Abstract
We have studied the turnover of an abundant chloroplast protein, ribulose-1,5-bisphosphate carboxylase/oxygenase (Rbu-P2 carboxylase/oxygenase), in plants (Spirodela oligorrhiza and Triticum aestivum L.) and algae (Chlamydomonas reinhardtii and C. moewusii) induced to senesce under oxidative conditions. Rbu-P2 carboxylase/oxygenase activity and stability in vivo were found to be highly susceptible to oxidative stress, resulting in intermolecular cross-linking of large subunits by disulfide bonds within the holoenzyme, rapid and specific translocation of the soluble enzyme complex to the chloroplast membranes, and finally protein degradation. The redox state of Cys-247 in Rbu-P2 carboxylase/oxygenase large subunit seems involved in the sensitivity of the holoenzyme to oxidative inactivation and cross-linking. However, this process did not drive membrane attachment or degradation of Rbu-P2 carboxylase/oxygenase in vivo. Translocation of oxidized Rbu-P2 carboxylase/oxygenase to chloroplast membranes may be a necessary step in its turnover, particularly during leaf senescence. Thus, processes that regulate the redox state of plant cells seem closely intertwined with cellular switches shifting the leaf from growth and maturation to senescence and death.
Highlights
We have studied the turnover of an abundant chlo- encoded small (12-18 kDa) subunits [1,2,3]
281 1 in the large subunit contributes to the sensitivity of Rbu-Pi carboxylase/oxygenase to oxidative inactivation and crosslinkingand that this process is not coupled to membrane translocation or degradation of Rbu-P2 carboxylase/oxygenase in vivo
The presence of lower molecular weight polypeptides immunoreactive with antibodies against Rbu-Pp carboxylase/oxygenase, in isolated wheat chloroplasts, is further evidence of degradation of large subunit of Rbu-P2 carboxylase/oxygenase under these conditions (Fig. lC, compare lanes 6-8 with lune 5 ) .Inability to detectdiscrete breakdown products of Rbu-P2carboxylase/oxygenasein Spirodela or intactwheat plants may becaused by faster ratesof degradation and/or loss of immunological epitopes in the breakdown products
Summary
2810”2816,1992 Printed in U.S.A. Oxidative Stress Causes RapidMembrane Translocation andin Vivo Degradation of Ribulose-1,&bisphosphate Carboxylase/Oxygenase*. Ribulose-1,s-bisphosphate carboxyl- oligomeric Rbu-P2 carboxylase/oxygenase in chloroplasts, or aseloxygenas(eRbu-Pz carboxylaseloxygenase), in in a heterologous system such as Escherichia coli, is promoted plants Studying Rbu-Ppcarboxylase/oxygenase degradation during whole plant senescence is problematic, in part because of the long duration of the process and difficulties in analyzing pulse-chase experiments in the face of contributions from changing developmental processes. We report here that oxidative conditions result in cross-linking of Rbu-Pz carboxylase/oxygenase via disulfide bridges, translocation of the protein to chloroplast membranes, and rapid degradation of the protein. 281 1 in the large subunit contributes to the sensitivity of Rbu-Pi carboxylase/oxygenase to oxidative inactivation and crosslinkingand that this process is not coupled to membrane translocation or degradation of Rbu-P2 carboxylase/oxygenase in vivo
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