Speaker
Description
Photosynthesis is a fundamental process in biology as it converts solar energy into chemical energy and thus, directly or indirectly, fuels all life on earth. The chemical energy is used to fix atmospheric CO2 and produce reduced carbon compounds in the Calvin-Benson-Bassham cycle. The key enzyme for this process in all photosynthetic organisms is ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), which is responsible for the conversion of an estimated amount of ~1011 tons of CO2 per annum into organic material. Rubisco is the most abundant enzyme in nature, owing in part to its low catalytic turnover rate and limited specificity for CO2 versus O2. Remarkably, Rubisco is also one of the most chaperone-dependent proteins, requiring the chaperonin system for folding and auxiliary factors for assembly.
To avoid the reaction with O2, cyanobacteria have evolved proteinaceous microcompartments called carboxysomes, in which the enzymes Rubisco and carbonic anhydrase (CA) are enclosed. Dissolved CO2 in the form of HCO3 diffuses through the proteinaceous carboxysome shell and is converted to CO2 by CA, generating a high concentration of CO2 for carbon fixation by Rubisco – the so-called CO2-concentrating mechanism (CCM). The shell also prevents access to reducing agents, generating an oxidizing environment inside the carboxysome. In beta-cyanobacteria the assembly of the carboxysome first involves the aggregation of Rubisco and CA, followed by shell formation. Recent advances have shown that early in the process of pro-carboxysome assembly, a specialized scaffolding protein called CM initiates phase-separation of both Rubisco and CA into condensates. I will describe our present understanding of the complex multivalent interactions that result in the sequestration of several proteins into a pro-carboxysome. Understanding carboxysome biogenesis will be important for efforts to engineer a CCM into crop plants.