A phase-separated CO2-fixing pyrenoid proteome determined by TurboID in Chlamydomonas reinhardtii.

Phase separation underpins many biologically important cellular events such as RNA metabolism, signaling and CO2 fixation. However, determining the composition of a phase-separated organelle is often challenging due to its sensitivity to environmental conditions, which limits the application of traditional proteomics techniques like organellar purification or affinity purification mass spectrometry to understand their composition. In Chlamydomonas reinhardtii, Rubisco is condensed into a crucial phase-separated organelle called the pyrenoid that improves photosynthetic performance by supplying Rubisco with elevated concentrations of CO2. Here, we developed a TurboID-based proximity labeling technique in which proximal proteins in Chlamydomonas chloroplasts are labeled by biotin radicals generated from the TurboID-tagged protein. By fusing two core pyrenoid components with the TurboID tag, we generated a high-confidence pyrenoid proxiome that contains most known pyrenoid proteins, in addition to new pyrenoid candidates. Fluorescence protein tagging of seven previously uncharacterized TurboID-identified proteins showed that six localized to a range of sub-pyrenoid regions. The resulting proxiome also suggests new secondary functions for the pyrenoid in RNA-associated processes and redox-sensitive iron-sulfur cluster metabolism. This developed pipeline can be used to investigate a broad range of biological processes in Chlamydomonas, especially at a temporally resolved sub-organellar resolution.