THE RUBISCO COMPLEX PROTEIN - A PROTEIN-INDUCED BY FRUIT REMOVAL THATFORMS A COMPLEX WITH RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE

Continuous removal of fruits from soybean plants (Glycine max [L.] Mer r.) causes a redistribution of ribulose-1,5-bisphosphate carboxylase/o xygenase (Rubisco, EC 4.1.1.39) from the soluble to the insoluble phas e of leaf extracts. The extent of this redistribution is genotype-depe ndent. We previously reported that insoluble Rubisco occurs in a high- molecular-mass complex together with a protein composed of 30-kDa subu nits (S.J. Crafts-Brander et al., Planta, 183, 300-306). In the presen t study, the Rubisco Complex Protein (RCP), was isolated from the Rubi sco-RCP complex by gel-filtration chromatography in 4 M urea. Under th ese conditions, RCP migrated with an apparent molecular mass of 120 kD a, indicating that the protein maintains a tetrameric structure even i n 4 M urea. Once freed of urea, purified RCP was soluble, but formed i nsoluble 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 whe n either component was in several-fold excess, but in this case the co mplex was soluble. Similarly, the amount of Rubisco sequestered as an insoluble Rubisco:RCP complex in leaf extracts of different soybean ge notypes was related to the relative amounts of Rubisco and RCP present in the extracts. Thus, with both purified components and in leaf extr acts, formation of an insoluble complex between Rubisco and RCP requir ed a precise stoichiometry. Antibodies directed against purified RCP d etected an accumulation of RCP in soybean leaves around the time of fl owering. The RCP was also detected in petioles, stems, and pod walls o f soybean, but not seeds. Fruit removal caused a marked increase in th e amount of RCP in the leaves to levels as high as 15% of the total so luble protein. The accumulation of RCP in response to source:sink mani pulations was similar to soybean vegetative storage proteins (VSPs). H owever, immunogold-localization showed that RCP was located in the cyt osol of leaves, compartmentalized separate from both Rubisco and the V SPs. Thus, the physiological relevance of the specific association bet ween RCP and Rubisco is obscure.