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Rubisco assembly: a model system for studying the mechanism of chaperonin action.

The chloroplast enzyme Rubisco (ribulose 1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39) catalyzes the carboxylation of ribulose bisphosphate in the Calvin cycle and the oxygenation of the same substrate in the photorespiratory pathway. Because these two competing reactions determine the net photosynthetic yield of many higher plants, it is important to understand their mechanisms. This requires not only a model of Rubisco atomic structure, but also experimental systems that allow the structure to be manipulated by in vitro mutagenesis. The requirement for a structural model has been fulfilled recently (Chapman et al., 1987, 1988; Andersson et al., 1989). Unfortunately, however, in vitro mutagenesis of higher plant Rubisco has not been possible because the subunits of higher plant Rubisco, either when dissociated from each other chemically in vitro (Voordouw, Van der Vies, and Boumeister, 1984) or when synthesized in Escherichia coli (Bradley, Van der Vies, and Gatenby, 1986), fail to assemble into active enzyme. Severa1 groups have attempted to overcome this problem by studying Rubisco assembly from either native subunits (Roy, Cannon, and Gilson, 1988a), prokaryotic subunits expressed in E. coli (Gatenby, 1988), or alga1 subunits (Newman and Cattolico, 1988). These studies indicate that the post-translational assembly of Rubisco holoenzyme in both prokaryotes and eukaryotes is a complex process that may require helper proteins called “chaperonins” (Gatenby, 1988; Hemmingsen et al., 1988; Roy and Cannon, 1988; Roy et al., 1988a; Ellis, Van der Vies, and Hemmingsen, 1989). Chaperonins are functionally analogous to other molecular chaperones such as nucleoplasmin, which is involved in nucleosome assembly, and the bovine microsomal binding protein, which participates in immunoglobulin transport (Pelham, 1986; Ellis et al., 1989). The molecular chaperones, including the chaperonins, are supposed to interact transiently with other proteins, promoting assembly of those proteins into functional complexes. The molecular chaperones do not form part of the final structure of the protein complexes whose assembly they promote. In this article I review recent experiments that persuasively support the idea that chaperonins are required for Rubisco assembly.