Influence of the activation status and of ATP on phosphoribulokinase degradation |
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Authors: | Kamber L; Feller U |
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Institution: | Institute of Plant Physiology, University of Bern, Altenbergrain 21, CH-3013 Bern, Switzerland; Corresponding author e-mail: feller@pfp.unibe.ch |
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Abstract: | The light-regulated chloroplast enzyme phosphoribulokinase (EC 2.7.1. 19)
exists in two forms. In darkness this enzyme is present in an oxidized
form, which is inactive. It is activated in the light by a
thioredoxin-mediated reduction. In extracts from young wheat leaves
(Triticum aeestivum L.) phosphoribulokinase as well as
some other thioredoxin-modulated enzymes can be activated by the artificial
reductant dithiothreitol (DTT). The influence of the activation status and
of the substrate ATP on phosphoribulokinase stability was investigated in
the presence of endogenous endopeptidases from senescing wheat leaves.
Similar experiments were performed with purified phosphoribulokinase from
spinach in the presence of exogenous, purified endopeptidases (chymotrypsin
and trypsin). Phosphoribulokinase stability was analysed by immunoblotting
and activity measurements. Both systems led to similar conclusions. DTT
(reductant and ATP (substrate) stabilized phosphoribulokinase in wheat leaf
extracts as well as partially purified phosphoribulokinase from spinach.
The combination of both effectors was far more protective than either
effector alone. DTT had hardly any effect on the degradation of
thioredoxin-independent chloroplast enzymes such as glutamate synthase and
glutamine synthetase. These results suggest that the activation status and
substrate concentrations are not only important for the activity of
phosphoribulokinase, but are also relevant for the susceptibility of this
enzyme to proteolysis. |
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