Role of molecular chaperone HSP90C in regulating plant photosynthesis

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The heat shock protein 90 (HSP90) is a molecular chaperone which aids in folding a variety of proteins (termed HSP90 substrates or clients), many of which are protein kinases and transcription factors playing key roles in cellular signaling pathways. In higher plants, HSP90 isoforms are localized in both cytoplasm and organelles. Compared with those of the cytosolic HSP90 isoforms (termed HSP90A), the mechanism of action and especially the client spectrum of the chloroplast localized HSP90C are still poorly understood. My long-term research goal is to understand the role and mechanism of action of HSP90 in plant development, organelle functions, and in plant adaptation to environmental changes. A short-term goal of this research program is to study the mechanism of action of the chloroplast HSP90C. Our research has demonstrated that down-regulation of HSP90C by transgene-induced gene silencing results in a variegated phenotype and impaired chloroplast biogenesis. However, the mechanism that underlies the malfunction of chloroplast in the transgenic plants, and whether overexpression of HSP90C is able to stimulate photosynthetic capacity is unclear. The objective of this proposed research program is to investigate the mechanism by which HSP90C regulates the photosynthetic capacity through a battery of biochemical, cellular, physiological and proteomics approaches. We hypothesize that higher expression of HSP90C enhances photosynthesis capacity. Research focus will be mainly on two broad goals: 1) to understand the mechanism of HSP90C in modulating the function of its specific client proteins; 2) to understand whether and how overexpression of HSP90C enhances the overall photosynthesis rate. First, to understand the mechanism of action of HSP90C, we will investigate the role of HSP90C in mediating the thylakoid lumen targeting of an oxygen evolving complex protein, PsbO (also termed OE33), which we previously demonstrated to interact with HSP90C. We hypothesize that HSP90C chaperones OE33 in the chloroplast stroma and aids its thylakoid lumen translocation. We will investigate the chloroplast targeting efficiency of OE33 in both HSP90C overexpression and cosuppression lines by means of biochemical and cellular approaches. Second, to understand the role of HSP90C in promoting plant photosynthesis capacity, we will use the HSP90C overexpression transgenic lines. The photosynthetic capacity of transgenic plants will be thoroughly investigated using gas exchange and chlorophyll fluorescence analyses. Moreover, the accumulation of end products or intermediates of photosynthesis will be analyzed. Chloroplast is the unique organelle of a plant cell where the photosynthesis takes place. This proposed research employs chloroplast HSP90C to study the role of molecular chaperones which regulate the chloroplast development, function and photosynthetic component activity. The study will reveal the mechanism of action of HSP90 in thylakoid protein targeting and protein complex assembly, thus providing a critical view of the complexity of life. By analyzing the role of HSP90C in regulating photosynthesis capacity, this study investigates new master regulators of the complicated photosynthesis process. The results of this research will potentially provide novel strategies to genetically engineer plant lines with higher photosynthetic capacity, thus better yield for grain crops. This study will also provide additional information for research in mammalian cells since HSP90 is a highly conserved protein and is being used as a target in cancer research.