Self-assembling organelles for CO2 fixation: stoichiometry and structural plasticity

Sun et al. investigate how carboxysomes are constructed and regulated in cyanobacteria. Plant Cell

By Luning Liu

Background: All cells are composed of well-defined compartments to encase enzymes and reactions to increase the efficiency of biological processes. In bacteria that can perform photosynthesis, such as cyanobacteria, the specialized microcompartments are carboxysomes. The carboxysome contains a nanoscale polyhedral protein shell filled with the primary carbon-fixation enzyme RuBisCO. The self-assembling organelle is generated by hundreds of proteins and plays important roles in converting carbon dioxide to sugar – a process known as carbon fixation.

Question: We aimed to address the exact stoichiometry of all building components in the functional carboxysome and how carboxysomes manipulate their compositions, organizations and functions to cope with environmental changes.

Findings: Using advanced microscopic techniques, in collaboration with Professor Mark Leak at the University of York, we explored the exact abundance of individual building proteins that form a single carboxysome in the model cyanobacterium Synechococcus elongatus PCC7942. This information allowed us to build a model of the icosahedral carboxysome structure. Moreover, we revealed that the protein stoichiometry, size, and mobility of carboxysomes in cyanobacteria are sensitive to the microenvironmental environment, such as CO2 levels and light intensity. This provides insight into the cellular strategies of dynamic regulation.

Next steps: This study advanced our understanding of the structure and physiological regulation of natural carboxysomes in cyanobacteria. As there is increasing interest in installing functional carboxysomes into crop plants to enhance photosynthesis, the knowledge gleaned from the natural carboxysome structure should suggest feasible ways to produce functional carboxysomes in plants, with the overarching goal of boosting crop yields.

Figure caption: Carboxysome protein stoichiometry and structural variability in response to environmental changes

Yaqi Sun, Adam J. M. Wollman, Fang Huang, Mark C. Leake, and Lu-Ning Liu. (2019). Single-organelle quantification reveals the stoichiometric and structural variability of carboxysomes dependent on the environment. Plant Cell.

Key words: self-assembly, photosynthesis, carbon fixation, cyanobacteria