SUMOylation of chloroplast GrpE homolog 1 regulates chloroplast biogenesis through maintaining TIC complex stability

In Arabidopsis, chloroplast protein import relies on the stromal cpHsp70 motor system, whose nucleotide-exchange factor CGE1 is indispensable, yet its regulatory mechanism remains poorly defined. We integrated bioinformatics prediction with in vitro and in vivo SUMOylation assays, biochemical analyses, confocal microscopy, and genetic complementation to elucidate the functional impact of CGE1 SUMOylation in chloroplast development. CGE1 undergoes SUMO3 conjugation, and Lys-214 is required for efficient CGE1 SUMOylation. The SUMOylation-defective variant CGE1^K214R retained stromal localization but caused marked chlorosis of emerging leaves, reduced photosynthetic efficiency, and impaired chloroplast development. Moreover, import of cytosol-synthesized preproteins is compromised in CGE1^K214R-complemented plants, accompanied by decreased abundance of TIC40 and TIC110. Loss of SUMOylation simultaneously strengthens the CGE1-cpHsp70-1 and CGE1-TIC40 interactions, and overexpression of either cpHsp70-1 or TIC40 in CGE1^K214R-complemented plants restores TIC40 and/or TIC110 levels and the photosynthetic phenotype. In conclusion, these findings reveal that SUMOylation of CGE1 plays an important role in regulating TIC complex abundance and precursor import efficiency through cpHsp70- and TIC40-related routes. This study uncovers a SUMOylation-mediated regulatory layer that fine-tunes chloroplast biogenesis.