In-cell Structure and Variability of Pyrenoid Rubisco.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a key enzyme in the global carbon cycle, catalyzing CO fixation during photosynthesis. To overcome Rubisco's inherent catalytic inefficiency, many photosynthetic organisms have evolved CO -concentrating mechanisms. Central to these mechanisms is the pyrenoid, a protein-dense organelle within the chloroplast of eukaryotic algae, which increases the local concentration of CO around Rubisco and thereby enhances its catalytic efficiency. Although the structure of Rubisco has been extensively studied by methods such as X-ray crystallography and single particle cryo-EM, its native structure within the pyrenoid, its dynamics, and its association with binding partners remain elusive. Here, we investigate the structure of native pyrenoid Rubisco inside the green alga by applying cryo-electron tomography (cryo-ET) on cryo-focused ion beam (cryo-FIB) milled cells, followed by subtomogram averaging and 3D classification. Reconstruction at sub-nanometer resolution allowed accurate modeling and determination of a closed (activated) Rubisco conformation. Comparison to other reconstructed subsets revealed local variations at the complex active site and at the large subunit dimers interface, as well as association with binding proteins. The different structural subsets distribute stochastically within the pyrenoid. Taken together, these findings offer a comprehensive description of the structure, dynamics, and functional organization of Rubisco within the pyrenoid, providing valuable insights into its critical role in CO fixation.