The Role of HSP90C in Chloroplast Maturation and Thylakoid Protein Transport
The chloroplast is an organelle that has a primary role in performing photosynthesis in green plants and contains thousands of proteins. Since most chloroplast proteins are synthesized outside of the chloroplast on cytosolic ribosomes, they are imported into chloroplasts in mostly unfolded states. A protein quality control system in the chloroplast is then responsible for their correct folding, assembly, and repair, or facilitate their removal if terminally damaged. The composition and translocation process of the protein import machineries on the chloroplast envelopes and the thylakoid membranes have been studied in-depth; however, how stromal factors are involved in bridging chloroplast membrane import and thylakoid transport of thylakoid proteins is still poorly understood. In this thesis, I elucidated a role and mechanism of action for the essential plastid HSP90 family heat shock protein HSP90C in this process. Using a thylakoid lumen-localized subunit of the oxygen evolving complex, I demonstrated that PsbO1 is tightly regulated by the HSP90C complex in the stroma. By analyzing the chloroplast maturation process, the essential role of HSP90C and its ATPase activity in regulating and balancing the influx and efflux of PsbO1 fusion proteins within the stroma was demonstrated. A specific role of HSP90C in forming a guiding complex that interacts with the SEC translocation system and facilitates ATP-dependent thylakoid protein targeting and transport was proposed. These studies reveal critical roles of HSP90C in the chloroplast and thus provide new insights into the molecular mechanism of HSP90 family molecular chaperones in regulating organellar protein homeostasis.
Chaperone, Chloroplast, Hsp90, Protein Homeostasis, Protein Translocation, Sec Translocon
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