Regulation of Rubisco activase and its interaction with Rubisco.

The large, alpha-isoform of Rubisco activase confers redox regulation of the ATP/ADP response of the ATP hydrolysis and Rubisco activation activities of the multimeric activase holoenzyme complex. The alpha-isoform has a C-terminal extension that contains the redox-sensitive cysteine residues and is characterized by a high content of acidic residues. Cross-linking and site-directed mutagenesis studies of the C-terminal extension that have provided new insights into the mechanism of redox regulation are reviewed. Also reviewed are new details about the interaction between activase and Rubisco and the likely mechanism of 'activation' that resulted from mutagenesis in a 'Sensor 2' domain of activase that AAA(+) proteins often use for substrate recognition. Two activase residues in this domain were identified that are involved in Rubisco recognition. The results directly complement earlier studies that identified critical residues for activase recognition in the large subunit of Rubisco.