A Trajectory of Discovery: Metabolic Regulation by the Conditionally Disordered Chloroplast Protein, CP12

Abstract

The chloroplast protein CP12, which is widespread in photosynthetic organisms, belongs to the intrinsically disordered proteins family. This small protein (80 amino acid residues long) presents a bias in its composition; it is enriched in charged amino acids, has a small number of hydrophobic residues, and has a high proportion of disorder-promoting residues. More precisely, CP12 is a conditionally disordered proteins (CDP) dependent upon the redox state of its four cysteine residues. During the day, reducing conditions prevail in the chloroplast, and CP12 is fully disordered. Under oxidizing conditions (night), its cysteine residues form two disulfide bridges that confer some stability to some structural elements. Like many CDPs, CP12 plays key roles, and its redox-dependent conditional disorder is important for the main function of CP12: the dark/light regulation of the Calvin-Benson-Bassham (CBB) cycle responsible for CO2 assimilation. Oxidized CP12 binds to glyceraldehyde-3-phosphate dehydrogenase and phosphoribulokinase and thereby inhibits their activity. However, recent studies reveal that CP12 may have other functions beyond the CBB cycle regulation. In this review, we report the discovery of this protein, its features as a disordered protein, and the many functions this small protein can have.

Location of the photosynthetic carbon metabolism in microcompartments and separated phases in microalgal cells

“…CP12 is a conditionally disordered protein with multiple functions but no catalytic activity [12]. A high number of proteins that belong to the IDP family such as CP12 play a role in the microcompartmentalisation of the cells.…”

“…The N‐terminal hairpin of CP12 that harbours the disulphide‐bond Cys23‐Cys31 occupies the active site groove of PRK [84,85]. The formation of the ternary complex is mediated by the redox‐dependent structural transition of CP12 that we have described using NMR and small‐angle X‐ray scattering (SAXS) experiments: It is intrinsically disordered in reducing conditions and becomes partially ordered in oxidising conditions [12,93]. This protein is thus conditionally disordered and is a hub for the ternary complex association.…”

“…The recent molecular description of the GAPDH‐CP12‐PRK complex can be confronted to 30‐year‐old reports of copurification of active proteins (Table 1) [12]. For example in spinach, GAPDH and PRK were shown to associate with phosphoribose isomerase (PRI), RuBisCO, sedoheptulose‐1,7‐bisphosphatase and ferredoxin‐NADP + reductase close to the thylakoid membranes [97].…”

“…The molecular mechanisms underlying the regulation of carbon acquisition, assimilation and storage pathways are well-described in the literature. These involve physicochemical transitions such as changes in pH, redox potential, metabolites and metal ion concentrations within the cellular compartments where photosynthesis occurs [1][2][3][4], post-translational modifications [5][6][7], structural transitions of key enzymes [8][9][10] and reorganisations within supramolecular complexes [11][12][13][14]. Recent high-resolution cryo-electron microscopy images and tomograms from the cyanobacterium Synechocystis 6803 [15], the green alga Chlamydomonas reinhardtii [16][17][18] and the diatom Phaeodactylum tricornutum [19] enabled the cellular localisation of some metabolic pathways.…”

Ribulose-1,5-bisphosphate regeneration in the Calvin-Benson-Bassham cycle: Focus on the last three enzymatic steps that allow the formation of Rubisco substrate

“…Regardless of the photosynthetic organism ( e.g. , cyanobacteria, algae, bryophytes, and land plants), this pathway is comprised on the same 11 enzymes and two regulatory proteins, namely the molecular chaperone Rubisco activase and the intrinsically disordered scaffold CP12 ( Michelet et al., 2013 ; Bhat et al., 2017 ; Gerard et al., 2022 ). The first step of the CBB cycle involves the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco).…”