|Portis Jr, Archie|
Submitted to: Photosynthesis Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/16/2000
Publication Date: 12/1/2001
Citation: ZHANG, N., SCHURMANN, P., PORTIS JR, A.R. CHARACTERIZATION OF THE REGULATORY FUNCTION OF THE 46-KDA ISOFORM OF RUBISCO ACTIVASE FROM ARABIDOPSIS. PHOTOSYNTHESIS RESEARCH. 2001. v. 68. p. 29-37. Interpretive Summary: The activity of Rubisco, the enzyme that captures carbon dioxide, often limits photosynthesis, the process by which plants use light energy from the sun to make carbohydrates for growth from carbon dioxide and water. In order to have activity, Rubisco requires modification by a process called activation, which is determined and hence regulated by another protein known as Rubisco activase. Rubisco activity might be increased to improve plant growth by altering the regulation of activation. Previously, we found that the activity of Rubisco activase is increased or decreased by the addition of two electrons to a special site that is mediated by a small protein called thioredoxin-f. In this study we further characterized the regulation of activase by this mechanism and compared it with the regulation of other chloroplast enzymes. This information will benefit scientists attempting to understand this type of regulation and to modify the properties and regulation of Rubisco in ways beneficial for increased photosynthesis by crop plants.
Technical Abstract: Arabidopsis Rubisco activase was recently shown to be regulated by redox changes in the larger (46-kDa) isoform specifically mediated by thioredoxin-f. Reduction greatly increases the activity of the 46-kDa isoform and the native protein at physiological ATP/ADP ratios. In this study we conducted additional experiments to characterize the regulation of Rubisco activase by thioredoxin-f. The Km for both ATP hydrolysis and Rubisco activation by the 46-kDa isoform was lowered by 4 to 5-fold after reduction, but the maximum activity was increased by only ten percent. Only 0.35 uM thioredoxin-f was required for half-maximal activity change after a 10 min preincubation and activation with 1 uM was complete after 10 min. Equal amounts of 46 k-Da and 43-kDa isoforms were required for a complete inhibition of the Rubisco activation activity after a reduction-oxidation cycle and assay at an ATP/ADP ratio of 3:1, whereas activity was only inhibited by 50 percent at a 2:1 ratio (43-/46-kDa) of the isoforms. This requirement is consistent with the fact that Arabidopsis normally contains about a 1:1 ratio of the two isoforms at both the mRNA and protein levels. Redox titrations indicated a midpoint potential of -344 mV for the 46-kDa isoform as compared to -342 mV for spinach fructose 1,6-bisphosphatase at pH 7.9, consistent with previous reports indicating that these proteins are co-regulated by light intensity in a similar manner.