Submitted to: American Society of Plant Biologists Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 3/14/2010
Publication Date: N/A
Technical Abstract: The inhibition of photosynthesis at moderately high temperatures has been linked to a decrease in Rubisco activation, thought to be a consequence of the thermal instability of Rubisco's chaperone, Rubisco activase. To determine the structural basis for inactivation of Rubisco activase, the effects of ADP or ATP and/or Mg2+ on the thermal stability of the protein were determined and changes in protein structure were related to the loss of activity. Both ADP and ATP protected Rubisco activase from inactivation, whereas Mg2+ promoted inactivation. Prolonged or more intense heating caused aggregation characterized by formation of insoluble protein. In contrast, heating under more mild conditions or incubation at room temperature without ATP or ADP caused the active ~660 kD Rubisco activase to aggregate, forming a soluble, but inactive complex with an apparent molecular mass of greater than 2 x106 D. Formation of this high molecular mass aggregate correlated with an irreversible loss of Rubisco activase activity. Circular dichroism and intrinsic fluorescence established that the secondary structure of Rubisco activase was altered when subunits aggregated in the high molecular mass complex compared with those assembled in the active holoenzyme. Differences in thermal stability between wild type Rubisco activase and a structurally-altered mutant were observed both for the recombinant proteins in vitro and when the proteins were expressed in transgenic Arabidopsis plants. That photosynthesis and Rubisco activation in plants with the mutant Rubisco activase were more sensitive to inhibition by moderate heat stress indicates that the inherent structural instability of Rubisco activase is the main determinant of the temperature-sensitivity of Rubisco activation.