PLANT PHYSIOLOGY , Vol 105, Issue 2 731-737, Copyright © 1994 by American Society of Plant Biologists
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METABOLISM AND ENZYMOLOGY |
Evidence That 2-Carboxyarabinitol 1-Phosphate Binds to Ribulose-1,5-Bisphosphate Carboxylase in Vivo
Bd. Moore and J. R. Seemann
Department of Biochemistry, University of Nevada, Reno, Nevada 89557-0014
An important question concerning the role of carboxyarabinitol 1-phosphate
(CA1P) metabolism in the light-dependent regulation of
ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity is the
extent to which CA1P is bound to Rubisco in vivo. We report here the
development of an extraction procedure using ammonium sulfate that
stabilizes CA1P bound to Rubisco. This procedure exploits the ability of
sulfate to bind at the catalytic site of Rubisco and to competitively
balance the binding and release of CA1P from Rubisco. In darkened bean
leaves about 75% of the Rubisco catalytic sites were found to be bound with
CA1P. This confirms previous indirect estimates from gas exchange
measurements. We have used this extraction procedure to examine
CA1P-Rubisco interactions in bean during a natural transition from darkness
to light. With increasing light intensity following sunrise, CA1P
degradation proceeded in two distinct phases: first, a majority of the
unbound CA1P pool was degraded at very low light levels ([less than or
equal to]30 [mu]mol quanta m-2 s-1); second, CA1P initially bound to
Rubisco was then degraded at increasing light levels (>30 [mu]mol quanta
m-2 s-1). These results indicate that there is a low-fluence activation of
CA1P phosphatase that can occur prior to CA1P release by Rubisco activase.
This activation may be mediated by NADPH. During sunrise in bean, the level
of the catalytically competent form of Rubisco was regulated by CA1P
metabolism.
