Requirements for the light-stimulated degradation of stromal proteins in isolated pea (Pisum sativum L.) chloroplasts

Chloroplasts from 17-d-old pea leaves (Pisum sativum L.) were isolated to elucidate the requirements for the light-induced degradation of stromal proteins, The influence of electron transport through the thylakoids and the influence of ATP on protein degradation were investigated. When chloroplasts were incubated in the light (45 mu mol m(-2) s(-1)), glutamine synthetase, the large subunit of ribulose-1,5-bisphosphate carboxylase and glutamate synthase were degraded, whereas phosphoribulokinase, ferredoxin-NADP(+) reductase and the 33 kDa protein of photosystem II remained more stable, Major protein degradation was not observed over 240 min in darkness, The electron transport inhibitor dichlorophenyldimethylurea reduced protein degradation in the light over several hours, whereas dibromothymoquinone was less effective. Inhibiting the production of ATP with tentoxin or by destroying the Delta pH with the ionophores valinomycin and nigericin had no effector even a stimulating influence on protein degradation when chloroplasts were exposed to light. Furthermore, adding ATP to chloroplasts incubated in the dark had no effect on proteolysis. From these results it is concluded that the transport of electrons through the thylakoids or photooxidative processes associated with it (especially in presence of DTT), rather than the availability of ATP caused the acceleration of stromal protein degradation by light in isolated pea chloroplasts.