Characterization of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase activase isoforms reveals hexameric assemblies with increased thermal stability

Most plants contain two isoforms of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activase (Rca), a chloroplast protein that maintains the activity of Rubisco during photosynthesis. The longer (alpha-) Rca isoform has previously been shown to regulate the activity of Rubisco in response to both the ADP:ATP ratio and redox potential via thioredoxin-f. We have characterized the arrangement of the different spinach (Spinacia oleracea) isoforms in solution, and show how the presence of nucleotides changes the oligomeric state. Although the shorter (beta-) isoform from both tobacco (Nicotiana tabacum) and spinach tend to form a range of oligomers in solution, the size of which are relatively unaffected by the addition of nucleotide, the spinach a-isoform assembles as a hexamer in the presence of adenosine 5'-[gamma-thio]triphosphate (ATP gamma S). These hexamers have significantly higher heat stability, and may play a role in optimizing photosynthesis at higher temperatures. Hexamers were also observed for mixtures of the two isoforms, suggesting that the alpha-isoform can act as a structural scaffold for hexamer formation by the beta-isoform. Additionally, it is shown that a variant of the tobacco beta-isoform acts in a similar fashion to the alpha-isoform of spinach, forming thermally stable hexamers in the presence of ATP gamma S. Both isoforms had similar rates of ATP hydrolysis, suggesting that a propensity for hexamer formation may not necessarily be correlated with activity. Modelling of the hexameric structures suggests that although the N-terminus of Rca forms a highly dynamic, extended structure, the C-terminus is located adjacent to the intersubunit interface.