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| Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubsico) activase (RCA) is a nuclear-encoded chloroplast protein that modifies the conformation of Rubisco, releases inhibitors from the active sites, and increases enzymatic activity. It appears to have other functions, which are related to its distribution within the chloroplast. The aim of this research was to resolve uncertainty about the localization of RCA, and to determine whether the distributions of Rubisco and RCA were altered when RCA content was reduced. Gas exchange and Rubisco were measured, and the sub-cellular locations of Rubisco and RCA were determined using immunogold-labeling electron microscopy, in wild-type and antisense rca rice plants. Net photosynthetic rate and the initial Rubisco activity in the antisense rca plants decreased much less than RCA content in the antisense plants. Immunocytolocalization showed that Rubisco in wild-type and antisense plants was localized in the stroma of chloroplasts. However, the amount of Rubisco in the antisense rca plants was greater than in the wild type plants. RCA was detected in both the stroma and in the thylakoid membranes of wild-type plants. We show that the percentage of RCA labeling in the thylakoid membrane was substantially decreased, while the fraction in the stroma was increased, by the antisense rca treatment. From the changes in RCA distribution and alterations in Rubisco activity, RCA in stroma of chloroplast probably contributes to the activation of Rubisco, and RCA in thylakoids compensates for the reduction of RCA in the stroma, allowing steady-state photosynthesis to be maintained when RCA is depleted. RCA may also have a second role in protecting membranes against environmental stresses as a chaperone. |
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| Keywords:Oryza sativa L., Rubisco, Rubsico activase, Cellular localization |
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