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Title: ATP HYDROLYSIS AND POLYMERISATION STATE OF RUBISCO ACTIVASE: EFFECTS OF SALT AND PROTEIN CONCENTRATION INDICATE A FUNCTIONAL SIMILARITY TO ACTIN

Author
item ROSS, MAC - UNIV WOLLONGONG AUSTRALIA
item PORTIS JR, ARCHIE

Submitted to: Australian Society of Plant Physiologists Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 10/2/1996
Publication Date: N/A
Citation: N/A

Interpretive Summary:

Technical Abstract: Rubisco activase enhances the rate and extent of activation of carbon fixing-enzyme rubisco by facilitating the release of sugar phosphates from decarbamylated and catalytically inactive rubisco. It also exhibits an intrinsic ATPase activity which is loosely coupled to rubisco activation. The ATPase and intrinsic fluor-escence (an indicator of extent of polymerisation) of rubisco activase were determined over a range of MgCl2 and Kcl concentrations. Both MgCl2 and KCl promoted ADP release and fluorescence enhancement by ATP-g-S, but Mg++ was more effective than K+ by a factor of about 10 and KCl was ineffective unless 0.1mM MgCl2 is present. With 5mM Mg++ and 22mM K+, specific activity of ADP release and specific intrinsic fluorescence enhancement with ATP-g-S were maximal and unaffected by changes in activase concentration above 1uM activase. However, both fell rapidly as activase was decreased below 1uM. This response is remarkably similar to the behavior of actin, except that the specific ATPase activity of actin is two orders of magnitude lower than rubisco activase. These results show that, in low Mg++, K+ and activase concentrations, the smaller oligomer present is inactive in releasing ADP from ATP. However, studies measuring the quenching of fluorescence of 1-anilinonapthaline-8-sulphonate showed that ADP and ATP-g-S bind equally well to activase at low and high Mg++ and K+ concentrations. This is consistent with an actin-like mechanism for ATP hydrolysis with constant treadmilling of subunits occurring under high Mg++, K+ and activase concentrations. Specific activation of substrate-bound decarbamylated rubisco by activase was not decreased at low activase concentrations, consistent with previous findings. This suggests a small oligomeric form of activase interacts with rubisco.