ATP HYDROLYSIS ACTIVITY AND POLYMERISATION STATE OF RUBISCO ACTIVASE. DO THE EFFECTS OF MG**2+, K**+ AND ACTIVASE CONCENTRATIONS INDICATE A FUNCTIONAL SIMILARITY TO ACTIN?

Authors: LILLEY, ROSS - UNIV WOLLONGONG AUSTRALIA; PORTIS JR, ARCHIE

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/1997
Publication Date: N/A
Citation: N/A

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. The activity of Rubisco is determined and hence regulated by another enzyme known as Rubisco activase. Rubisco activity might be increased to improve plant growth by altering its regulation. Regulation by Rubisco activase is believed to involve binding of the proteins to each other and the use, by Rubisco activase, of the energy provided by the hydrolysis of a compound called ATP. However, our understanding is clouded by the binding of Rubisco activase with itself and ATP hydrolysis in the absence of Rubisco. In this study we documented conditions under which ATP hydrolysis did not occur and self-binding was similarly reduced, which indicates that these processes may be intimately related. This information will benefit scientists attempting to modify the properties and regulation of Rubisco in ways beneficial for increased photosynthesis by crop plants.

Technical Abstract: ATPase and fluorescence of rubisco were determined over a range of MgC12, KC1 and activase concentrations. Both salts promoted ADP release from ATP and intrinsic fluorescence enhancement by ATP-y-S, but Mg**2+ was about ten times more effective than K**+. ATPase and fluorescene enhancement both increased from zero to saturation within the same Mg**2+ and K**+ concentration ranges. At saturating concentrations (5mM Mg**2+ and 22mM K**+), ATPase specific activity (turnover time about 1 second) and specific intrinsic fluorescence enhancement were maximal and unaffected by activase concentration above 1uM activase. Below 1uM activase, both fell sharply. These responses are remarkably similar to the behavior of actin. Intrinsic fluorescene enhancement of rubisco activase reflects the extent of polymerisation, showing that the smaller oligomer or monomer present in low salt and activase concentrations is inactive in ATP hydrolysis. However, fluorescence quenching by 1-anilinonapthaline-8-sulphonate revealed that ADP and ATP-y-S bind equally well to activase at low and high salt concentrations. This is consistent with an actin-like mechanism requiring a dynamic equilibrium between monomer and oligomers for ATP hydrolysis. The specific activation rate of substrate-bound decarbamylated rubisco decreased at activase concentrations below 1uM. This suggests that a large oligomeric form of activase, rather than monomer, interacts with rubisco when performing the release of bound ribulose-1,5bisphosphate from the inactive enzyme.