Location: Plant Physiology and Genetics ResearchTitle: Atomic resolution x-ray structure of the substrate recognition domain of higher plant rubisco activase) Author
Submitted to: Journal of Biological Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/20/2011
Publication Date: 8/31/2011
Citation: Henderson, J.N., Kuriata, A., Fromme, R., Salvucci, M.E., Watcher, R.M., 2011. Atomic resolution x-ray structure of the substrate recognition domain of higher plant rubisco activase. Journal of Biological Chemistry. 286:35683-35688. Interpretive Summary: In the process of photosynthesis, plants convert light into chemical energy. The energy produced by photosynthesis is then used to synthesize the carbon compounds that are harvested for food, fuel, fiber or other natural products. Heat stress inhibits photosynthesis, reducing the overall yield of the plant. Previous research from our group identified an enzyme called Rubisco activase as the component of photosynthesis that is most sensitive to inhibition by heat. Rubisco activase is a regulatory enzyme that controls the activity of Rubisco, the major carbon dioxide-fixing enzyme in plants. In this manuscript, we present for the first time data about the atomic structure of Rubisco activase. Specifically, an important part of Rubisco activase, the region that interacts with Rubisco, was crystallized. X-ray bombardment of the crystals was used to identify the three-dimensional positions of the various atoms that comprise the Rubisco activase enzyme. From the positions of the atoms, a three-dimensional structure of the protein was determined. Knowledge of the structure of Rubisco activase is a major development in understanding how Rubisco activase functions to control the activity of Rubisco. In addition, the information about the structure will help elucidate the mechanism for heat inactivation of photosynthesis since knowing the positions of the various atoms reveals how the stability of Rubisco activase is maintained and how it can be strengthened to increase the temperature tolerance of the enzyme. This information eventually can be used to make changes that improve the activity and stability of Rubisco activase in order to improve the efficiency of photosynthesis under heat stress.
Technical Abstract: The rapid release of tight-binding inhibitors from dead-end Rubisco complexes requires the activity of Rubisco activase, an AAA+ ATPase that utilizes chemo-mechanical energy to catalyze the reactivation of Rubisco. Activase is thought to play a central role in coordinating the rate of CO2 fixation with the light reactions of photosynthesis. Here, we present a 1.9 Angstrom crystal structure of the C-domain core of creosote activase. The fold consists of a canonical four-helix bundle, from which a paddle-like extension protrudes that entails a 9-turn helix lined by an irregularly structured peptide strand. The residues Lys313 and Val316 involved in the species-specific recognition of Rubisco are located near the tip of the paddle. An ionic bond between Lys313 and Glu309 appears to stabilize the glycine-rich end of the helix. Structural superpositions onto the distant homolog FtsH imply that the paddles extend away from the hexameric toroid in a fan-like fashion, such that the hydrophobic sides of each blade bearing Trp302 are facing inward, and the polar sides bearing Lys313 and Val316 are facing outward. Therefore, we speculate that upon binding, the activase paddles embrace the Rubisco cylinder by placing their hydrophobic patches close to the partner protein. This model suggests that conformational adjustments at the remote end of the paddle may relate to selectivity in recognition, rather than specific ionic contacts involving Lys313. Additionally, the superpositions predict that the catalytically critical Arg293 does not interact with the bound nucleotide. Ring-ring stacking and peptide threading models for Rubisco reactivation are briefly discussed.