Rubisco activase - Rubisco's catalytic chaperone.

The current status of research on the structure, regulation, mechanism and importance of Rubisco activase is reviewed. The activase is now recognized to be a member of the AAA(+) family, whose members participate in macromolecular complexes that perform diverse chaperone-like functions. The conserved nucleotide-binding domain of AAA(+) family members appears to have a common fold that when applied to the activase is generally consistent with previous site-directed mutagenesis studies of the activase. Regulation of the activase in species containing both isoforms can occur via redox changes in the carboxy-terminus of the larger isoform, mediated by thioredoxin-f, which alters the response of activase to the ratio of ADP to ATP in the stroma. Studies of Rubisco activation in transgenic Arabidopsis plants demonstrated that light modulation is dependent on redox regulation of the larger isoform, providing a model for the regulation in other species. Further insights into the mechanism of the activase have emerged from an analysis of the crystal structures of Rubisco conformational variants and the identification of Rubisco residues that confer specificity in its interaction with the activase. The physiological importance of the activase is reinforced by recent studies indicating that it plays a vital role in the response of photosynthesis to temperature. Rubisco activase is one of a new type of chaperone, which in this case functions to promote and maintain the catalytic activity of Rubisco.