Abstract
Ribulose-bisphosphate carboxylase/oxygenase (Rubisco) activase uses the energy from ATP hydrolysis to remove tight binding inhibitors from Rubisco, thus playing a key role in regulating photosynthesis in plants. Although several structures have recently added much needed structural information for different Rubisco activase enzymes, the arrangement of these subunits in solution remains unclear. In this study, we use a variety of techniques to show that Rubisco activase forms a wide range of structures in solution, ranging from monomers to much higher order species, and that the distribution of these species is highly dependent on protein concentration. The data support a model in which Rubisco activase forms an open spiraling structure rather than a closed hexameric structure. At protein concentrations of 1 μM, corresponding to the maximal activity of the enzyme, Rubisco activase has an oligomeric state of 2-4 subunits. We propose a model in which Rubisco activase requires at least 1 neighboring subunit for hydrolysis of ATP.
Highlights
Ribulose-bisphosphate carboxylase/oxygenase (Rubisco) activase optimizes photosynthesis in plants, yet the arrangement of subunits is unclear
To directly correlate the solution structure of Rubisco activase with biological function, we have used a variety of techniques to investigate the quaternary structure of Rubisco activase at different protein concentrations
The results show that the size of the Rubisco activase oligomer in solution is strongly dependent on protein concentration and ranges from dimeric/monomeric at low concentrations (Ͻ0.5 M) up to complexes larger than a hexamer at higher concentrations (Ͼ10 M) (Fig. 1)
Summary
Rubisco activase optimizes photosynthesis in plants, yet the arrangement of subunits is unclear. The crystal structure of tobacco Rubisco activase has a helical structure, with 6 subunits/turn, it is suggested that the enzyme forms closed hexamers in solution [1]. The structure of tobacco Rubisco activase forms a helical arrangement in the crystal structure, with 6 subunits/turn [1], and a hexameric arrangement would be consistent with many AAAϩ proteins. Stepwise addition of monomers could generate dimeric, trimeric, tetrameric, and pentameric intermediates, which may be similar to isodesmic systems in which the dissociation constant for monomer addition to any species is considered to be the same This mechanism could lead to the formation of helical units with 6-fold symmetry, such as those observed for the tobacco Rubisco activase crystal structure. A better understanding of this process will provide insights into how photosynthesis may be optimized and appreciating how plants will adapt to climate change [40]
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