Pyrenoid proteomics reveals independent evolution of the CO2-concentrating organelle in chlorarachniophytes

From algae to plants: understanding pyrenoid-based CO2-concentrating mechanisms

Pyrenoids are the key component of one of the most abundant biological CO2 concentration mechanisms found in nature. Pyrenoid-based CO2-concentrating mechanisms (pCCMs) are estimated to account for one third of global photosynthetic CO2 capture. Our molecular understanding of how pyrenoids work is based largely on work in the green algae Chlamydomonas reinhardtii. Here, we review recent advances in our fundamental knowledge of the biogenesis, architecture, and function of pyrenoids in Chlamydomonas and ongoing engineering biology efforts to introduce a functional pCCM into chloroplasts of vascular plants, which, if successful, has the potential to enhance crop productivity and resilience to climate change.

Hornworts reveal a spatial model for pyrenoid-based CO2-concentrating mechanisms in land plants

Pyrenoid-based CO2-concentrating mechanisms (pCCMs) turbocharge photosynthesis by saturating CO2 around Rubisco. Hornworts are the only land plants with a pCCM. Owing to their closer relationship to crops, hornworts could offer greater translational potential than the green alga Chlamydomonas, the traditional model for studying pCCMs. Here we report a thorough investigation of a hornwort pCCM using the emerging model Anthoceros agrestis. The pyrenoids in A. agrestis exhibit liquid-like properties similar to those in Chlamydomonas, but they differ by lacking starch sheaths and being enclosed by multiple thylakoids. We found that the core pCCM components in Chlamydomonas, including BST, LCIB and CAH3, are conserved in A. agrestis and probably have similar functions on the basis of their subcellular localizations. The underlying chassis for concentrating CO2 might therefore be shared between hornworts and Chlamydomonas, and ancestral to land plants. Our study presents a spatial model for a pCCM in a land plant, paving the way for future biochemical and genetic investigations.

π-π Interactions Drive the Homotypic Phase Separation of the Prion-like Diatom Pyrenoid Scaffold PYCO1

CO2 fixation in most unicellular algae relies on the pyrenoid, a biomolecular condensate, which sequesters the cell's carboxylase Rubisco. In the marine diatom Phaeodactylum tricornutum, the pyrenoid tandem repeat protein Pyrenoid Component 1 (PYCO1) multivalently binds Rubisco to form a heterotypic Rubisco condensate. PYCO1 contains prion-like domains and can phase-separate homotypically in a salt-dependent manner. Here we dissect PYCO1 homotypic liquid-liquid phase separation (LLPS) by evaluating protein fragments and the effect of site-directed mutagenesis. Two of PYCO1's six repeats are required for homotypic LLPS. Mutagenesis of a minimal phase-separating fragment reveals tremendous sensitivity to the substitution of aromatic residues. Removing positively charged lysines and arginines instead enhances the propensity of the fragment to condense. We conclude that PYCO1 homotypic LLPS is mostly driven by π-π interactions mediated by tyrosine and tryptophan stickers. In contrast π-cation interactions involving arginine or lysine are not significant drivers of LLPS in this system.