The stickers and spacers of Rubiscondensation: assembling the centrepiece of biophysical CO2-concentrating mechanisms

Abstract

Aquatic autotrophs that fix carbon using ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) frequently expend metabolic energy to pump inorganic carbon towards the enzyme’s active site. A central requirement of this strategy is the formation of highly concentrated Rubisco condensates known as carboxysomes and pyrenoids, which have convergently evolved multiple times in prokaryotes and eukaryotes respectively. Recent data indicates these condensates form by the mechanism of liquid- liquid phase separation (LLPS). LLPS requires networks of weak multivalent interactions typically mediated by intrinsically disordered scaffold proteins. Here we comparatively review recent rapid developments that detail the determinants and precise interactions that underlie diverse Rubisco condensates. The burgeoning field of biomolecular condensates has few examples where LLPS can be linked to clear phenotypic outcomes. When present, Rubisco condensates are essential for photosynthesis and growth, and they are thus emerging as powerful and tractable models to investigate the structure function relationship of phase separation in biology.

A carboxysome‐based CO2 concentrating mechanism for C3 crop chloroplasts: advances and the road ahead

“…Additionally, under certain conditions, each disordered scaffold has been shown to phase‐separate with its cognate Rubisco, forming liquid‐like droplets in solution reminiscent of the liquid‐like state observed between Rubisco and essential pyrenoid component 1 (EPYC1) in the pyrenoid system of the microalga Chlamydomonas (Freeman‐Rosenzweig et al., 2017). While the functional relevance of carboxysomal Rubisco droplet‐like structures in vivo remains elusive, the propensity for carboxysomal Rubiscos to form phase‐separated condensates with disordered binding partners (CsoS2/CcmM) appears a widespread adaptation among bacterial and algal CCMs (Ang et al., 2023). Such interactions may delineate a major adaptation in the emergence of these systems, creating a diffusional barrier that would enable greater CO 2 elevation in ancestral CCM‐like Rubisco compartments (Flamholz et al., 2022; Long et al., 2021).…”

The challenge of engineering Rubisco for improving photosynthesis

“…advances in understanding the complex structure and assembly of carboxysomes and pyrenoids [27][28][29][30]…”

“…Both of these biophysical mechanisms involve the creation of a localized high CO 2 environment through the activity of carbonic anhydrase (CA), the enzyme that catalyzes the interconversion of CO 2 and bicarbonate (HCO 3 ‐ ). Recent advances in understanding the complex structure and assembly of carboxysomes and pyrenoids [27–30] have laid the groundwork towards engineering CCMs into plants [31,32]. Notably, to form a functional CCM in plants, bicarbonate transporters (BT) will have to be expressed in the chloroplast inner membrane to pump HCO 3 ‐ into the stroma [29].…”

Rubisco and its regulation—major advances to improve carbon assimilation and productivity

“…This CO 2 -rich environment enabled the evolution of faster Rubiscos in C 4 plants, algae, and cyanobacteria ( Sharwood, 2017 ). Rubisco condensation within pyrenoids and carboxysomes by liquid–liquid phase separation has been a major step forward in understanding Rubisco aggregation, mediated by disordered linkers such as EPYC1 in Chlamydomonas ( Ang et al ., 2023 ), and unlocking the potential to transfer these solutions from nature into improving crops.…”