The Rubisco complex protein: A protein induced by fruit removal that forms a complex with ribulose-1,5-bisphosphate carboxylase/oxygenase

Abstract

Continuous removal of fruits from soybean plants (Glycine max [L.] Merr.) causes a redistribution of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco, EC 4.1.1.39) from the soluble to the insoluble phase of leaf extracts. The extent of this redistribution is genotype-dependent. We previously reported that insoluble Rubisco occurs in a high-molecular-mass complex together with a protein composed of 30-kDa subunits (S.J. Crafts-Brander et al., Planta, 183, 300–306). In the present study, the Rubisco Complex Protein (RCP), was isolated from the Rubisco-RCP complex by gel-filtration chromatography in 4 M urea. Under these conditions, RCP migrated with an apparent molecular mass of 120 kDa, indicating that the protein maintains a tetrameric structure even in 4 M urea. Once freed of urea, purified RCP was soluble, but formed insoluble complexes with Rubisco from soybean, tobacco and spinach when RCP and Rubisco were incubated in a ratio of 1∶1 by weight. Purified Rubisco and RCP also associated into a high-molecular-mass complex when either component was in several-fold excess, but in this case the complex was soluble. Similarly, the amount of Rubisco sequestered as an insoluble Rubisco:RCP complex in leaf extracts of different soybean genotypes was related to the relative amounts of Rubisco and RCP present in the extracts. Thus, with both purified components and in leaf extracts, formation of an insoluble complex between Rubisco and RCP required a precise stoichiometry. Antibodies directed against purified RCP detected an accumulation of RCP in soybean leaves around the time of flowering. The RCP was also detected in petioles, stems, and pod walls of soybean, but not seeds. Fruit removal caused a marked increase in the amount of RCP in the leaves to levels as high as 15% of the total soluble protein. The accumulation of RCP in response to source:sink manipulations was similar to soybean vegetative storage proteins (VSPs). However, immunogold-localization showed that RCP was located in the cytosol of leaves, compartmentalized separate from both Rubisco and the VSPs. Thus, the physiological relevance of the specific association between RCP and Rubisco is obscure.