Degradation of glutamine synthetase in intact chloroplasts isolated from pea (Pisum sativum) leaves

Chloroplast proteins can be degraded in the intact organelle, but the relative rates of degradation vary considerably. To investigate regulatory aspects of glutamine synthetase (GS) degradation, isolated pea (Pisum sativum L.) chloroplasts were used as a suitable system. Since chloroplasts were re-isolated after incubation, only chloroplasts remaining intact throughout the incubation and processes occurring therein were analysed. Net changes of nuclear-encoded proteins indicate degradation because protein synthesis is no longer possible under these conditions. Incubation of intact chloroplasts in a medium which promotes CO2 -assimilation led to the stabilisation of GS and several other chloroplast proteins. This general effect suggests that photosynthetic metabolism is relevant for the stability of stromal proteins. In addition, GS was specifically stabilised by methionine sulfoximine (MSO), a potent inhibitor of GS. However, other enzymes tested were not affected by MSO. When chloroplasts were incubated with methyl viologen, the degradation of several stromal enzymes including GS was accelerated, most likely by active oxygen species. Again, MSO specifically delayed the degradation of GS. As MSO is a substrate analogue, it appears likely that substrates or other ligands influence the susceptibility of enzymes against degradation systems present in the same compartment.