The Pyrenoid: A Liquid-Liquid Phase Separated CO2 Fixing Organelle

Approximately one-third of global carbon-fixation occurs in an overlooked algal organelle called the pyrenoid. The pyrenoid contains the CO₂-fixing enzyme Rubisco, and enhances carbon-fixation by supplying Rubisco with a high concentration of CO₂. Since the discovery of the pyrenoid over 130 years ago, the molecular structure and biogenesis of this ecologically fundamental organelle have remained enigmatic. To improve our understanding of the pyrenoid, we have developed high-throughput fluorescent protein-tagging pipelines to determine the localizations of over 250 candidate proteins in the green alga Chlamydomonas reinhardtii. Combining localization data with protein-protein interaction data has allowed us to generate a spatial interactome of the pyrenoid. This has led to the discovery that the pyrenoid is a highly dynamic non-membrane bound organelle that has liquid-like properties and can undergo rapid liquid-liquid phase separation. This Rubisco condensation is mediated by a low complexity repeat protein that links Rubisco molecules and enables phase separation. These new insights are guiding the engineering of the pyrenoid into higher plants with a goal to enhance crop carbon fixation efficiency.