Poster: Photosynthesis (carbon)

Abs # 231: In vitro analyses of inactivation of Arabidopsis Rubisco and Rubisco activase at high temperature

Presenter:	Kim, Kangmin , kkim1@uiuc.edu
Authors	Kim, Kangmin  (A)   Portis, Archie R (A)
Affiliations:	(A): USDA-ARS-Photosynthesis Research Unit

A reduced activation state of Rubisco is considered to be the primary cause for the reversible inhibition of net photosynthesis of plants at elevated temperature. We examined the in vitro temperature dependence of Rubisco and Rubisco activase. The initial activity of fully activated Rubisco increased with temperature but was inhibited as a function of time. After reaching a plateau, the residual activity of Rubisco at 25 and 40 ^oC was 38 and 51 %, respectively. The half times of inhibition at 25, 30, 35, and 40 ^oC were 790, 600, 350 and 130 seconds, respectively. Low carbon dioxide and high oxygen concentration facilitated this process, especially at high temperature. Heat stabilities of the two Rubisco activase isoforms were also investigated by continuously monitoring ATPase activity at different temperature. Similar to Rubisco, the initial activity increased with temperature. ATPase activities of both isoforms were maintained for at least 30 min. In contrast, the 43 KDa isoform assayed at 34.5, 37, and 40 ^oC showed 43, 65, and 95 % loss of the initial activity within 10 min. Inhibition of the 46 KDa isoform by temperature was similar. Light-scattering revealed that activase (small isoform) formed aggregates above 36 ~ 37 ^oC. Incubation with nucleotide (ADP or ATP w/o Mg⁺⁺) greatly increased the heat stability of activase. This implies that nucleotide binding may protect enzyme conformation whereas catalytic turnover makes activase heat-labile at high temperature. These results provide further support for the concept that in planta activase loses its capability to prevent a faster inactivation of Rubisco at high temperature, due to loss of activase activity. -Supported, in part, by DOE grant 97ER20268