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Structure, function and assembly of Rubisco activase

In higher plants, the P‐loop ATPase Rubisco activase (Rca) helps coordinate the light and dark reactions of photosynthesis by catalyzing the release of inhibitors from Rubisco. When ATP levels drop or the temperature rises, Rca activity falls off, Rubisco inactivates, and atmospheric carbon fixation ceases. We have used X‐ray crystallography to show that the Rubisco recognition site on Rca involves an insertion into the canonical AAAplus domain that places the specificity‐determining residues around the periphery of ring‐like assemblies. Kinetic assays provide a Michaelis constant for ATP hydrolysis of 15 micromolar and a Ki for ADP of 6 micromolar, in line with moderate product inhibition. In the presence of ADP, positive cooperativity of 2 subunits is indicated. However, when ADP is lacking, positive cooperativity of 3 or more subunits is observed, suggesting an allosteric regulatory mechanism based on the ADP/ATP ratio in the stroma. Using fluorescence correlation spectroscopy, we have determined that ADP‐bound Rca self‐associates in a step‐wise fashion to form oligomeric and higher‐order forms from dimeric building blocks. We propose that in vitro, ADP‐bound Rca self‐associates by forming spiral arrays that grow along the helical axis. This work was supported by the US DOE‐BES, Photosynthetic Systems Grants No. DE‐FG02–09‐ER16123 and No. DE‐FG02–08ER‐20268.