Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Proceedings
Publication Acceptance Date: May 1, 1996
Publication Date: N/A
Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy, energy that is then used to synthesize sugars and other food stuffs. The first and rate-limiting step in photosynthesis involves the fixation of atmospheric carbon dioxide by the enzyme Rubisco. The activity of Rubisco is regulated by another enzyme called Rubisco activase. Rubisco oactivase controls the overall process of photosynthesis by making Rubisco activity responsive to light intensity. It is not known how Rubisco activase interacts with Rubisco to control Rubisco activity. In this paper, we used genetic engineering to identify a portion of the Rubisco activase enzyme that is involved in the interaction with Rubisco. The results of this study provide new information about the interaction between Rubisco and Rubisco activase and how this interaction influences Rubisco activity. This information eventually can be used to make changes that improve the activity of Rubisco activase and ultimately the efficiency of photosynthesis.
The role of the N-terminal region of Rubisco activase in ATP hydrolysis and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activation was examined by construction of mutant proteins. Deletion of the first 50 amino acids of Rubisco activase almostly completely eliminated the ability to activate Rubisco, without changing the ATP hydrolyzing and self- associating properties of the enzyme. Thus, the N-terminus of Rubisco activase is distinct from the ATP-hydrolyzing domain and is required for Rubisco activation. Directed mutagenesis of the species-invariant tryptophan at position 16 inhibited Rubisco activation, but not the binding or hydrolysis of ATP. Changes in the intrinsic fluorescence of truncated and Trp16 mutants upon addition of ATP were similar to the wild-type, evidence that Trp16 is not the residue reporting the conformational change that accompanies subunit association. However, modification of the W16C mutant with a fluorescent adduct showed that position 16 becomes more solvent accessible in response to nucleotide binding.