phase and its level decreased substantially as cells entered the S/M phase in wild-type cells. Even though cytosolic glutathione of the small-sized mutants, smt15-1 and GSH2 overexpressors, followed the pattern of wild-type cells being accumulated at G1 and declined at the S/M phase, the basal body-specific accumulation of glutathione was associated with only the small-sized mutants. Therefore, we propose that glutathione-mediated redox in the basal bodies may regulate mitotic division number in Chlamydomonas reinhardtii . Our results support the link between glutathione-mediated redox regulation and mitotic cell division and suggest a new mechanism through which glutathione regulates the cell cycle. One sentence summary Glutathione-mediated redox regulation in basal bodies is important for cell division control