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Review
. 2018 Mar 13:5:24.
doi: 10.3389/fmolb.2018.00024. eCollection 2018.

Rubisco Assembly in the Chloroplast

Affiliations
Review

Rubisco Assembly in the Chloroplast

Anna Vitlin Gruber et al. Front Mol Biosci. .

Abstract

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyzes the rate-limiting step in the Calvin-Benson cycle, which transforms atmospheric carbon into a biologically useful carbon source. The slow catalytic rate of Rubisco and low substrate specificity necessitate the production of high levels of this enzyme. In order to engineer a more efficient plant Rubisco, we need to better understand its folding and assembly process. Form I Rubisco, found in green algae and vascular plants, is a hexadecamer composed of 8 large subunits (RbcL), encoded by the chloroplast genome and 8 small, nuclear-encoded subunits (RbcS). Unlike its cyanobacterial homolog, which can be reconstituted in vitro or in E. coli, assisted by bacterial chaperonins (GroEL-GroES) and the RbcX chaperone, biogenesis of functional chloroplast Rubisco requires Cpn60-Cpn20, the chloroplast homologs of GroEL-GroES, and additional auxiliary factors, including Rubisco accumulation factor 1 (Raf1), Rubisco accumulation factor 2 (Raf2) and Bundle sheath defective 2 (Bsd2). The discovery and characterization of these factors paved the way for Arabidopsis Rubisco assembly in E. coli. In the present review, we discuss the uniqueness of hetero-oligomeric chaperonin complex for RbcL folding, as well as the sequential or concurrent actions of the post-chaperonin chaperones in holoenzyme assembly. The exact stages at which each assembly factor functions are yet to be determined. Expression of Arabidopsis Rubisco in E. coli provided some insight regarding the potential roles for Raf1 and RbcX in facilitating RbcL oligomerization, for Bsd2 in stabilizing the oligomeric core prior to holoenzyme assembly, and for Raf2 in interacting with both RbcL and RbcS. In the long term, functional characterization of each known factor along with the potential discovery and characterization of additional factors will set the stage for designing more efficient plants, with a greater biomass, for use in biofuels and sustenance.

Keywords: Rubisco; assembly; chaperone; chaperonin; chloroplast; folding.

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Figures

Figure 1
Figure 1
Phylogenetic tree of green-type RbcL subunits together with factors involved in Rubisco holoenzyme formation. Phylogenetic tree of RbcL sequences represents organisms mentioned in this review. The variety of folding and assembly factors and their involvement in Rubisco biogenesis are shown for each clade and discussed in the text. Species full names: Thiomonas intermedia K 12, Halothiobacillus neapolitanus, Synechococcus PCC6301, Nostoc sp. PCC7120, Anabaena sp. CA, Thermosynechococcus elongatus, Synechocystis PCC 6803, Chlamydomonas reinhardtii, Arabidopsis thaliana, Zea mays, Nicotiana tabacum. The phylogenetic tree was created using phylogeny.fr (http://www.phylogeny.fr; Dereeper et al., 2008, 2010).
Figure 2
Figure 2
Model summarizing the roles of different chaperones in Rubisco assembly. From top; Newly-synthesized RbcL (L) interacts with the chaperonin complex, which leads to correct folding (Native L). After import into chloroplast and cleavage of its transit peptide, RbcS (S) folds spontaneously, or with the help of a chaperone. Raf1, Raf2, RbcX, and Bsd2 form dynamic intermediates with the folded RbcL. RbcS subunits could either displace the chaperones in a final chaperone-RbcL intermediate to form the holoenzyme (L8S8), or interact with chaperones and RbcL in earlier stages of the assembly. Continuous and dashed arrows indicate certain and speculative nature of each step, respectively.

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